aa: a term of hawaiian origin. used in reference to a basaltic lava that occurs in flows with a fissured, rough and jagged surface.

accretionary: wedge a mass of sediment and oceanic lithosphere that is transferred from a subducting plate to the less dense, overriding plate with which it converges.

ablation:occurs when more glacier ice is lost by melting and evaporation each year than is added by snowfall.

(or)

loss of ice in a glacier or ice sheet from melting, sublimation, or calving of bergs into a body of water.

ablation complex: assemblage of sediments deposited during the ablation of a glacier, generally by being let down from at or near the glacier surface by melting of underlying ice.

ablation hummock: mound of till-like sediment deposited in a depression in the ice. the sediment becomes a positive topographic feature after the surrounding ice has melted.

abrasion: a form of mechanical weathering that occurs when loose fragments or particles of rocks and minerals that are being transported, as by water or air, collide with each other or scrape the surfaces of stationary rocks.

absolute age:the approximate age of a geologic event, feature, fossil, or rock in years. 'absolute' ages are determined by using natural radioactive 'clocks'. the preferred term is radiometric age.

accretion:a process that adds part of one tectonic plate to a larger plate along a convergent (collisional) plate boundary.

actinolite: a bright to gray-green member of the amphibole mineral family. in addition to silica, it contains calcium, magnesium, and iron. actinolite is a non-hazardous relative of asbestos and is a common mineral in metamorphic rocks.

active volcano:a volcano that has erupted within historical time and is likely to do so again in the future.

acid rain: rain that contains such acidic compounds as sulfuric acid and nitric acid, which are produced by the combination of atmospheric water with oxides released when hydrocarbons are burned. acid rain is widely considered responsible

for damaging forests, crops, and human-made structures, and for killing aqua-tic life.

acidic rock: an igneous rock that has a relatively high silica content. examples are granite and rhyolite. also see entries for basic, intermediate and ultrabasic rocks.

acre-foot: the volume of water needed to flood one acre of land to a depth of one foot. equivalent to 43,560 cubic feet, 1,233 cubic meters or 325,851 gallons. one of the most common units of measure used for reservoir capacity. also used in mineral resource calculations (an acre-foot of coal is a block of coal one acre in area and one foot thick - it weighs approximately 1,800 tons).

aftershock: a ground tremor caused by the repositioning of rocks after an earthquake. aftershocks may continue to occur for as long as two years after the initial earthquake. the intensity of an earthquake's aftershocks decreases over time.

aggradation: the process by which a stream's gradient steepens due to increased deposition of sediment.

a horizon: the top layer of soil. plant and other organic debris builds up in this layer. this is the part of the soil generally referred to as 'top soil'.

alkali: used in reference to materials that are rich in sodium and/or potassium.

alloy: a metal that is manufactured by combining two or more molten metals. an alloy is always harder than its component metals. bronze is an alloy of copper and tin.

alluvial: pertaining to material or processes associated with transportation and or subaerial deposition by concentrated running water.

alluvial fan: a fan-shaped wedge of sediment that typically accumulates on land where a stream emerges from a steep canyon onto a flat area. in map view it has the shape of an open fan. typically forms in arid or semiarid climates.

(or)a fan-shaped pile of sediment that forms

where a rapidly flowing mountain stream enters a relatively flat valley. as water slows down, it deposits sediment (alluvium) that gradually builds a fan.

(or)

a triangular deposit of sediment left by a stream that has lost velocity upon entering a broad, relatively flat valley.

alluvial valley: river or stream valley flanked by floodplains that are frequently inundated by seasonal floods and underlain by alluvium.

alluvium: an unconsolidated accumulation of stream-deposited sediments, including sands, silts, clays or gravels.

(or)sand, gravel, and silt deposited by rivers and

streams in a valley bottom.

(or)

unconsolidated clastic material subaerially deposited by running water, including gravel, sand, silt, clay, and various mixtures of these.

(or)

a deposit of sediment left by a stream on the stream's channel or floodplain.

(or)

sand, gravel, silt, and clay deposited adjacent to modern streams and derived from erosion of surface sediments elsewhere in the watershed or from valley walls.

alpine glacier: a mountain glacier that is confined by highlands.

amphibole:a family of silicate minerals forming prism or needlelike crystals. amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts, along with water. hornblende always has aluminum and is a most common dark green to black variety of amphibole; it, forms forming in many igneous and metamorphic rocks. actinolite has no aluminum; it and is needle-shaped and light green. blue amphibole contains sodium and, of course, is bluish in color.

amphibolite:a rock made up mostly amphibole and plagioclase feldspar. although the name amphibolite usually refers to a type of metamorphic rock, an igneous rock composed dominantly of amphibole can be called an amphibolite too.

andesite:fine-grained, generally dark colored, igneous volcanic rock with more silica than basalt. commonly with visible crystals of plagioclase feldspar. generally occurs in lava flows, but also as dikes. the most common rock in volcanic arcs.

(or)

the dark, aphanitic, extrusive rock that has a silica content of about 60% and is the second most abundant volcanic rock. andesites are found in large quantities in the andes mountains.

angle of repose: the maximum angle that a soil, sediment or other loose material can be placed or accumulate and be stable. the angle of repose varies for different types of materials and different moisture conditions.

(or)

the maximum angle at which a pile of unconsolidated material can remain stable.

angular unconformity: an erosional surface that separates rock units of differing dips. the rocks below the surface were deposited, deformed and eroded. the younger rocks above then accumulated upon the erosional surface.

anhydrous:literally, "without water". refers to minerals or other materials which do not have water as an primary constituent.

anion: negatively charged ion in solution. when minerals dissolve in water, they form ions that have a tiny positive or negative electrical charge.

annual snowline:a term used by glaciologists (scientists who study glaciers) for the boundary where the amount of snow loss from melting equals the amount of snow accumulation from snowfall (also called firn limit).

annular seal: cement or bentonite clay pumped into the space between the borehole wall and well casing to seal out water and contaminants.

annular space: the space between a well casing pipe and the drilled borehole into which the casing is inserted.

anthracite: the highest rank of coal. by definition, a coal with a fixed carbon content of over 91% on a dry ash-free basis. anthracite coals have a bright luster, break with a conchoidal fracture, a semi-metallic luster and are difficult to ignite. frequently referred to by the layman as "hard coal".

(or)

a hard, jet-black coal that develops from lignite and bituminous coal through metamorphism, has a carbon content of 92% to 98%, and contains little or no gas. anthracite burns with an extremely hot, blue flame and very little smoke, but it is difficult to ignite and both difficult and dangerous to mine.

anticline:a upward-curving (convex) fold in rock that resembles an arch. the central part contains the oldest section of rock.

(or)

a convex fold in rock, the central part of which contains the oldest section of rock. see also syncline.

(or)

a fold where the rocks are bent convex upward.

aphanitic: an igneous rock texture in which individual mineral grains are too small to be distinguished with the naked eye.

aplite: a light-colored igneous rock with the same mineral composition as granite: quartz, plagioclase feldspar, and potassium feldspar, but with a fine-grained, almost sugary texture.

aquiclude: a subsurface rock, soil or sediment unit that does not yield useful quanties of water.

(or)

an impermeable body of rock that may absorb water slowly but does not transmit it.

aquifer: a subsurface rock or sediment unit that is porous and permeable. to be an aquifer it must have these traits to a high enough degree that it stores and transmits useful quantities of water.

(or)

a permeable body of rock or regolith that both stores and transports groundwater.

(or)

a permeable body of rock, such as fractured bedrock or glacial till, that is saturated with ground water and is capable of providing significant quanties of water to wells and springs.

aquifer (artesian): an aquifer that is bounded above and below by impermeable rock or sediment layers. the water in the aquifer is also under enough pressure that, when the aquifer is tapped by a well, the water rises up the well bore to a level that is above the top of the aquifer. the water may or may not flow onto the land surface.

aquifer (confined): an aquifer that is bounded above and below by impermeable rock or sediment layers. there may or may not be enough pressure in the aquifer to make it an "artesian aquifer".

aquifer (unconfined): an aquifer that is not overlain by an impermeable rock unit. the water in this aquifer is under atmospheric pressure and is recharged by precipitation that falls on the land surface directly above the aquifer.

aquitard: a layer of rock having low permeability that stores groundwater but delays its flow.

(or)

slowly permeable stratum that retards water movement into and out of adjacent or underlying aquifers .

arc rocks: volcanic arc rocks.

arch: a broad open anticlinal fold on a regional scale.

archean eon: the time interval between 3800-2500 million years ago. the archean is one of the precambrian time intervals.

archeology:the science that focuses on the study of past human cultures.

arête: a sharp ridge of erosion-resistant rock formed between adjacent cirque glaciers.

argillaceous: a term used to describe clay-rich rocks.

argillic horizon: a clay-rich layer of soil. clay often forms in overlying soil layers from the decomposition of feldspars and other minerals. the extremely fine clay particles are gradually carried down by water to accumulate into the argillic horizon.

argillite: name used for unusually hard, fine-grained sedimentary rocks, such as

shale, mudstone, siltstone, and claystone. commonly black.

aridity: index the ratio of a region's potential annual evaporation, as determined by its receipt of solar radiation, to its average annual precipitation.

arkose: a sandstone that contains at least 25% feldspar. easily recognized because the feldspar grains are typically pink and angular in shape.

arroyo: a flat-bottom gully with steep sides that is a channel for an intermittent stream.

(or)

a small, deep, usually dry channel eroded by a short-lived or intermittent desert stream.

artesian: of, being, or concerning an aquifer in which water rises to the surface due to pressure from overlying water.

arthropod: a large group of invertebrate animals having segmented bodies, jointed legs, and exoskeleton (e.g., insects, trilobite).

aseismic: a region without earthquakes (seismic activity).

ash: fine particles of volcanic rock and glass blown into the atmosphere by a volcanic eruption.

asthenosphere: a portion of the upper mantle that is directly below the lithosphere. a zone of low strength in the upper mantle defines the top of the asthenosphere. this weak zone allows the plates of the lithosphere to slide across the top of the asthenosphere.

(or)the uppermost layer of the mantle, located below

the lithosphere. this zone of soft, easily deformed rock exists at depths of 100 kilometers to as deep as 700 kilometers.

(or)

a layer of soft but solid, mobile rock comprising the lower part of the upper mantle from about 100 to 350 kilometers beneath the earth's surface. see also lithosphere.

astrobleme: an ancient circular scar on earth's surface produced by the impact of a meteorite or comet. use our google maps page to get close up images of these meteor impact structures.

atoll: a ring-shaped group of coral islands that are surrounded by deep ocean water and that enclose a shallow lagoon. (or) a circular reef that encloses a relatively shallow lagoon and extends from a very great depth to the sea surface. an atoll forms when an oceanic island ringed by a barrier reef sinks below sea level.

atom: the smallest particle that retains all the chemical properties of a given element.

atomic mass: the sum of protons and neutrons in an atom's nucleus. 2. the combined mass of all the particles in a given atom.

atomic number: the number of protons in the nucleus of a given atom. elements are distinguished from each other by their at-omic numbers.

attenuation: the chemical, physical, and(or) biological processes that restrict the migration of contaminants through geologic media.

augen: augen are relatively large, eye-shaped mineral grains in certain types of metamorphic rocks, especially schist and gneiss. (augen = eyes in german)

avalanche: masses of rock or ice that fall or slide suddenly under the force of gravity.

backslope: the hillslope profile position that forms the steepest and generally linear, middle portion of the slope.

backwash: the seaward rush of water down a beach that occurs with a receding wave.

banded gneiss: see gneiss.

banded iron ore: a rock that consists of alternating layers of chert and iron oxide mineral (usually hematite) with the iron oxide in high enough concentration to be of economic value.

bankfull stage: a height of water in a stream that completely fills the natural channel. if the water rises any higher a flood will occur.

bank storage: water that seeps into the ground along the banks of a stream during a time of high flow. this loss of water into the ground slightly reduces the height that the stream will attain and then slowly seeps into the stream as the high water level subsides - hence the term "bank storage".

bar: an underwater ridge, usually of sand and/or gravel, that forms from the deposition and reworking of sediments by currents and/or waves. bars occur in rivers, river mouths and in offshore waters.

(or)

a generic term for any of various elongate offshore ridges, banks, or mounds of sand, gravel, or other unconsolidated material submerged or built up by the action of waves or currents.

barchan: a sand dune that is crescent-shaped in map view. barchan dunes form in areas of limited sand supply. they move across the desert floor with their gently sloping convex sides facing upwind and their steeply sloping concave sides facing downwind.

barrier island: a long, narrow island that parallels a shoreline.

basalt: a dark-colored fine-grained extrusive igneous rock composed largely of plagioclase feldspar and pyroxene. similar in composition to gabbro. basalt is thought to be one of the main components of oceanic crust.

(or)

a dark, fine-grained, extrusive (volcanic) igneous rock with a low silica content (40% to 50%), but rich in iron, magnesium and calcium. generally occurs in lava flows, but also as dikes. basalt makes up most of the ocean floor and is the most abundant volcanic rock in the earth’s crust.

basal outwash: sand and gravel that were deposited as outwash in front of an advancing glacier and subsequently overridden by the ice and buried by other kinds of deposits. basal outwash commonly forms the base of glacial depositional sequences.

basal till: glacial till deposited by melting of ice at the base of a glacier and little reworked by meltwater or mass movement.

base flow: water that seeps into a stream through a permeable rock or sediment unit that outcrops in the bottom or banks of the stream.

base level: the lower limit of erosion by a stream. sea level is the ultimate base level. however, lakes can serve as a temporary base level in upstream areas. (or)

basement: the igneous and metamorphic rocks that exist below the oldest sedimentary cover. in some areas such as shields the basement rocks may be exposed at the surface.

basic rock: an igneous rock that has a relatively low silica content. examples are gabbro and basalt. also see entries for acid, intermediate and ultrabasic rocks.

basin: in tectonics, a circular, syncline-like depression of strata. in sedimentology, the site of accumulation of a large thickness of sediments.

(or)a depression in the earth’s surface that

collects sediment.

(or)

a low area on the earth's crust where sediments have accumulated. these sediments may be consolidated or unconsolidated.

basin and range province: this province extends from eastern california to central utah, and from southern idaho into the state of sonora in mexico. within the basin and range province the earth’s crust and uppermost mantle have been stretched, creating large faults. along these faults linear mountain ranges were uplifted and flat valleys down-dropped, producing the distinctive topography of the basin and range province.

batholith: a very large intrusive igneous rock mass that has been exposed by erosion and with an exposed surface area of over 100 square kilometers. a batholith has no known floor.

(or) very large mass of intrusive (plutonic) igneous rock that forms

when magma solidifies at depth. a batholith must have greater than 100 square kilometers (40 square miles) of exposed area. see pluton, stock.

bathymetry: the measurement of ocean depths and the preparation of topographic maps of the ocean floor.

bauxite: the principal ore of aluminum. a mixture of aluminum oxides and hydroxides that forms from intense chemical weathering of a soil in tropical environments.

beach ridge: a low, essentially continuous mound of beach or beach-and-dune material heaped up by the action of waves and currents on the backshore of a beach, which is beyond the present limit of storm waves. these ridges roughly parallel the relict or present shoreline.

(or)

an arcuate ridge of sand that parallels or sub-parallels a coast. beach ridges are formed by fluctuation in water level that create the core of the ridge. later they increase in size by the addition of dune sand.

bed:a layer of sediment or sedimentary rock.

bedding: the characteristic structure of sedimentary rocks in which layers of different composition, grain size or arrangement are stacked one on top of another in a sequence with oldest at the bottom and youngest at the top.

(or)parallel layers of sediment or sedimentary rock (beds)

that can be distinguished from each other by characteristics such as grain size and chemical composition.

(or)sedimentary layers in a rock. the beds are distinguished

from each other by grain size and composition, such as in shale and sandstone. subtle changes, such as beds richer in iron-oxide, help distinguish bedding. most beds are deposited essentially horizontally.

bedding plane: a distinct surface of contact between two sedimentary rock layers.

(or)

surface marking the break between two distinct pulses (beds) of sediment deposition.

bed-load: the larger heavier particles that are being transported by a stream. instead of being dissolved or suspended, these are being rolled or bounced along, spending at least part of their time in contact with the stream bottom. see also: load, suspended load, dissolved load.

bedrock: solid rock present beneath any soil, sediment or other surface cover. in some locations it may be exposed at earth's surface.

(or) the solid rock that lies beneath soil and other

loose surface materials.

(or)

a general term for the solid rock that underlies the soil and other unconsolidated material or that is exposed at the surface.

(or)

consolidated rock composed of cemented or lithified sediments (such as sandstone, shale, limestone) or crystalline rock such as granite or slate. underlies all surfical soil, sand, gravel, clay, etc.

bentonite: type of clay derived from weathered volcanic ash that expands when wet; commonly used as well drilling mud and annular seal.

beta-particle: an electron emitted with high energy and velocity from the nucleus of an atom during radioactive decay.

b-horizon: a layer in the soil, below the a-horizon, where materials leached from above accumulate. typically enriched in clay and oxides.

billion: in north america, 1,000,000,000.

biochemical rocks: a sedimentary rock that forms from the chemical activities of organisms. organic (reef and fossiliferous) limestones and bacterial iron ores are examples.

biodegradable: subject to decomposition by biological means, especially by micro-organisms.

biotite: a common rock-forming mineral of the mica family. biotite is a black or dark brown silicate rich in iron, magnesium, potassium, aluminum, and, of course, silica. like other micas, it forms flat book-like crystals that peal apart into individual sheets on cleavage planes.

bioturbated: an adjective used in reference to a sediment or sedimentary rock. bioturbated sediments have been disturbed by animals (such as burronwing worms or shell fish) or plant roots. these have penetrated the sediment and disturbed any or all original sedimentary laminations and structures. bioturbated rocks were disturbed in this way while still in the soft sediment phase of their formation.

bituminous coal: a rank of coal that falls between anthracite and semi-bituminous. the most abundant rank of coal. frequently referred to by the layman as "soft coal".

bivalve: any mollusk having a shell in two parts, hinged together so it will open and close like a book.

block fault mountain: a linear mountain that is bounded on both sides by normal faults.

blow count: the unit of measure for the standard penetration test and representing the number of blows required to drive a 2-inch diameter core sampler one foot through unconsolidated material by dropping a 140-pound hammer from a distance of 30 inches.

blowout: a shallow, round or trough-shaped depression in sand or dry soil that is formed by wind erosion. the material removed by the wind may also be referred to as "blowout".

blueschist: metamorphic rock rich in blue amphibole.

bog: waterlogged, spongy ground, consisting of mosses containing acidic, decaying vegetation such as spaghnum, sedges, and heaths, that may develop into peat.

borrow pit: a pit or excavation area used for gathering earth materials

(borrow) such as sand or gravel.

boulder: any loose rock (sediment) larger than 256 millimeters (10 inches).

brachiopod: a group of sea animals with hinged half shells and a soft body.

brackish: containing salt, brine.

breccia: rock made up of angular fragments of other rocks held together by mineral cement or a fine-grained matrix. volcanic breccia is made of volcanic rock fragments, generally blown from a volcano or eroded from it. fault breccia is made by breaking and grinding rocks along a fault.

butte: a conspicuous hill with steep sides and a flat top. the top is usually a cap-rock of resistant material. this structure is frequently an erosional remnant in an area of flat-lying sedimentary rocks.

calcareous: a descriptive term used for rocks and other earth materials that have an abundance of calcium carbonate (caco3). for example, a calcareous sandstone has up to 50% calcium carbonate.

calcic horizon: a soil layer at least 15 cm thick that has been enriched with calcium carbonate (caco3).

calcite: mineral made of calcium carbonate (caco3). generally white, easily scratched with knife. most seashells are made of calcite or related minerals. this is the lime of limestone.

caldera: a large, bowl-shaped crater associated with a volcanic vent. a caldera can form from a volcanic blast or the collapse of a volcanic cone into an emptied magma chamber.

(or) large, generally circular, fault-bounded

depression caused by the withdrawal of magma from below a volcano or volcanoes. commonly, the magma erupts explosively as from a giant volcano and, falling back to earth as volcanic ash, fills the caldera so formed.

carbonate: a sedimentary rock made mainly of calcium carbonate (caco3). limestone and dolomite are common carbonate sedimentary rocks.

carbonate rock: a rock made up primarily of carbonate minerals (minerals containing the co3 anionic structure). limestone (made up primarily of calicite - caco3) and dolostone (made up primarily of dolomite - camg (co3)2 are the most common examples.

carbonic acid: a weak acid (h2co3) that forms from the reaction of water and carbon dioxide. most rain water is a very weak carbonic acid solution formed by the reaction of rain with small amounts of carbon dioxide in the atmosphere.

(or) a mild acid formed when water and carbon dioxide chemically combine in the atmosphere and soil.this acid is a very important component in the development of cave decorations (speleothems).

cataclastic rock: a breccia of powdered rock formed by crushing and shearing during tectonic movements.

cave: a natural opening in the ground extending beyond the zone of light and large enough to permit the entry of an average human.

cave system: a cave or caves having a complex network of interconnected chambers and passages that constitute an underground drainage system.

cavernous weathering: a combination of chemical and mechanical weathering processes act on rock surfaces to produce hollows and caverns. this is also called honeycomb weathering.

cement: a solid precipitate of calcium carbonate, silica, iron oxide, clay minerals or other materials that forms within the pore spaces of a sediment and binds it into a sedimentary rock.

cementation: the processes through which chemical precipitates form within the pore spaces of a sediment and help bind it into a sedimentary rock.

(or)one of the processes that work together to turn

sediment into sedimentary rock (lithification). mineral-laden water percolates through sediment with open pore spaces. the spaces are gradually filled by minerals precipitating from the water, binding the grains together.

cenozoic era: the time span between 66.4 million years ago to the present.

chemical sedimentary rock: a rock that forms from the precipitation of mineral material from solution. examples are chert and rock salt.

(or)sedimentary rock composed of minerals that were

precipitated from water. this process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source. eventually these minerals are redeposited, or precipitated, when the water evaporates away or when the water becomes over-saturated.

chemical weathering: the breaking down of surface rock material by solution or chemical alteration. common alteration processes are oxidation and hydrolysis.

(or)the process that changes the chemical makeup of

a rock or mineral at or near the earth’s surface. chemical weathering alters the internal structure of minerals by the removing and/or adding elements. compare with mechanical weathering.

chert: a microcrystalline or cryptocrystalline sedimentary rock material composed of sio2. occurs as nodules and concretionary masses and less frequently as a layered deposit.

(or)

a very fine-grained sedimentary rock made of quartz. usually made of millions of globular siliceous skeletons of tiny marine plankton called radiolarians. black chert is called flint.

chlorite: family of platy silicate minerals containing various amounts of magnesium, iron, aluminum, water, and small amounts of other elements. some mineralogists include chorites in the mica family because the crystals form small flakes. commonly green.

c-horizon: the lowest horizon of a soil profile. it is below the b-horizon and is made up of weathered bedrock.

cinder: a bubbly (vesicular) volcanic rock fragment that forms when molten, gas-filled lava is thrown into the air, then solidifies as it falls.

cinder cone: a cone-shaped hill that consists of pyroclastic materials ejected from a volcanic vent.

(or)

a volcanic cone built almost entirely of loose volcanic fragments, ash, and pumice (pyroclastics or tephra).

cirque: a bowl-shaped depression with very steep sides that forms at the head of a mountain glacier. forms from cold-climate weathering processes including frost wedging and plucking.

clast: a fragment of a pre-existing rock or fossil embedded within another rock.

clastic: a sedimentary rock (such as shale, siltstone, sandstone or conglomerate) or sediment (such as mud, silt, sand, or pebbles). an accumulation of transported weathering debris.

(or)

a sedimentary rock composed of fragments (clasts) of pre-existing rock or fossils. (=detrital sedimentary rocks).

clay: a clastic mineral particle of any composition that has a grain size smaller than 1/256 mm. the term is also used in reference to a broad category of hydrous silicate minerals in which the silica tetrahedrons are arranged into sheets.

(or)

a family of platy silicate minerals that commonly form as a product of rock weathering. also, any particle smaller than 1/256 of a millimeter in diameter.

cleavage: the tendency of a mineral to break along weak planes.

coal: a brown or black sedimentary rock that forms from accumulated plant debris. a combustible rock that contains at least 50% (by weight) carbon compounds.

coastal plain: an area of low relief along a continental margin that is underlain by thick, gently dipping sediments.

cobble: loose particles of rock or mineral (sediment) that range in size from 64 - 256 millimeters in diameter. cobbles are a size of gravel larger than pebbles, but smaller than boulders.

compaction: a compression process that reorients and reshapes the grains of a sediment in response to the weight of overlying deposits.

(or)

occurs when the weight of overlying material compresses more deeply buried sediment. along with cementation, this process converts sediments to solid rock.

composite cone: a cone-shaped volcanic mountain composed of alternating layers of cinders and lava flows. also known as a stratovolcano.

composite volcano: see stratovolcano.

cone of depression: a cone-shaped lowering of the water table around a producing well.

conformable: rock layers that were deposited in sequence without episodes of erosion between deposition of layers.

conglomerate: a sedimentary rock rock made of rounded rock fragments, such as pebbles, cobbles, and boulders, in a finer-grained matrix. to call the rock a conglomerate, some of the consituent pebbles must be at least 2 mm (about 1/13th of an inch) across.

contact metamorphism: alteration of a rock, mainly by heat, which occurs adjacent to a dike, sill, magma chamber or other magma body.

(or)

metamorphism caused by heat from an igneous intrusion.

continental collision: convergence of two continental plates. such a convergence between the indian and eurasian plates is responsible for producing the himalayas.

continental crust: the rigid, outer layer of relatively low density rock that makes up the continents.

continental drift: a hypothesis proposed by alfred wegener suggesting that the continents are not stationary, but have 'drifted' through time. plate tectonics is the name for the theory that provided the evidence necessary to support wegener’s hypothesis.

contour line: a line on a map that traces locations where the value of a variable is constant. for example, contour lines of elevation trace points of equal elevation across the map. all points on the "ten foot" contour line are ten feet above sea level.

contour map: a map that shows the change in value of a variable over a geographic area through the use of contour lines. for example, a contour map of elevation has lines that trace points of equal elevation across the map. see also: contour line and topographic map.

convergent plate boundary: a boundary in which two plates collide. the collision can be between two continents (continental collision), an relatively dense oceanic plate and a more buoyant continental plate (subduction zone) or two oceanic plates (subduction zone).

core: the innermost layer of the earth, made up of mostly of iron and nickel. the core is divided into a liquid outer core and a solid inner core. the core is the most dense of the earth’s layers.

cordilleran ice sheet: ice cap that grew in western north america during the pleistocene epoch. it began growing first in canada, eventually covering much of british columbia, alaska, the northern u.s., and parts of several western states.

crater: the depression produced by a meteorite impact or at the summit of a volcano.

craton: the relatively stable nucleus of a continent. cratons are made up of a shield-like core of precambrian rock and a buried extension of the shield.

crossbedding: cross-stratification in which the cross-beds are more than 1 cm in thickness.

crust: the rocky, relatively low density, outermost layer of the earth.

crystallization: growth of minerals (crystalline solids) from a liquid or gas.

cubic feet per second: a unit of measure frequently used to quantify the rate of flow of a stream. it is equal to a volume of water one foot high and one foot wide moving a linear distance of one foot in one second.

data base :a set of words, numbers, locations, or other data put into a computer program. data bases are set up so that related pieces of information can be easily retrieved and compiled.

datum: a reference location or elevation which is used as a starting point for subsequent measurements. sea level is a datum for elevation measurements. datums can also be arbitrary such as the starting point for stream stage measurements or based upon a physical feature such as the base of a rock unit.

daughter element: the element produced through the radioactive decay of a parent element.

daughter product:an isotope produced by decay of a radioactive element.

debris avalanche: the sudden downslope movement of rock and soil on a steep slope.

debris flow:a type of landslide made up of a mixture of water-saturated rock debris and soil with a consistency similar to wet cement. debris flows move rapidly downslope under the influence of gravity. sometimes referred to as earth flows or mud flows.

deflation: the removal of clay- and silt-size particles from a soil by wind erosion. the term can also be used in reference to the removal by wind of any unconsolidated material. (or) removal of loose material by wind.

deformation:general term for folding, faulting, and other processes resulting from shear, compression, and extension of rocks.

delta: a deposit of sediment that forms where a stream enters a standing body of water such as a lake or ocean. the name is derived from the greek letter "delta" because these deposits typically have a triangular shape in map view.

(or) a fan-shaped deposit that forms where a stream

enters a lake or ocean and drops its load of sediment.

(or)

a body of alluvium, nearly flat and fan-shaped at or near the mouth of a stream where it enters a body of relatively quiet water, usually a lake.

dendritic drainage: a stream drainage pattern that resembles the veins of a leaf in map view. occurs mainly where the rocks below have a uniform resistance to erosion.

density:the weight per unit volume of a material.

density current: a gravity-driven flow of dense water down an underwater slope. the increased density of the water is a result of a temperature difference, increased salinity or suspended sediment load.

deposit:any accumulation of sediment.

deposition: the settling from suspension of transported sediments. also, the precipitation of chemical sediments from mineral rich waters.

depression: any relatively sunken part of the earth's surface, especially a low-lying area surrounded by higher ground.

desiccate:to dry out, usually by evaporation of water.

desert:a region with an average annual rainfall of 10 inches or less.

desert pavement: a ground cover of granule-size and larger particles that is typically found in arid areas. this ground cover of coarse particles is a residual deposit - formed when the wind selectively removes the sand-, silt- and clay-sized materials. (or) a closely-packed surface layer of coarse pebbles and gravel.

detrital: a word used in reference to sediments or sedimentary rocks that are composed of particles that were transported and deposited by wind, water or ice.

diagenesis: all of the changes which happen to a sediment after deposition, excluding weathering and metamorphism. diagenesis includes compaction, cementation, leaching and replacement. (or) a group of processes that cause physical and chemical changes in sediment after it has been deposited and buried under another layer of sediment. diagenesis may culminate in lithification of sediment, turning it into solid rock.

diapir: forceful, upward intrusion of a rock mass into overlying rock. in the case of an igneous diapir, the intruding rock may be magma or a crystal-rich mush, either of which is less dense than the surrounding rock.

diatom: a one celled plant that lives in the shallow waters of lakes, streams or oceans. many of these secrete a shell or internal parts composed of silica. diatoms can occur in very large numbers and can make significant contributions to sea-floor or lake sediment.

diatomite: a light colored, fine-grained siliceous sedimentary rock that forms from a sediment rich in diatom remains.

diatom ooze: a seafloor sediment that consists of at least 30% diatom remains.

differentiated planet: a planet that has layers composed of elements and minerals of different densities. as an example, earth is a differentiated planet because it has a metal-rich core, surrounded by a rocky mantle, and covered by a crust of low-density minerals.

dike:a sheet-like or tabular-shaped igneous intrusion that cuts across the sedimentary layering, metamorphic foliation, or other texture of a pre-existing rock.

diorite:intrusive igneous rock made of plagioclase feldspar and amphibole and/or pyroxene. similar to gabbro only not as so dark, and containing less iron and magnesium.

(or)

intrusive igneous rock made of plagioclase feldspar and amphibole and/or pyroxene. similar to gabbro only not as so dark, and containing less iron and magnesium.

dip: the angle that a rock unit, fault or other rock structure makes with a horizontal plane. expressed as the angular difference between the horizontal plane and the structure. the angle is measured in a plane perpendicular to the strike of the rock structure.

(or)

a measure of the angle between the flat horizon and the slope of a sedimentary layer, fault plane, metamorphic foliation, or other geologic structure.

disappearing stream:in karst areas, streams often disappear into the ground usually at a sinkhole.

discharge: the volume of water in a flowing stream that passes a given location in a unit of time. frequently expressed in cubic feet per second or cubic meters per second. calculated by the formula q = a x v where q is the discharge, a is the cross sectional area of the channel and v is the average velocity of the stream.

(or)

the amount of water issuing from a spring or in a stream that passes a specific point in a given period of time.

discontinuity: a surface separating rock layers of differing properties or compositions. (see seismic discontinuity)

dissolution:the process of chemical weathering of bedrock in which the combination of water and acid slowly removes mineral compounds from solid bedrock and carries them away in liquid solution. also called chemical solution.

dissolved load: the dissolved material being carried by a stream. see also: load, suspended load, dissolved load.

distal: material that is deposited farthest from the source.

divergent plate boundary:a boundary in which two tectonic plates move apart.

divide: a ridge that separates two adjacent drainage basins.

doline: see sinkhole.

dolomite: a magnesium-rich carbonate sedimentary rock. also, a magnesium-rich carbonate mineral (camgco3). (or) a carbonate sedimentary rock consisting chiefly (more than 50% by weight) of the mineral dolomite.

dome: an uplift that is round or elliptical in map view with beds dipping away in all directions from a central point.

drainage: any channel that carries water.

drainage basin: the geographic area that contributes runoff to a stream. it can be outlined on a topographic map by tracing the points of highest elevation (usually ridge crests) between two adjacent stream valleys. (or)

the land area drained by a stream.

drainage divide: the boundary between two adjacent drainage basins. drainage divides are ridge crests (or less obvious locations where slope of the landscape changes direction). runoff produced on one side of the ridge flows into stream "a" and runoff on the other side of the ridge flows into stream "b".

drainageway: a general term for a course of channel which water moves in draining an area.

drawdown: a lowering of the water table around a producing well. the drawdown at any given location will be the vertical change between the original water table and the level of the water table reduced by pumping.

drift: a general term for all sedimentary materials deposited directly from the ice or melt water of a glacier.

(or)

a general term applied to all mineral material (clay, sand, silt, boulders) transported by a glacier and deposited directly by or from the ice, or by running water emanating from the glacier. generally applies to pleistocene glacial deposits.

drumlin: a low, smoothly rounded, elongate hill. drumlins are deposits of compacted till that are sculpted beneath the ice of a flowing glacier. the long axis of a drumlin parallels the flow direction of the ice.

dune: a mound or ridge of wind-blown sand. typically found in deserts and inland from a beach. many dunes are moved by the wind. (or)

a usually asymmetrical hill of wind-deposited sand.

dune, eolian: a low mound, ridge, bank, or hill of loose, windblown, granular material (generally sand), either bare or covered by vegetation, that is capable of movement from place to place but always maintaining its characteristic shape.

durable crust: an outer rind or crust formed on a rock. durable crusts form when rock chemically reacts with water and possibly atmospheric dust, producing a hard outer surface that resists weathering.

erratic: a rock fragment carried by glacial ice, or by floating ice, and subsequently deposited at some distance from the outcrop of which it was derived.

earthflow: a detached mass of soil that moves downslope over a curved failure surface under the influence of gravity. an earthflow is more complex than a slump; it has a higher moisture content and the moving mass of soil has some internal movement or "flow". rates of movement are typically a few inches per year but faster rates can occur.

earthquake: a trembling of the earth caused by a sudden release of energy stored in subsurface rock units.

(or)

a sudden ground motion or vibration of the earth. produced by a rapid release of stored-up energy along an active fault.

ebb tide: a tidal current that generally moves seaward and occurs during the part of the tide cycle when sea level is falling. (see also: flood tide)

effluent stream: a stream that gains water from ground water flow. these streams are typical of humid climates where water tables are high. the discharge of an effluent stream can be sustained by ground water flow for long periods of time between runoff-producing rainfall or snowmelt. effluent streams generally increase in discharge downstream and contain water throughout the year. the opposite is an influent stream.

elastic limit: the maximum stress that can be applied to a body without resulting in permanent deformation - the rock reverts to its original shape after the stress is removed. in the case of a fault or a fold the elastic limit is exceeded and the deformation becomes a permanent structure of the rock.

elastic rebound theory: a theory that explains the earthquake process. in this theory, slowly accumulating elastic strain builds within a rock mass over an extended length of time. this strain is suddenly released through fault movement, producing an earthquake.

electron: a subatomic particle with a negative charge and of negligible mass that orbits the nucleus of an atom.

elevation: the vertical distance between mean sea level and a point or object on, above or below earth's surface.

eolian: a term used in reference to the wind. eolian materials or structures are deposited by or created by the wind. (or) term describing the

process of wind erosion, transport, and deposition, and wind-created deposits and structures such as sand dunes.

eon: the major divisions of the geologic time scale. eons are divided into intervals know as "eras". two eons of the geologic time scale are the phanerozoic (570 million years ago to present) and the cryptozoic (4,600 million years ago until 570 million years ago).

(or)

the largest time unit on the geologic time scale.

ephemeral stream: a stream drainage that is usually dry and fills with water only during brief episodes of rainfall. many desert streams ephemeral.

epicenter: the point on the earth's surface directly above the focus of an earthquake.

(or)

the point on the earth’s surface located directly above the focus of an earthquake.

epidote: family of silicate minerals containing mostly calcium, aluminum, iron and magnesium along with water. epidote is apple green and generally forms very small, stubby, prismatic crystals. it often occurs in veins or as a green coating on fracture surfaces. most common in metamorphic rocks, but occasionally forms in igneous plutons that crystallize very deep in the crust.

epoch: a subdivision of geologic time that is longer than an age but shorter than a period. the tertiary period is divided into five epochs. from most recent to oldest they are: pliocene, miocene, oligocene, eocene and paleocene.

era: a subdivision of geologic time that is longer than a period but shorter than an eon. precambrian, paleozoic, mesozoic, and cenozoic are the eras of the time scale from oldest to youngest.

erosion: a general term applied to the wearing away and movement of earth materials by gravity, wind, water and ice. (or) removal of material by water, wind, or ice.

eruption: occurs when solid, liquid, or gaseous volcanic materials are ejected into the earth’s atmosphere or surface by volcanic activity. eruptions may occur as quiet lava flows or violent explosive events.

esker: a long winding ridge of sorted sands and gravel. thought to be formed from sediment deposited by a stream flowing within or beneath a glacier.

eustatic sea level change: a rise or fall in sea level that affects the entire earth. thought to be caused by an increase/decrease in the amount of available water or a change in the capacity of ocean basins.

evaporation: the process of liquid water becoming water vapor. includes vaporization from water surfaces, land surfaces and snow/ice surfaces.

evaporite: a chemical sediment or sedimentary rock that has formed by precipitation from evaporating waters. gypsum, salt, nitrates and borates are examples of evaporite minerals.

evapotranspiration: all methods of water moving from a liquid to water vapor in nature. includes both evaporation and transpiration.

exfoliation: a physical weathering process in which concentric layers of rock are removed from an outcrop.

expansive clay (expansive soil): a clay soil that expands when water is added and contracts when it dries out. this volume change when in contact with buildings, roadways, or underground utilities can cause severe damage.

extension: in geology, the process of stretching the earth’s crust. usually cracks (faults) form, and some blocks sink, forming sedimentary basins.

extrusive: igneous rocks that crystallize at earth's surface.

(or)

igneous rocks that cool and solidify rapidly at or very near the earth’s surface. also known as volcanic rocks.

facies: the characteristics of a rock mass that reflect its depositional environment. these characteristics enable the rock mass to be distinguished from rocks deposited in adjacent environments.

fan: a fan-shaped sedimentary deposit that forms where rapidly flowing water enters a relatively open, flat area. as water slows down, it deposits sediment and gradually builds a fan. see alluvial fan.

fault: a fracture or fracture zone in rock along which movement has occurred.

(or)

a fracture in the earth along which one side has moved in relative to the other. sudden movements on faults cause earthquakes.

fault-block mountain: a linear mountain that is bounded on both sides by normal faults.

fault scarp: a steep slope or cliff formed when movement along a fault exposes the fault surface.

faunal succession: a principle of relative dating that is based upon the observed sequence of organisms in the rock record. the relative age of two rock units can frequently be determined by matching the fossils found in those rocks to their positions in the rock record.

feldspar: family of silicate minerals containing varying amounts of potassium, sodium and calcium along with aluminum, silicon and oxygen. potassium feldspars contain considerable potassium. plagioclase feldspars contain considerable sodium and calcium. feldspar crystals are stubby prisms, generally white, gray, or pink.

felsic: a term used to describe an igneous rock that has a large percentage of light-colored minerals such as quartz, feldspar, and muscovite. also used in reference to the magmas from which these rocks crystallize. felsic rocks are generally rich in silicon and aluminum and contain only small amounts of magnesium and iron. granite and rhyolite are examples of felsic rocks. (see mafic to contrast.)

(or)

a term used to describe light-colored igneous rocks with an abundance of light-colored minerals, especially feldspars and quartz.

fen: waterlogged, spongy ground containing alkaline vegetation, characterized by reeds, that develops into peat.

firn: partially compacted snow that survives the summer melting season.

firn limit: a term used by glaciologists (scientists who study glaciers) for the boundary where the amount of snow loss from melting and evaporation equals the amount of snow accumulation from snowfall (also called the annual snowline).

fission tracks: microscopic tunnels made in crystals by escaping nuclear particles emitted by radioactive elements. most commonly studied are fission tracks

in zircon crystals made by the radioactive decay of uranium, present as an impurity.

fissure: elongate, narrow fractures.

fjord: a deep, narrow, steep-walled, u-shaped valley that was carved by a glacier and is now occupied by the sea.

flaggy: term used to describe sedimentary or metamorphic rocks that tend to split into layers that are 1-10 cm thick.

flood: an overflow of water onto lands that are normally above local water levels. can be caused by stream discharge exceeding the capacity of the stream channel, storm winds and reduced pressure drawing water from a lake or ocean onto the coastline, dam failure, lake level increase, local drainage problems or other reasons.

(or)

a lake, stream, or other body of water that flows over its natural confining boundaries. during a flood, water flows out over land not normally covered with water.

flood basalt: a sequence of parallel to subparallel basalt flows that were formed during a geologically brief interval of time and which covered an extensive geographic area. thought to have formed from simultaneous or successive fissure eruptions.

flood plain: an area of alluvium-covered, relatively level land along the banks of a stream that is covered with water when the stream leaves its channel during a time of high flow.

(or)

a relatively flat surface next to a stream. during floods, when the stream overflows its banks, water flows over the flood plain. streams construct flood plains that accommodate their maximum flood capacity.

(or)

the nearly level plain that borders a stream and is subject to inundation under floodstage conditions unless protected artificially.

flood stage: a water height that is reached when the discharge of a stream exceeds the capacity of the channel.

flood tide: a tidal current that generally moves landward and occurs during the part of the tide cycle when sea level is rising. (see neap tide for contrast.)

flowing well: a well that taps an aquifer that is under enough pressure to force water to the surface. caused when the aquifer has a recharge area at a higher elevation.

flowstone: a general term for a type of cave decoration or speleothem that encrusts floors or walls of caves.

flowtill: a supraglacial till that is modified and transported by mass flow.

fluid inclusion: a small amount of fluid (liquid and/or gas) trapped within a rock and which is thought to represent the fluid from which the rock crystallized.

fluvial: term used to describe river or stream-related features or processes. fluvial deposits are sediments deposited by the flowing water of a stream.

(or)

of or pertaining to rivers; produced by river action.

focus: a point beneath earth's surface where the vibrations of an earthquake are thought to have originated. also known as a hypocenter.

(or)

the location where an earthquake begins. rock ruptures at this spot, then seismic waves radiate outward in all directions.

fold: a bend or flexure in a rock unit or series of rock units that has been caused by crustal movements.

foliation: the planar or layered characteristics of metamorphic rocks that are evidence of the pressures and/or temperatures to which the rock was exposed. these can be structural such as cleavage, textural such as mineral grain flattening or elongation, or compositional such as mineral segregation banding.

(or)

aligned layers of minerals characteristic of some metamorphic rocks. foliation forms in metamorphic rocks when pressure squeezes flat or elongates minerals so that they become aligned. these rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.

footslope: the hillslope position that forms the inner, gently inclined surface at the base of a hillslope.

formation: a rock formation is a body of rock of considerable extent with distinctive characteristics that allow geologists to map, describe, and name it.

foraminifer: a group of single-celled organisms, mostly marine, that produce a calcium carbonate shell. their shells can make up a significant portion of the carbonate sediment in some areas.

foraminiferal ooze: a calcareous sea-floor sediment composed of foraminifer shells.

forset beds: the distinctly dipping sediment layers deposited on the front of a prograding delta or on the lee side of a sand dune.

formation: a laterally continuous rock unit with a distinctive set of characteristics that make it possible to recognize and map from one outcrop or well to another. the basic rock unit of stratigraphy.

fossil: remains, imprints or traces of an ancient organism that have been preserved in the rock record. bones, shells, casts, tracks and excrement can all become fossils.

(or)

mineralized remains or traces of organisms.

fossil fuel: a carbon-rich rock material or fluid, of organic origin that can be produced and burned as a fuel. coal, oil and natural gas are examples of fossil fuels.

(or)

general term for any hydrocarbon used as fuel, including coal, oil, natural gas, and oil shale.

fracture: any break in rock along which no significant movement has occurred.

freeze-thaw cycle: in colder temperate regions, water trapped in fractures and between grains of rocks repeatedly freezes, then thaws during the winter months. in some areas this occurs on a daily basis as water freezes at night, then melts in warmer daytime temperatures. only in the coldest regions does water remain frozen throughout the winter.

frost wedging: a process that mechanically breaks apart rock caused by expansion of water as it freezes in cracks and crevices.

fumarole: a vent that emits hot gases, usually associated with past or current magmatic activity below.

(or)

a volcanic vent that emits hydrogen sulfide or other gases.

gabbro: a black, coarse-grained intrusive igneous rocks that is the compositional equivalent of basalt. composed of calcium-rich feldspars, pyroxene and possibly olivine, but containing little if any quartz.

(or)

a dark, coarse-grained intrusive igneous rock. gabbro is made of calcium-rich plagioclase, with amphibole and/or pyroxene, and is chemically equivalent to basalt.

gage height: a measured height of water above a reference datum. frequently used to describe the height of water in a stream, lake, well, canal or other water body.

gaging station: a facility on a stream, lake, canal, reservoir or other water body where instruments are installed to automatically monitor the water. measurments such as stage, discharge, water temperature and ph are automatically taken and transmitted to hydrologists via satellite, radio or telephone. measurements from these stations are useful for a wide variety of flood prediction, water management, recreation and navigation purposes.

garnet: family of silicate minerals containing varying amounts of aluminum, iron, magnesium, and calcium. schist and gneiss often have tiny, glassy red garnet dodecahedrons.

geochemistry: a branch of geology that focuses on the chemical composition of earth materials.

geochronology: a study of the time relationships of rock units. includes methods of both relative and absolute dating.

geomorphology: the science of earth's landforms, their description, classification, distribution, origin and significance.

(or)a branch of geology and geography that studies

the development of landforms.

(or)

the science that treats the general configuration of the earth's surface; specifically the study of the classification, description, nature, origin, and development of landforms and their relationship to underlying structures, and of the history of geologic changes as recorded by these surface features.

geosyncline: a major trough or downwarp of the earth's crust, in which great thicknesses of sedimentary and/or volcanic rocks have accumulated.

geothermal energy: power generation using natural steam derived from the earth’s internal heat.

geothermal gradient: the progressive increase of temperature with depth into the earth.

geyser: a hot spring that intermittently erupts a spray of steam and hot water. caused by the heating of ground water within a confined opening in hot rock.

glaciation: the formation, movement, and recession of glaciers or ice sheet; geologic processes of glacial activity.

glacial lake: a lake that derives much or all of its water from the meltingof glacier ice, fed by meltwater, and lying outside the glaciers margin.

glacial rebound: a very gradual uplift of earth's crust that occurs after the weight of a thick continental ice sheet (which produced subsidence) has melted away.

glacial striations: grooves and scratches on a bedrock surface that were produced the movement of a glacier. the orientation of the striations gives evidence to the direction of glacial movement.

glacial valley: a valley with a u-shaped cross section that was cut by an alpine glacier.

glaciofluvial deposits: material moved by glaciers and subsequently sorted and deposited by streams flowing from the melting ice. materials are commonly stratified.

glacier: a thick mass of ice that forms on land from an accumulation and recrystallization of snow significant enough to persist through the summer and grow year by year. there are two basic types of glaciers: 1) valley (or alpine) glaciers that creep downslope under the influence of gravity, and 2) continental glaciers that flow outward from a thick central area under their own weight.

(or)

a long-lived sheet or mass of ice made of recrystallized snow. glaciers move downhill due to the stress of their own weight.

glass: an amorphous (without crystal structure) igneous rock that forms from very rapid cooling of magma. the rapid cooling does not provide enough time for crystal growth.

glass (volcanic): natural glass (obsidian) that forms when molten lava cools too rapidly to permit crystal growth.

gneiss: a coarse-grained, foliated rock produced by regional metamorphism. the mineral grains within gneiss are elongated due to pressure and the rock has a compositional banding due to chemical activity.

(or)

a coarse-grained, foliated metamorphic rock that commonly has alternating bands of light and dark-colored minerals.

gondwana: a continent formed in the southern hemisphere during the late paleozoic. it included most of south america, africa, india, austrailia, and antarctica.

graben: an elongated, downthrown block bounded by two steeply dipping normal faults. produced in an area of crustal extension. (or) an elongate block of rock down-dropped along roughly parallel faults.

graded bed: a sediment layer with a gradation of grain size from large grains to finer grains.

graded bedding: a rock layer that has a progressive change in particle size from top to bottom. most common is a sequence with coarse grains at the bottom and fining upwards, which is typically caused by a declining current velocity within the depositional environment.

grain size: refers to the size of individual mineral crystals or particles within a rock or sediment deposit.

granite: a coarse-grained, intrusive igneous rock composed primarily of light colored minerals such as quartz, orthoclase, sodium plagioclase and muscovite mica. granite is thought to be one of the main components of continental crust.

(or)

a coarse-grained intrusive igneous rock with at least 65% silica. quartz, plagioclase feldspar and potassium feldspar make up most of the rock and give it a fairly light color. granite has more potassium feldspar than plagioclase feldspar. usually with biotite, but also may have hornblende.

granitic: a general term for intrusive igneous rocks that look similar to granite but may range in composition from quartz-diorite to granite. all granitic rocks are light colored; feldspar and quartz are visible in hand specimen.

granodiorite: an intrusive igneous rock similar to granite, but contains more plagioclase than potassium feldspar.

gravel: clastic sedimentary particles of any composition that are over 2 mm in diameter.

(or)

all sedimentary particles larger than two millimeters is called gravel. gravel is subdivided into pebbles, cobbles, and boulders.

gravity anomaly: a geographic area where the gravitational attraction is significantly higher or significantly lower than normal.

greenhouse effect: a warming of the atmosphere caused by carbon dioxide and water vapor in the lower portions of the atmosphere capturing heat that is radiated from and reflected by earth's surface.

greenstone: a low-grade metamorphic rock that frequently contains green minerals such as chlorite, epidote and talc. (or) a metamorphic rock derived from basalt or chemically equivalent rock such as gabbro. greenstones contain sodium-rich plagioclase feldspar, chlorite, and epidote, as well as quartz. the chlorite and epidote make greenstones green.

ground moraine: a blanket of till that is deposited during the retreat of a glacier.

ground water: water that exists below the water table in the zone of saturation. ground water moves slowly in the same direction that the water table slopes.

(or)

water found beneath the earth’s surface where all empty space in the rock is completely filled with water.

ground water recharge area: a location where surface water or precipitation can infiltrate into the ground and replenish the water supply of an aquifer.

grus: coarse sand and gravel that forms from weathering of granitic rocks.

gully: a very small channel formed by running water. gullies hold water for brief periods of time after a rain storm or snow melt.

guyot: a seamount with a flat top.

half-life: the time required for one half of a radioactive substance to decay into its daughter material.

(or)

the time required for one-half of the atoms of a radioactive substance to decay.

hanging valley: a tributary to a u-shaped glacial valley which, instead of entering the valley at the same level as the main stream, enters at a higher elevation, frequently as a waterfall. these different stream levels are a result of the rapid downcutting of the glacier being much faster than the slower downcutting of the tributary stream.

hardness: a measure of a mineral’s resistance to scratching. the hardness of a mineral is measured by scratching it against another substance of known hardness.

hard water: water that has a significant amount of dissolved calcium and magnesium ions. this water performs poorly with most soaps and detergents and leaves a scaly deposit in containers where it is heated or evaporates. it can frequently be improved through the use of home-based water treatment systems.

headland: headlands are projections of land that stick out into a sea or lake.

headwater(s): the upper portions of a drainage basin where the tributaries of a stream first begin flow.

heat flow: the movement of heat energy from the core of the earth towards the surface.

hogback: a narrow ridge with steeply inclined sides of nearly equal slopes. formed by differential erosion of steeply dipping rock units.

holocene: an epoch of the quaternary period beginning 10,000 years ago and continuing today.

hornfels: a nonfoliated metamorphic rock that is typically formed by contact metamorphism around igneous intrusions.

horst: an elongated block of high topographic relief that is bounded on two sides by steep normal faults. produced in an area of crustal extension.

(or)

an elongate block of rock uplifted along roughly parallel faults.

hornblende: see amphibole.

hornblende schist: a schist rich in hornblende. generally with abundant plagioclase feldspar as well. grades into amphibolite.

hornfels: a dark, very fine-grained metamorphic rock produced by the recrystallization of a fine-grained rock by heat from a nearby igneous intrusion. from the german, meaning horn rock.

hot spot: a volcanic center located within a lithospheric plate that is thought to be caused by a plume of hot mantle material rising from depth.

(or)

an area of concentrated heat in the mantle that produces magma that rises to the earth’s surface to form volcanic islands. the volcanic activity of the hawaiian islands is one example. hot spots generally persist for millions of years.

hot spring: a natural spring that delivers water to the surface that is of higher temperature than the human body.

humus: the dark portion of a soil that consists of organic material that is well enough decayed that the original source material can not be identified.

hydraulic conductivity: the ability of a porous material to transmit a fluid. permeability.

hydrocarbon: any organic chemical compound (gaseous, liquid or solid) that is composed of carbon and hydrogen. the term is frequently used in reference to fossil fuels, specifically crude oil and natural gas.

hydrograph: a graph that shows the change of a water-related variable over time. example: a stream discharge hydrograph shows the change in discharge of a stream over time.

hydrologic cycle: the natural cycling of earth's water between the atmosphere, surface and subsurface through the processes of evaporation, transpiration, percolation, infiltration, runoff and precipitation.

hydrology: the science of earth's water, its movement, abundance, chemistry and distribution on, above and below earth's surface. (or) the science that deals with water on and beneath the earth surface.

hydrolysis: a chemical reaction involving water that results in the breakdown of mineral material.

hydrothermal: pertaining to hot water, the actions of hot water or the products produced by the actions of hot water.

hydrothermal deposits: mineral deposits that are formed by the actions of hot water or gases associated with a magmatic source.

hydrothermal metamorphism: alteration of rock by hot waters or gases associated with a magmatic source.

hydrothermal vein: a deposit of minerals precipitated in a fracture by the actions of hot water or gases associated with a magmatic source.

hydrous: literally, "with water". refers to minerals or other materials which have water as a primary constituent.

hypocenter: a point beneath earth's surface where the vibrations of an earthquake are thought to have originated. also known as the focus.

igneous rock: a rock formed by the crystallization of magma or lava.

(or)

rock formed when molten rock (magma) that has cooled and solidified (crystallized). see intrusive (plutonic) and extrusive (volcanic) igneous rock.

ignimbrite: an igneous rock formed by the lithification of ash flow or pyroclastic flow deposits.

illite: a potassium-rich clay mineral

impermeable: rock or sediment that does not allow passage of water.

impermeable layer: a layer of rock, sediment or soil that does not allow water to pass through. this could be caused by a lack of pore space or pore spaces that are so small that water molecules have difficulty passing through.

incised: a term used to describe down-cutting (downward erosion) by a stream. incision deepens and often steepens the stream channel.

infiltration: the movement of surface water into porous soil.

injection well: a well that is used to force a fluid into the ground. the injection could be done for disposal or to place the fluid (such as natural gas) into a subsurface reservoir.

inner core: the innermost layer of earth. consists of solid iron and nickel.

interior drainage: a system of streams that flow into a land-locked basin and evaporate.

internal drainage: an area in which surface water cannot reach the ocean. any water that falls into an area with internal drainage as rain or snow does not escape out of it; not one of the streams that originate within these basins ever find an outlet to the ocean. (see drainage basin)

intermediate rock: an igneous rock that has an intermediate silica content. examples are syenite and diorite. also see entries for acidic, basic and ultrabasic rocks.

intensity: one way to measure the strength of an earthquake. intensity measures of the effects of an earthquake on buildings and the reactions of people. compare with magnitude.

intrusion: a igneous rock body that formed from magma that forced its way into, through or between subsurface rock units.

(or)

emplacement of magma (molten rock) into preexisting rock. dikes, sills, and batholiths are intrusions.

intrusive: igneous rocks that crystallize below earth's surface.

intrusive rock:

igneous rock that cools and solidifies beneath the earth’s surface. (= plutonic rock).

ion: an atom or group of atoms that have gained or lost one or more electrons and as a result have an electrical charge.

ionic bond: a chemical bond formed by the electrostatic attraction between oppositely charged ions.

iron formation: a layered deposit of chemical sedimentary rocks containing at least 15 percent (by weight) iron in the form of sulfide, oxide, hydroxide, or carbonate minerals.

island arc: an arc-shaped chain of volcanic islands produced where an oceanic plate is sinking (subducting) beneath another.

isograd: a line on a map that represents a specific degree of metamorphism. rocks on one side of the line have been subjected to a greater level of metamorphism and on the other side of the line a lower level of metamorphism.

isostasy: a condition of gravitational balance (similar to floating) in which a mass of lighter crustal rocks are buoyantly supported from below by denser mantle rocks. the crustal rocks above subside into the mantle until they have displaced an adequate amount of mantle material to support them.

isotope: one of several forms of an element. these different forms have the same number of protons but varying numbers of neutrons.

(or)

different forms of a single element that have the same number of protons but different numbers of neutrons in their nuclei. some radioactive isotopes are unstable and shed nuclear particles over time until they become stable. for instance, unstable isotopes of uranium break down to become lead.

jade: a translucent gemstone consisting of either jadeite or nephrite that is typically green in color. jade is a very durable stone and is used for a variety of jewelry and ornamental objects. typically cut in the cabochon shape or carved.

jadeite: a high pressure clinopyroxene that is frequently carved and polished as a gemstone.

jasper: a variety of colored chert, typically red or green and often found in association with iron ores. jasper is frequently used as a gemstone or in the production of ornaments.

jet: a variety of coal that is frequently cut and polished for jewelry or ornaments.

jetty: a human made structure built at right angles to a coastline and extending into the water. jetties are built to protect an area of shoreline from the effects of currents, erosion or deposition.

joint: a fracture in rock along which there has been no displacement.

(or)

a narrow crack in rock along which there has been no significant movement of either side. joints commonly form in parallel sets.

joint set: a group of joints that are parallel or nearly parallel. they are frequently formed at the same time interval from a common process.

jolly balance: a spring balance used in the determination of specific gravity.

juvenile water: water that is new to the hydrologic cycle. brought to earth's surface through volcanic eruptions.

kame: a low mound, knob, hummock, or short irregular ridge, composed of stratified sand and gravel deposited by a subglacial stream as a fan or delta at the margin of a melting glacier; or as a ponded deposit on the surface or at the margin of stagnant ice; or by a supraglacial stream in a low place or hole on the surface of the glacier.

karst: a landscape that is characterized by the features of solution weathering and erosion in the subsurface. these features include caves, sinkholes, disappearing streams and subsurface drainage.

(or)

a distinctive landscape (topography) that can develop where the underlying bedrock, often limestone or marble, is partially dissoved by surface or ground water.

kerogen: solid organic substances frequently found in shales. the organic component of an oil shale.

kettle: a depression formed in glacial deposits when a buried block of ice, left behind by a retreating glacier, melts. (or) a steep-sided, bowl-shaped depression commonly without surface drainage; usually formed by a large detached block of stagnant ice that had been partially or wholly buried in the drift.

kettle lake: a lake that forms in a kettle.

k-feldspar: a potassium feldspar such as orthoclase, microcline, sanidine or adularia. also referred to as potash feldspar.

kilobar: a unit of pressure equal to 1000 bars (the mean atmospheric pressure at 100 meters above sea level is one bar).

kimberlite: a variety of peridotite that is found in volcanic pipes which are thought to be intrusions from the upper mantle. many diamond deposits are found in kimberlite pipes.

knickpoint: an abrupt change in slope. a point on a stream profile where a change in gradient occurs. this could be caused by a change in underlying bedrock or bedrock structure.

knob: a small hilltop that is round in shape.

kyanite: an aluminum-rich, blue to light green silicate mineral. kyanite forms in metamorphic rocks at moderate temperature and high pressure.

laccolith: an igneous intrusion that has been forced between two layered rock units. the top of the intrusion is arched upwards and the bottom of the intrusion is nearly flat.

lacustrine deposit: clastic sediment and chemical precipitates deposited in lakes.

lahar: a mudflow composed of water and volcanic ash. lahars can be triggered by the flash melting of the snow cap of a volcanic mountain or from heavy rain. lahars are very dangerous because they can occur suddenly and travel at great speeds.

(or)

a type of mudflow that originates on the slopes of volcanoes when volcanic ash and debris becomes saturated with water and flows rapidly downslope.

lakebed: the flat to gently undulating ground underlain or composed of fine-grained sediment deposited in a former lake.

lake plain: a nearly level surface marking the floor of an extinct lake filled with well-sorted generally fine-textured sediments that are commonly stratified.

laminar flow: a state of uniform flow within a fluid in which the moving particles travel along parallel paths (compare with turbulent flow).

lamination: very thin layers of less than 1 cm thickness.

landform: any physical, recognizable form or feature on the earth's surface, having a characteristic shape and range in composition, and produced by natural causes.

landslide: a downslope movement of rock and soil over a failure surface and under the influence of gravity. slumps, earthflows, debris flows and debris slides are examples.

(or)

downslope movement of rock, soil, and mud.

land-use: present and historical uses of land, such as for agriculture, mining,

recreation and grazing.

lapilli: volcanic rock materials which are formed when magma is ejected by a volcano. typically used for material that ranges between 2 and 64 millimeters in diameter.

lateral moraine: an accumulation of till along the sides of a valley glacier that is produced by ice action.

lava: molten rock material on earth's surface.

(or)

magma that reaches the earth’s surface through a volcanic eruption. when cooled and solidified, forms extrusive (volcanic) igneous rock.

lava tube: a tunnel below the surface of a solidified lava flow, formed when the exterior portions of the flow solidify and the molten internal material is drained away.

lawsonite: a metamorphic mineral that forms only under very high pressure. it is a calcium aluminum silicate and usually forms microscopic crystals.

leaching: the removal of soluble constituents from a rock or soil by moving ground water or hydrothermal fluids.

left-lateral fault: a fault with horizontal movement. if you are standing on one side of the fault and look across it the block on the opposite side of the fault has moved to the left. (also see right-lateral fault.)

levee: a long continuous ridge built by people along the banks of a stream to contain the water during times of high flow. natural levees can also be built along the banks of a stream. when the flood water decelerates upon leaving the channel, sediments quickly drop out of suspension and build a ridge over time.

lichenometry: a dating method that uses the growth rate of certain lichen species as an indicator of the age of the surface the lichen is growing on.

limb: one side of a fold. the dipping rock units between the crest of an anticline and the trough of a syncline.

limestone: a sedimentary rock consisting of at least 50% calcium carbonate (caco2) by weight.

(or)

a sedimentary rock made mostly of the mineral calcite (calcium carbonate). limestone is usually formed from shells of once-living organisms or other organic processes, but may also form by inorganic precipitation.

(or)

a sedimentary rock consisting chiefly of calcium carbonate, primarily in the form of calcite.

limonite: a mineral composed of iron oxides and water. rust. very common in many rocks after weathering at the earth’s surface. imparts brown or yellow colors to many rocks.

lineament: a straight topographic feature of regional extent which is thought to represent crustal structure. a fault, line of sinkholes, straight stream stretch or a line of volcanoes can be considered linear features.

(or)

a linear (relatively straight) topographic feature or features such as a fault, line of dense vegetation, or a chain of aligned volcanoes.

lineation: parallel arrangement of elongate minerals or groups of minerals. to envision lineation, imagine packages of spaghetti or pencils.

lithification: the processes through which sediments are converted into sedimentary rock, including compaction and cementation.

(or)

the conversion of loose sediment into solid sedimentary rock. several processes, including compaction of grains, filling of spaces between grains with mineral cement, and crystallization act to solidify sediment.

lithology: the study and description of rocks, including their mineral composition and texture. also used in reference to the compositional and textural characteristics of a rock.

lithosphere:

the rigid outer shell of the earth which includes the crust and a portion of the upper mantle.

(or)

the outer layer of solid rock that includes the crust and uppermost mantle. this layer, up to 100 kilometers (60 miles) thick, forms the earth’s tectonic plates. tectonic plates float above the more dense, flowing layer of mantle called the asthenosphere.

lithospheric plate: a large slab of the lithosphere that can be moved by convection current motion within the mantle.

load: the total amount of sediment being carried by a stream or a glacier. includes suspended materials, dissolved materials and materials moved along earth's surface. (also see: bed load, dissolved load, suspended load.)

lode: a rich accumulation of minerals in solid rock. frequently in the form of a vein, layer or an area with a large concentration of disseminated particles. (see placer for contrast.)

loess: a wind-blown deposit of sediment made mostly of silt-sized grains.

(or)

material transported and deposited by wind and consisting primarily of silt size particles.

longitudinal dune: a long narrow sand dune that has its long dimension oriented parallel to the direction of the wind.

longitudinal profile: a cross section of a stream or valley beginning at the source and continuing to the mouth. these profiles are drawn to illustrate the gradient of the stream.

longshore bar: a narrow, elongate, coarse-textured ridge tht once rose near to, or barely above, a pluvial or glacial lake and extending generally parallel to the shore.

longshore current: a flow of water parallel to a coastline that is caused by waves striking the coast at an oblique angle.

longshore drift: the movement of sediment along a coastline caused by waves striking the coast at an oblique angle. the waves wash sediment particles up the beach at an oblique angle and the swash back to the sea carries the particles down the gradient of the beach. this produces a zig-zag path of particle movement along the beach.

lowland: a relatively flat area in the lower levels of regional elevation.

low-velocity zone: a zone within the upper mantle where seismic wave velocities are relatively low. this zone is located about 35 to 155 miles below the surface.

luster: the manner in which light reflects from a mineral surface. metallic, submetallic and non-metallic are the basic types of luster.

(or)

the appearance of the reflection of light from the surface of a mineral. luster is described as metallic, glassy, dull, etc.

mafic: a term used to describe an igneous rock that has a large percentage of dark-colored minerals such as amphibole, pyroxene and olivine. also used in reference to the magmas from which these rocks crystallize. mafic rocks are generally rich in iron and magnesium. basalt and gabbro are examples of mafic rocks. (see felsic to contrast.)

(or)

a term used to describe minerals or igneous rocks that are rich in iron and/or magnesium. mafic igneous rocks have a high percentage of dark-colored (mafic) minerals.

magma: molten rock material that occurs below earth's surface.

(or)

molten rock. magma may be completely liquid or a mixture of liquid rock, dissolved gases and crystals. molten rock that flows out onto the earth’s surface is called lava.

magma chamber: a full or emptied magma reservoir in the shallow portion of the lithosphere.

(or)

a body of molten rock and solid crystal mush beneath the earth’s surface. when this chamber cools and solidifies, it is called a pluton.

magmatic water: water that is dissolved in a magma or water that is released from a magma. some magmas can contain up to several percent dissolved water by weight.

magnetic anomalies: see magnetic reversals.

magnetic anomaly: an increase or decrease in the local magnetic field compared to the normally expected value.

magnetic declination: the horizontal angular difference between true north and magnetic north.

magnetic inclination: the vertical angular difference between a horizontal plane and the orientation of earth's magnetic field.

magnetic north: the direction that a compass points. the location where earth's magnetic field dips vertically into the earth.

magnetic reversal: a change in the polarity of earth's magnetic field in which the north magnetic pole becomes the south magnetic pole and vice versa. also known as geomagnetic reversal or polarity reversal. earth's magnetic field has reversed many times in the past and the time intervals between these changes are known as polarity epochs.

magnetic reversals: earth’s magnetic field occassionally "flips" or reverses polarity. this means that, if a polarity reversal happened today, your compass would point south instead of north!

magnetic stratigraphy: the correlation of rock units and study of earth's history using magnetic events and magnetic epochs as a time reference.

magnetite: iron oxide mineral (fe3o4). usually tiny black, metallic crystals. magnetite will attract a magnet and sometimes, in a rock, a hiker’s compass needle.

magnetometer: an instrument designed to measure the strength and character of earth's magnetic field.

(or)

device for measuring magnetism.

magnitude: a measure of earthquake strength based upon the amount of ground motion experienced and corrected for the distance between the observation point and the epicenter. there are several magnitude scales in use.

(or)

a measure of the total amount of energy released by an earthquake.

manganese nodule: a rounded concretion, rich in manganese minerals with minor concentrations of cobalt, copper and nickel. these occur in abundance on some parts of the deep ocean floor and have been considered as a potential source of manganese.

mantle: a major subdivision of earth's internal structure. located between the base of the crust and overlying the core.

(or)

the layer of the earth below the crust and above the core. the uppermost part of the mantle is rigid and, along with the crust, forms the 'plates' of plate tectonics. the mantle is made up of dense, iron and magnesium rich (ultramafic) rock such as dunite and peridotite.

mantle plume: a rising mass of hot mantle material that can create an area of volcanic activity in the center of a lithospheric plate.

marble: a metamorphic rock of made of calcium carbonate. marble forms from limestone by metamorphic recrystallization.

massive: a term used in reference to a rock unit that is homogeneous in texture, fabric and appearance.

mass wasting (also mass movement): a general term used for any downslope movement of rock, soil, snow or ice under the influence of gravity. includes: landslides, creep, rock falls and avalanches.

(or)

movement of rock and soil downslope under the influence of gravity.

matrix: fine-grained material surrounding larger grains in a sedimentary rock.

meandering stream: a stream that has many bends (meanders). this type of drainage pattern usually develops on a nearly level landscape and where the banks of the stream are easily eroded.

mechanical weathering: a general term applied to a variety of weathering processes that result in the particle size reduction of rock materials with no change in composition. frost action, salt crystal growth and pressure relief fracturing are examples. also known as physical weathering.

(or)

the mechanical break-up or disintegration of rock into smaller fragments.

medial moraine: a streak of till in the center of a glacier. these are found downslope from the junction of two glaciers and are a merging of their lateral moraine deposits.

medical geology: the study of human health related to geology. examples would include the correlation of disease or vitality with residences over specific types of bedrock or health problems associated with exposure to specific mineral materials.

melange: mixture of rocks formed by tectonic disruption, such as multiple faulting, which brings disparate rock types together. usually consists of a matrix of weak material, like shale, with hard pieces of exotic rocks, such as gneiss or igneous rocks.

metaconglomerate: metamorphosed conglomerate.

metamorphic rock: a rock that has undergone chemical or structural changes produced by increase in heat or pressure, or by replacement of elements by hot, chemically active fluids.

metamorphism: alteration of the minerals, textures and composition of a rock caused by exposure to heat, pressure and chemical actions.

meteoric water: water from the atmosphere, such as rain, snow, hail, or sleet.

meteor: a meteoroid that penetrates earth's atmosphere, producing a streak of bright light caused by incineration.

meteorite: a particle of iron or rock that has fallen to earth's surface from inter-planetary space.

meteoroid: a particle of iron or rock found in inter-planetary space. distinguished from planets or asteroids by its much smaller size.

mica: group of silicate minerals composed of varying amounts of aluminum, potassium, magnesium, iron and water. all micas form flat, plate-like crystals. crystals cleave into smooth flakes. biotite is dark, black or brown mica; muscovite is light-colored or clear mica.

micaceous: a general term for mica-rich rocks.

micro-high: a generic microrelief term applied to slightly elevated areas relative to the adjacent ground surface; changes in relief range from several centimeters to several meters.

micro-low: a generic microrelief term applied to slightly depressed areas relative to the adjacent ground surface; changes in relief range from several centimeters to several meters.

microrelief: slight variations in the height of a land surface that are too small to delineate on a topographic or soil map; applies to local and slight irregularities in the land surface.

microseism: a vibration of the earth that is unrelated to earthquake activity - instead it is caused by wind, moving trees, ocean waves or human activity.

midden: in geology, a mound of organic debris or organic-rich soil created by an animal. in archeology, a mound of human refuse.

migmatite: "mixed rock". a metamorphic rock that forms in one of two ways. the metamorphic rock may be heated enough to partially melt, but not completely. the molten minerals resolidify within the metamorphic rock, producing a rock that incorporates both metamorphic and igneous features. migmatites can also form

when metamorphic rock experiences multiple injections of igneous rock that solidify to form a network of cross-cutting dikes.

mineral: a naturally occurring, inorganic solid with a definite chemical composition and an ordered internal structure.

(or)

a naturally occurring chemical compound or limited mixture of chemical compounds. minerals generally form crystals and have specific physical and chemical properties which can be used to identify them.

mineralization: the formation of minerals. new minerals may be added to fractures and empty spaces in a rock or by replacing preexisting minerals with different ones.

mineralogy: the study of minerals - their composition, structure, formation, uses, properties, occurrence and geographic distribution. (or)

the study of minerals.

miocene: an epoch that includes the time interval of about 23.7 to 5.3 million years ago.

mohorovicic discontinuity: the boundary between the crust and the mantle. frequently referred to as the moho.

moho: the boundary separating the base of the earth’s crust and the top of the mantle. the moho occurs at a depth of 5-10 kilometers beneath oceanic crust and about 35-65 kilometers below continental crust. the term moho is an abbreviation for mohorovicic discontinuity, named for andrija mohorovicic, a croatian seismologist.

mohs hardness scale: a collection of minerals ranging from very soft to very hard. use as a comparison scale during mineral identification. from softest to hardest, the ten minerals are: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, and diamond 10. developed by friedrich mohs, a german mineralogist in the early 1800's.

monadock: an isolated hill or mountain of resistant rock rising conspicously above the general level of a lower erosion surface in a temperate climate representing an isolated remnant of a former erosional cycle in an area that has been beveled to its base level.

monocline: an area of increased dip in otherwise gently dipping strata.

moraine: a mound, ridge or ground covering of unstratified and unsorted till, deposited by ice action or by melting away of a glacier.

(or)

a hill-like pile of rock rubble located on or deposited by a glacier. an end moraine forms at the terminus of a glacier. a terminal moraine is an end moraine at the farthest advance of the glacier. a lateral moraine forms along the sides of a glacier. see till

moraine, end: a ridge-like accumulation that is being or was produced at the outer margin of an actively flowing glacier at any given time.

moraine, ground: an extensive, fairly even layer of till, having an uneven or undulating surface; a deposit of rock and mineral debris dragged along, in, on, or beneath a glacier and emplaced by process including basal lodgement.

moraine, kame: an end moraine that contains numerous kames.

moraine, lateral: a ridge-like moraine carried on and deposited at the side margin of a valley glacier.

moraine, recessional: an end or lateral moraine, built during a temporary but significant halt in the final retreat of a glacier.

moraine, terminal: an end moraine that marks the farthestadvance of a glacier and usually has the form of a massive arcuate or concentric ridge or complex of ridges, underlain by till and other types of drift.

morphology: the study of shape or form. see geomorphology.

mountain: a general term used in reference to an area that is at a conspicuously higher elevation than surrounding lands. mountains are larger than hills and are significant enough in relief that they are given names by local residents.

mud: wet clay and silt-rich sediment.

mudflow: a type of mass movement composed mainly of clay-size materials with a high enough water content that it flows readily. (or) same as debris flow.

mudstone: a sedimentary rock composed of clay-size particles but lacking the stratified structure that is characteristic of a shale.

(or)

a very fine-grained sedimentary rock formed from mud.

muscovite: one of the mica family of minerals. muscovite is light-colored or clear mica, sometimes called isingglass.

m.y.: million years - abbreviation.

m.y.a.: million years ago - abbreviation.

mylonite: a brecciated metamorphic rock frequently found in a fault zone. the fractured texture is thought to form by the crushing actions of fault movement.

nannofossils: a generic term used in reference to very small fossils that are at the limit of resolution by a light microscope. they are therefore studied with electron microscopes and are frequently fossil discoasters and coccoliths.

nappe: a large slab of earth's surface that has been moved in a horizontal or near horizontal direction over a plane of separation. this motion can be produced by faulting or sliding. the term is generally used for very large slabs which are many square kilometers or miles in surface area.

native metal: a natural deposit of a metallic element such as gold, silver, copper or iron in a pure form.

natural bridge: an arch-shaped rock formation produced by weathering and/or erosion.

natural gas: naturally occurring hydrocarbons that exist in subsurface rock units in the gaseous state. methane is the most abundant but ethane, propane and others also occur.

natural levee: a mound of sediment that parallels a stream channel forming a levee-like deposit. when flood waters leave the normal stream channel and enter the flood plain there is a reduction of velocity that causes suspended sediments to fall to the bottom, producing this deposit.

(or)

a long, broad low ridge or embankment of sand and coarse silt, built by a stream on its flood plain and along both sides of its channel, especially in time of flood when water overflowing the normal banks is forced to deposit the coarsest part of its load.

neap tide: a daily tidal range of minimal amplitude that occurs when the moon and sun are positioned at 90 degrees to one another. in this moon-earth-sun configuration, the gravitational attraction of the moon and sun compete for earth's water. occurs at the first and third quarters of the moon. see spring tide for contrast.

nebula: a cloud of interstellar dust that is faintly visible from earth.

neutron: a subatomic particle, contained in the nucleus of an atom. it has no electrical charge and a mass similar to that of a proton.

nodule: a mineral mass that has a different composition or is more weathering resistant than its surrounding rock. these are normally rounded in shape. examples include: chert masses in a limestone rock unit, pyrite masses in a coal seam, or carbonate masses in a shale. in most cases these "nodules" have formed within the rock unit or its former sediment mass. the term is also applied to rounded masses of manganese minerals that occur on some parts of the ocean floor.

nonconformity: an type of unconformity in which young sedimentary rocks lie on top of older metamorphic or intrusive igneous rocks.

non-point source pollution: pollution that does not originate at a single location. in an urban area runoff water can be polluted as it flows to a stream by gasoline, antifreeze, road salt or other contaminants. in rural areas runoff can be contaminated by insecticides, manure, or fertilizer. this contamination can be significant but can not be traced back to a specific source.

nonsilicate minerals: a mineral without silicon (si). see silicate minerals.

normal fault: a fault with vertical movement and an inclined fault plane. the block above the fault has moved down relative to the block below the fault.

(or)

a fault that drops rock on one side of the fault down relative to the other side.

normal polarity: a magnetic field produced by the earth that is the same as exists today.

oblique-slip fault: a fault that has both horizontal and vertical elements of displacement.

obsidian: a glassy igneous rock with a composition similar to granite. the glassy texture is a result of cooling so fast that mineral lattices were not developed.

(or)

dark-colored volcanic glass. usually has the same chemical composition as the extrusive igneous rock, rhyolite.

oceanic crust: the relatively thin, dense crust that forms the ocean basins.

oceanic rocks: rocks formed in the deep ocean. includes sedimentary rocks

deposited on the deep ocean floor as well as the basalt of the oceanic crust. commonly include some slices of the underlying mantle (ultramafic rocks) as well.

oil field: the geographic area above an underground accumulation of oil and natural gas.

oil shale: a dark-colored shale containing an unusual amount of solid organic material. this shale can be crushed and heated to liberate gaseous and liquid hydrocarbons. at present the expenditure required to process oil shale into a fuel makes this effort marginally profitable or unprofitable.

old age: a stage in the development of a landscape when streams have a low gradient and meander back and forth across broad floodplains. the landscape is marked by meander scars and oxbow lakes.

olivine: silicate mineral containing iron and magnesium. a green glassy mineral formed at high temperature. common in basalt, especially ocean-floor basalt, and in ultramafic rocks. gem-quality olivine is called peridote. rock made up entirely of olivine is called dunite.

oolite: a small sphere of calcium carbonate no more than a few millimeters in diameter and with a concentric internal structure . these spheres are thought to have formed by inorganic precipitation of calcium carbonate in very thin layers around a grain of sand or a particle of shell or coral. a rock composed primarily of oolites.

oolitic: a limestone texture that is characterized by spherical grains of calcium carbonate with a concentric internal structure. these grains are thought to form by inorganic precipitation of calcium carbonate around a sand grain or shell particle nucleus.

opaque: an adjective used in reference to a substance that does not allow light of visible wavelength to enter or pass through. minerals with a metallic or submetallic luster are normally opaque.

ophiolite suite: the typical sequence of rocks in the oceanic crust: from bottom to top: ultrabasic rocks, gabbro, sheeted dikes, pillow basalts, and sea-floor sediments. igneous rocks and deep-sea sediments associated with divergence zones and the sea-floor environment.

orbit: an elliptical or hyperbolic path traveled by a satellite object around a more massive body. for example, the earth orbits the sun.

ordovician: a period in the paleozoic era that includes the time interval from about 505 to 438 million years ago.

ore: a mineral deposit that can be mined at a profit.

ore deposit: a natural accumulation of a metal, gemstone or other valuable mineral substance, which is rich enough in concentration that it can be mined and processed at a profit.

ore mineral: a mineral that contains a high enough concentration of a useful element or compound that the element or compound can be extracted at a profit.

original horizontality: one of the principles of relative dating. based upon the good assumption that sedimentary rocks are deposited in horizontal or nearly horizontal layers; then if sedimentary layers are found in an inclined orientation the force that moved them to that orientation must have been applied at some time after their deposition.

orogenic belt: a linear or arcuate region of folded and uplifted rocks.

orogeny: a compressive tectonic process that results in intense folding, reverse faulting, crustal thickening, uplift and deep plutonic activity. a mountain-building episode.

(or)

an episode of mountain building and/or intense rock deformation.

orthogneiss: gneiss formed by squeezing (deformation and usually some recrystallization) of a granitic igneous plutonic rock.

oscillation ripple marks: symmetrical ridges in sand or other sediment that are caused by a back-and-forth wave action.

outcrop: an exposure of bedrock. outcrops can be formed naturally or by human action. stream erosion and highway construction can produce outcrops.

(or)

a mass of rock that appears at the earth surface.

outer core: the liquid outer layer of the core that lies directly beneath the mantle.

outfall: a location where water is discharged. normally used in reference to where a water treatment facility releases treated water into the environment.

outgassing: the release of juvenile gases and water to the surface from a magma source.

outwash: sorted and stratified sediment deposited in front of a glacier by meltwater streams.

(or)

glacial outwash is the deposit of sand, silt, and gravel formed below a glacier by meltwater streams and rivers. an outwash plain is an extensive, relatively flat area of such deposits.

(or)

stratified detritus (chiefly sand and gravel) removed or washed out from a glacier by melt-water streams and deposited in front or beyond the end moraine or the margin of an active glacier.

outwash plain: an extensive lowland area of coarse textured, glaciofluvial material.

overbank deposits: silt and clay deposited on a flood plain by a flooding stream.

overturned fold: a fold that has both limbs dipping in the same direction, resulting from one of those limbs being rotated through an angle of at least 90 degrees. overturned folds are found in areas of intense deformation. the name overturned is given because the strata on one limb of the fold are "overturned" or upside down.

oxbow lake: a crescent-shaped lake that forms when a meandering stream changes course. such changes in course frequently occur during flood events when overbank waters erode a new channel.

oxidation: a chemical reaction in which substances combine with oxygen. for example, the combination of iron with oxygen to form an iron oxide.

(or)

removal of electrons from an atom or ion. usually by combining with oxygen ions. minerals exposed to air may oxidize as a form of chemical weathering.

pahoehoe: a hawaiian term for a lava flow that has a surface flow structure appearance that looks like coiled rope or cord. see aa for contrast.

(or)

a lava flow with a smooth, ropy surface.

paleobiogeography: the science that studies the past distribution of plants and animals.

paleoclimate: the climate of a given area at a specific time in the past. paleoclimates can be read from the rocks much as areas with different types of climates produce sediments with specific characteristics today.

paleoclimatology: the study of how global climate has changed through time.

paleocurrent map: a map that shows the directions of currents at the time of sediment deposition. these directions can be determined through the study of cross bedding, ripple marks, tool marks and other sedimentary structures.

paleogeographic map: a map that shows the distribution of sedimentary environments at a specific time in the past. these maps are made by studying the rock record to correlate rock units that were deposited at the same time, then relating rock characteristics to specific sedimentary environments.

paleomagnetism: the study of earth's magnetic field over time. when rocks that contain magnetic minerals are deposited, the character (vertical and horizontal orientation) of earth's magnetic field is locked within the rocks. this information can be used to study changes in earth's magnetic field as well as the movement of plates over time.

(or)

the magnetism of an iron-bearing rock imparted to it by the earth’s magnetic field when the rock formed. literally, early magnetism; meaning magnetism formed in a past geologic era.

paleontology: the study of ancient life through fossils.

paleoseismology: the study of ancient seismic (earthquake) events.

paleozoic era: includes the time from about 570-245 million years ago.

palynology: the study of pollen, living and fossil.

pangaea: a large continental landmass that existed from about 300 million years ago through about 200 million years ago. it included most of the continental lithosphere present at that time. it has since broken up and the fragments have drifted to become the configuration of earth's present day continents.

(or)

the supercontinent which formed at the end of the paleozoic era and began breaking up about 200 million years ago to form today’s continents.

panthalassa: the ancient ocean that surrounded the pangaea landmass.

parent element: a radioactive element that spontaneously decays into a new substance. the product of this decay is known as a "daughter" element.

parent isotope: a radioactive isotope that changes to a different, daughter

isotope when its nucleus decays.

parent rock: the preexisting rock from which a metamorphic rock forms.

passive margin: a tectonically inactive continental margin characterized by a lack of earthquakes and volcanic activity.

pavement: a bare rock surface that provides a protective rock cover over the material beneath it.

peak flow: the maximum instantaneous discharge of a stream at a specific location. corresponds to the highest stage of a flood.

peat: an accumulation of unconsolidated plant debris that if buried and preserved could become coal. special conditions are required to accumulate and preserve plant materials. these conditions are most often found in a marsh or swamp where water cover prevents oxidation and attack by most organisms.

pebble: loose particles of rock or mineral (sediment) that range in size from 2 - 64 millimeters in diameter. pebbles are the smallest type of gravel.

pediment: a broad, gently sloping erosional surface of low local relief adjacent to an eroding cliff or mountain range. the area is likely covered with sediments.

(or)

a sloping bedrock surface at the base of a mountain, formed when erosion removes much of the mountain’s mass.

pegmatite: a very coarse grained igneous rock, normally of granitic composition. typically forms during the final states of magma chamber crystallization when the high water content solutions allow rapid crystal growth.

(or)

a very coarse-grained igneous rock, commonly with a granitic composition. usually forms from molten rock rich in water or other volatiles that facilitate the growth of large crystals. forms sills and dikes.

pelagic sediment: a ocean sediment that accumulates far enough from land that detrital materials are a minor component. these sediments are largely composed of the tiny shell debris of radiolarians and foraminifera.

pelite: a fine-grained sedimentary rock consisting mostly of clay and/or silt. mudstone, shale, siltstone, and claystone are all pelitic.

perched water table: a water table that is isolated from and higher than the regional water table. this can occur when a hilltop is underlain by an impermeable rock unit. infiltrating waters stack up on the impermeable unit, creating an isolated water table that is higher than the water table of the surrounding land.

peridotite: a dark-colored, coarse-grained igneous rock that is made up mainly of olivine and pyroxene, with very little quartz or feldspar.

permeability: a measure of how well a material can transmit water. materials such as gravel, that transmit water quickly, have high values of permeability. materials such as shale, that transmit water poorly, have low values. permeability is primarily determined by the size of the pore spaces and their degree of interconnection. permeability measures are expressed in units of velocity, such as centimeters per second, and assume a gradient of one vertical foot of drop per linear foot.

(or)

the ability of a rock or other material to allow water to flow through its interconnected spaces. permeable bedrock makes a good aquifer, a rock layer that yields water to wells. see porosity. (3 mb porosity animation available)

perennial stream: a stream that runs continuously throughout the year.

petrology: the study of rocks.

petrophile: lover of rocks. may be a petrologist, a stone mason, a rock climber.

ph: a relative measure of the acidity or alkalinity of a water based upon a scale that ranges between 0 and 14 with 7 being neutral. values of ph below 7 indicate acid solutions and values of ph above 7 indicate basic solutions.

phaneritic: an igneous rock texture in which the mineral grains are large enough to be seen with the unaided eye and are of approximately equal size.

phanerozoic eon: the eon beginning about 570 million years ago and continuing to the present. the portion of earth history with rocks containing abundant fossils.

phenocryst: a term used to describe large crystals embedded in a mass of finer crystals (groundmass) in an igneous rock. see 'porphyritic'.

phlogopite: a magnesium-rich member of the mica mineral family. phlogopite

is a yellowish-brown to coppery-colored mica. like all micas, phlogopite forms flat, plate-like crystals that cleave into smooth flakes.

phyllite: a very fine-grained, foliated metamorphic rock, generally derived from shale or fine-grained sandstone. phyllites are usually black or dark gray; the foliation is commonly crinkled or wavy. differs from less recrystallized slate by its sheen, which is produced by barely visible flakes of muscovite (mica).

physical weathering: a general term applied to a variety of weathering processes that result in the particle size reduction of rock materials with no change in composition. frost action, salt crystal growth and pressure relief fracturing are examples. also known as mechanical weathering.

physiographic province: a region of which all parts are similar in geologic structure and climate and which has consequently had a unified geomorphic history; a region whose relief features and landforms differ significantly from that of adjacent regions.

placer deposit: a mass of stream sediment that contains an economically significant concentration of mineral particles. this accumulation of mineral particles is a result of their being of high specific gravity or resistant to abrasion. gold, magnetite, and diamonds can be found in placer deposits.

plagioclase: a member of the feldspar mineral family. plagioclase feldspars are silicates that contain considerable sodium and calcium. feldspar crystals are stubby prisms, generally white to gray and a glassy luster.

plankton: generally tiny animals or plants that live floating in water.

plastic deformation: permanent deformation (change in size or shape) of soft, but solid rock by folding or flowing without fracturing.

plate: a slab of rigid lithosphere (crust and uppermost mantle) that moves over the asthenosphere.

plateau basalt: a sequence of parallel to subparallel basalt flows that were formed during a geologically brief interval of time and which covered an extensive geographic area. thought to have formed from simultaneous or successive fissure eruptions.

plate tectonics: the theory that the earth’s outer shell is made up of about a dozen lithospheric plates that move about and interact at their boundaries.

playa: playas are shallow, short-lived lakes that form where water drains into basins with no outlet to the sea and quickly evaporates. playas are common features

in arid (desert) regions and are among the flattest landforms in the world.

pleistocene: the epoch of the quaternary period of geologic time, followint the pliocene epoch and preceding the holocene (approximately 2 million to 10 thousand years ago).

pleistocene epoch: the earliest epoch of the quaternary period, beginning about 1.6 million years ago and ending 10,000 years ago. commonly known as the 'ice age', a time with episodes of widespread continental glaciation.

pliocene: the latest epoch of the tertiary period, beginning about 5.3 million years ago and ending 1.6 million years ago.

pluton: a large body of intrusive igneous rock that solidified within the crust. batholiths and stocks are types of plutons.

plutonic rock: any igneous rock that cools beneath the surface. (=intrusive rock).

pluvial lake: a lake formed in a land-locked basin during a period of increased rainfall associated with glacial advance elsewhere.

point-source pollution: water contamination that can be traced to a single point. a toxic material spill and a sewage discharge pipe are examples of point sources.

polarity epoch: an interval of time between reversals of earth's magnetic field.

polarity event: a specific event in the history of earth's magnetic field. usually used in reference to a specific polarity reversal.

polarity reversal: a change in the polarity of earth's magnetic field in which the north magnetic pole becomes the south magnetic pole and vice versa. also known as geomagnetic reversal or magnetic reversal. earth's magnetic field has reversed many times in the past and the time intervals between these changes are known as polarity epochs.

porosity: the volume of pore space in a rock, sediment or soil. usually expressed as a percentage. this pore space can include openings between grains, fracture openings and caverns.

(or)

the percentage of open spaces (pores) in rock or soil. when these spaces are interconnected, water, air, or other fluids can migrate from space to space. interconnected spaces make the soil or bedrock permeable.

porphyritic: an igneous rock texture characterized by larger crystals (phenocrysts) in a matrix of distinctly finer crystals (groundmass).

porphyroblasts: large mineral grains that grow during metamorphism.

porphyry: an igneous rock, usually a dike or sill, with larger, generally conspicuous, early-formed crystals contained within a matrix of much smaller crystals.

post-glacial: refer to the holocene.

potable water: water that is of adequate quality for drinking.

pothole: a cylindrical or hemispherical hold in the bedrock of a stream that is formed from the continual swirling motion of sand and gravel by swirling currents.

ppm: an abbreviation for parts per million.

precambrian: the 'unofficial' time period that encompasses all time from the

earth’s formation, 4.55 billion years ago to 570 million years ago, the beginning of the paleozoic era.

precarious boulder: a large rock resting on another in an unstable position. precarious boulders may remain in place for thousands of years until an earthquake or human-caused tremor dislodges them.

precipitate (verb): the process that separates solids from a solution.

precipitate (noun): mineral precipitate. a mineral deposited from a water solution in pores or other openings in rocks. chemical reaction with the surrounding rock, changes in pressure or temperature, or just drying up (evaporation) can cause a mineral to precipitate out of solution. quartz veins are common products of mineral precipitation.

precipitation: movement of water from the atmosphere to the land or to a surface water body. rain, hail, snow, dew, and sleet are all examples of precipitation.

primary seismic waves: the fastest set of earthquake vibrations - also known as p-waves. they move through the earth in compression and expansion motions (much like sound waves move through air). called primary because they are the first recorded at a seismograph. primary waves are able to travel through both solids and liquids.

proterozoic eon: the 'precambrian' time interval from 2.5 billion to 570 million years ago.

proto-sun: an intermediate stage in the development of a star in which a large cloud of dust and gases gradually condenses through gravitational actions.

proven reserves: mineral deposits that have been explored thoroughly enough to be quantified but which are still in the ground.

proximal: a deposit of material that is closest to the source area.

pumice: a vesicular volcanic glass of granitic composition. it has so many vesicles that it has a very low specific gravity - sometimes low enough to float on water.

(or)

a light-colored, frothy, glassy volcanic rock. the texture is formed by rapidly expanding gas in erupting lava.

p-wave: primary seismic waves. the fastest set of earthquake vibrations. they move through the earth in compression and expansion motions (much like sound waves move through air). called primary because they are the first recorded at a seismograph. primary waves are able to travel through both solids and liquids.

pyrite: iron sulfide mineral (fes). forms silvery to brassy metallic cubes or masses. common in many rocks. known as fool’s gold. weathered pyrite produces limonite (iron oxide) that stains rock brown. or yellow.

pyroclastic: an igneous rock texture produced from consolidation of fragmented volcanic material ejected during a violent eruption. also used to describe ash, bombs and other material forcefully ejected during a volcanic eruption (=tephra)

pyroclastic eruption: a volcanic eruption that produces a large volume of solid volcanic fragments (pyroclastics) rather than fluid lava. this type of eruption is typical of volcanoes with high silica, viscous, gas-rich magma.

pyroclastic flow: an extremely hot mixture of gas, ash and pumice fragments, that travels down the flanks of a volcano or along the surface of the ground at speeds of 50 to 100 miles per hour.

pyroclastic rock: a rock formed when small particles of magma are blown from the vent of a volcano by escaping gas.

pyroxene: family of silicate minerals containing iron, magnesium, and calcium in varying amounts. differ from amphibole family by lack of water in the crystals. the most common variety, augite, contains aluminum as well. generally forms very

dark green to black stubby prisms.

pyroxene granulite: a coarse-grained contact metamorphic rock that is formed at high temperatures and low pressures and which is rich in pyroxene minerals.

quarry: a surface mine usually for the extraction of construction stone.

quartz: one of the most abundant minerals in the earth's crust. has a chemical composition of sio2 and a hardness of seven. one of the index minerals in moh's hardness scale. occurs in sedimentary, metamorphic and igneous rocks.

(or)

one of the most common minerals in the earth’s crust (and in some new-age boutiques). made up of silicon dioxide (sio2),lso called silica. commonly in white masses. crystals are clear, glassy 6-sided prisms.

quartz arenite: a sandstone consisting of at least 95% quartz.

quartzite: a metamorphic rock formed by the alteration of sandstone by heat, pressure and chemical activity.

(or)a nonfoliated metamorphic rock formed from

pure, dominantly quartz sandstone.

(or)

hard, somewhat glassy-looking rock made up almost entirely of quartz. metamorphosed quartz sandstone and chert are quartzites.

quartzose: an adjective used in reference to a rock that is composted primarily of quartz.

quaternary: the most recent period of the cenozoic era. encompasses the time interval of 1.6 million years ago through today.

radiocarbon dating: the age of organic material determined by the amounts of carbon isotopes 12, 13 and 14. the ratio of 12 to 14 is about the same in all living things but when a plant or animal dies, no more carbon is taken on. carbon 12 and 13 are stable isotopes and the amounts remain the same even in dead material. carbon 14 is an radioactive isotope that decays radioactively until none is left; . thus, the ratio records the time elapsed since death. since carbon 14 decays relatively rapidly, the method is only reliable for the last 40,000 years. see

radiometric age.

radial drainage: a drainage pattern in which stream channels run away from a central high point such as a volcano or dome.

radiolarian: a group of one-celled marine animals with a siliceous skeleton that occupies shallow portions of the water column. radiolarians have a range from cambrian to present.

radiolarian chert: a rock made up of the spherical siliceous shells of radiolarians which are single-celled planktonic animals (protozoans).

radiolarian ooze: a deep-sea pelagic sediment that contains at least 30% siliceous radiolarian remains.

radiometric age: the approximate age of a geologic event, feature, fossil, or rock in years. radiometric ages, sometimes termed 'absolute' ages, are determined by using natural radioactive 'clocks'. see radiocarbon dating.

radiometric dating: a dating method that uses measurements of certain radioactive isotopes to calculate the ages in years (absolute age) of rocks and minerals.

rating curve: a plot that shows the relationship between the stage and discharge (streamflow) of a specific stream at a specific location. it is customary to plot stream stage on the y-axis of the plot and discharge on the x-axis. the resulting relationship is normally a curve. rating curves can be used to estimate discharge (which is time consuming and expensive to measure) using a single stage measurement (which can be collected with automatic equipment). the principle of a rating curve enables hydrologists to monitor the discharge of many streams simultaneously once gages have been placed to collect and report the stage of the stream.

reaction series: a series of interactions between a melt and mineral crystals in contact with the melt. in a reaction series the first formed crystals (highest temperature minerals) react with the melt to produce a new mineral.

recharge: water added to an aquifer or other water body. an aquifer is recharged by precipitation in an area where the aquifer has a porous connection to the surface.

recharge area: the geographic area where water infiltrates into the ground and enters an aquifer..

recrystallization: a solid state reaction in which the atoms of existing crystals within a rock are reorganized in response to heat and/or pressure. the recrystallized mineral grains are typically larger in size than the original crystals.

rectangular drainage: a drainage pattern in which stream channels develop within a large-scale network of intersecting joints. this drainage pattern is characterized by right-angle bends in the channels of streams and streams that intersect at right angles.

recumbent fold: an overturned fold that has two limbs which are nearly horizontal.

reef: a ridge-like or mound-like structure, layered or massive, builted by sedentary calcareous organisms; it is wave resistant and stands above the surrounding contemporaneously deposited sediment.

refraction: the bending of a seismic wave as it enters a material of different density, or, the bending of a beam of light as it enters a material of different refractive index.

regional metamorphism: metamorphism across a broad area caused by the elevated temperatures and pressures of plate collision or deep burial.

(or)

metamorphism affecting a large region that is associated with mountain building events.

regolith: a general term used in reference to unconsolidated rock, alluvium or soil material on top of the bedrock. regolith may be formed in place or transported in from adjacent lands.

regression: a retreat of the sea from land areas. possible causes include a drop in sea level or uplift.

relief: variations in the height and slope of earth's surface. also used in reference to the vertical difference between the highest and lowest elevations of an area.

(or)

refers to differences in elevation of different points in a region.

relative dating: the process of placing rocks and geologic structures in the correct chronological order. this process does not yield ages in number of years. see radiometric dating.

remote sensing: the collection of information about an object or area from a distance. methods employed include photography, radar, spectroscopy and magnetism.

replacement: the dissolving or disintegration of one material followed by precipitation of a new material in its place.

retrograde metamorphism: mineral changes within a rock that are caused by adjustments to conditions of reduced temperature and pressure.

reverse fault: a fault with vertical movement and an inclined fault plane. the block above the fault has moved upwards relative to the block below the fault.

rhyolite: the fine-grained volcanic or extrusive rocks that is equivalent in composition to granite. normally white, pink or gray in color.

(or)

a volcanic rock chemically equivalent to granite. usually light colored, very fine-grained or glassy-looking. may have tiny visible crystals of quartz and/or feldspar dispersed in a glassy white, green, or pink groundmass.

ribbon chert: chert and shale in thin alternating beds. the beds resemble parallel ribbons stretched over an outcrop.

richter magnitude scale: a scale that is used to compare the strength of earthquakes based upon the amount of energy released. the scale is logarithmic and an arbitrary earthquake was used as a starting point for creating the scale. as a result it is a continuous scale with no upper limit and negative numbers possible for very small earthquakes. an upper limit of approximately 9.0 is suspected as earth materials will most likely fail before storing enough energy for a larger magnitude earthquake.

ridge (mid-ocean): an elevated area of the sea floor in the center of an ocean basin with rugged topography, a central rift-valley and recurring seismic activity. ridges generally stand about 1000 meters to 3000 meters above the adjacent ocean floor and are about 1500 kilometers in width.

rift zone: a region of earth’s crust along which divergence is taking place. a linear zone of volcanic activity and faulting usually associated with diverging plates or crustal stretching.

right-lateral fault: a fault with horizontal movement. if you are standing on one side of the fault and look across it, the block on the opposite side of the fault has moved to the right. (also see left-lateral fault.)

ring of fire: a zone of volcanoes, earthquakes, and mountain-building encircling the pacific ocean formed where plates collide.

rip current: a strong, narrow current of high velocity and short duration that flows seaward through the breaker zone. caused when a build up of water pushed onto the beach by winds and waves returns seaward.

ripple marks: a series of parallel or sub-parallel ridges in sand or sediment that is caused by the rhythmic or directional movement of wind or water.

rock: rocks are made of different kinds of minerals, or broken pieces of crystals, or broken pieces of rocks. some rocks are made of the shells of once-living animals, or of compressed pieces of plants. rocks are divided into three basic types, igneous, sedimentary and metamorphic, depending upon how they were formed.

rock cycle: all rock at or near earth's surface is being modified by the processes of metamorphism, melting, crystallization, lithification and weathering. these processes move rock material through the states of metamorphic rock, igneous rock, sedimentary rock, melts and sediment. the natural and continuous cycling of rock materials through these states is known as the rock cycle.

rockfall: falling, bouncing, and rolling of debris down slope.

rock flour: finely pulverized rock material of silt or smaller size produced by abrasion at the base of a glacier.

rock glacier: a mass of rock material, cemented together by ice, that flows down a slope under the force of gravity much like the motion of a glacier.

rockslide: a type of mass wasting in which a large volume of rock debris slides down a slope under the influence of gravity.

root of a volcano: plutonic igneous rock formed from magma that crystallized beneath the volcano it once fed.

runoff: liquid water moving over the land surface as a sheet or channelized flow. the portion of precipitation that moves over the ground instead of evaporating or infiltrating.

rupture strength: the maximum amount of stress that a material can sustain without failure.

saltation: the transport of sediment in short jumps and bounces above the stream bed or ground by a current that is not strong enough to hold the sediment in continuous suspension. (see suspension and traction for comparison.)

sand: loose particles of rock or mineral (sediment) that range in size from 0.0625 - 2.0 millimeters in diameter.

sandstone: a sedimentary rock composed of sand-sized particles (1/16 to 2 millimeters in diameter).

(or)

sedimentary rock made mostly of sand-sized grains.

sanidine: a type of potassium feldspar that forms only at high temperature. common in potassium-rich volcanic rocks.

scarp: a cliff formed by faulting, erosion, or landslides. (=escarpment)

schist: a metamorphic rock containing abundant particles of mica, characterized by strong foliation, and originating from a metamorphism in which directed pressure plays a significant role.

(or)

metamorphic rock usually derived from fine-grained sedimentary rock such as shale. individual minerals in schist have grown during metamorphism so that they are easily visible to the naked eye. schists are named for their mineral constituents. for example, mica schist is conspicuously rich in mica such as biotite or muscovite.

schistosity: the parallel arrangement of platy or prismatic minerals in a rock that is caused by metamorphism in which directed pressure plays a significant role.

scoria: an igneous rock of basaltic composition and containing numerous vesicles caused by trapped gases.

(or)

very bubbly (vesicular) basalt or andesite. both scoria and pumice develop their bubbly textures when escaping gas is trapped as lava solidifies. scoria is more dense and darker than pumice.

sea-floor spreading: the process that occurs at mid-ocean ridges in which convection currents below pull the plates apart and create new sea floor.

seamount: a mountain on the sea floor that has at least 1000 meters of local relief. most seamounts are shield volcanoes. (see also guyot.)

sea stack: sea stacks are blocks of erosion-resistant rock isolated from the land by sea.

sediment: a loose unconsolidated deposit of weathering debris, chemical precipitates or biological debris that accumulates on earth's surface. (or)

loose, uncemented pieces of rock or minerals.

sedimentary: sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. they form from deposits that accumulate on the earth’s surface. sedimentary rocks often have distinctive layering or bedding.

sedimentary rock: a rock formed from the accumulation and consolidation of sediment, usually in layered deposits.

sedimentary structure: a structure in a sedimentary rock that forms at or near the time of deposition and reveals information about the depositional environment. examples include: ripple marks, cross-bedding, mud cracks, and graded bedding.

sedimentation: the process of sediment deposition from out of a suspension or solution.

seepage: the slow movement of water through the pore spaces of a solid material. this term is also applied to a loss of water by infiltration through the bottom of a stream, canal, irrigation ditch, reservoir or other body of water.

seif dune: a large sand dune that forms parallel to the direction of a strong wind that blows in a consistent direction throughout the year. also called a longitudinal dune.

seismic: refers to earthquakes.

seismic discontinuity: a surface separating rocks that transmit seismic waves at different velocities.

seismicity: the study of the worldwide distribution of earthquakes over time and the probability of an earthquake occurring in a specific location.

serpentine: a family of silicate minerals rich in magnesium and water, derived from low-temperature alteration or metamorphism of the minerals in ultramafic rocks. rocks made up of serpentine minerals are called serpentinite. serpentine minerals are light to dark green, commonly varied in hue, and greasy looking; the mineral feels slippery.

settling pond: an open pond where waste or process water is allowed to stand while suspended materials settle out.

shale: sedimentary rock derived from mud. commonly finely laminated (bedded). particles in shale are commonly clay minerals mixed with tiny grains of quartz eroded from pre-existing rocks. shaley means like a shale or having some shale component, as in shaley sandstone.

sheetwash: overland flow of water in thin sheets.

shrink-swell: refers to the property of many clays to swell when wetted and shrink when dried.

sillimanite: an aluminum-rich silicate found only in metamorphic rocks that form at high temperature and pressure.

silica: silicon dioxide (sio2). one of the most common compounds in the earth’s crust. common window glass is made of silica. the building block of the mineral quartz and other silicate minerals.

silicate: refers to the chemical unit silicon tetroxide, sio4, the fundamental building block of silicate minerals. silicate minerals make up most rocks we see at the earth’s surface.

siliceous: generally refers to a rock rich in quartz.

sill: see dike.

silt: loose particles of rock or mineral (sediment) that range in size from 0.002 - 0.0625 millimeters in diameter. silt is finer than sand, but coarser than clay.

siltstone: a sedimentary rock made mostly of silt-sized grains.

sinkhole: a depression in the land surface that results from the collapse or slow settlement of underground voids produced by solution weathering. the rock being dissolved is normally limestone but can also be salt, gypsum or dolostone.

(or)

a depression in the surface commonly found in in karst landscapes. sinkholes often form where limestone or some other soluble rock is partially dissolved by groundwater, then collapses to form a depression. sinkholes are often "bowl-shaped" and can be a few to many hundreds of meters in diameter. also known as dolines.

slump: a type of landslide in which a mass of rock breaks away along a curved surface and rotates more or less intact downslope. the sliding mass of rock is called a slump block.

smectite: group of clays, those most susceptible to shrink-swell.

soil: all loose, unconsolidated earth and organic materials above bedrock that support plant growth.

solution: a chemical weathering process in which a material is dissolved. also, the transport of dissolved ions by the water of a stream.

speleology: the exploration and study of caves.

speleothem: a deposit formed in caves when calcium carbonate (caco3) or some other mineral precipitates from drips or thin films of water. stalactites and stalagmites are common speleothems.

stalactite: a mineral deposit (speleothem) which hangs downwards from a roof or wall of a cave. see stalagmite.

stalagmite: a mineral deposit (speleothem) which projects upwards from a cave floor. see stalactite.

stitching pluton: plutons of roughly the same age which that intruded several tectonic terranes after the terranes were faulted together. the plutons do not really "sew" the terranes together, but they help record when terranes were assembled.

stock: relatively small globular or columnar-shaped pluton. like a batholith only smaller.

stope block: stope blocks form when injection of intrusive igneous rock weakens the solid rock surrounding it, causing blocks to loosen and sink into the molten mass.

storm sewer: a sewer system that collects surface runoff instead of waste water. these two types of water are kept separate because they require different processing before release to the environment.

storm surge: the piling up of water along a shoreline cause by the sustained winds of a strong storm - usually a hurricane..

strain: a change in the volume or shape of a rock mass in response to stress.

stratification: a layered structure of sedimentary rocks in which the individual layers can be traced a considerable distance. the layers can be caused by many differences which include materials of different composition, color, grain size or orientation.

stratified: formed, arranged, or laid down in layers. the term refers to geologic deposits.

stratigraphic sequence: the sequence of sedimentary rock layers found in a specific geographic area, arranged in the order of their deposition.

stratigraphy: the study of sedimentary rock units, including their geographic extent, age, classification, characteristics and formation.

stratovolcano: a volcanic cone made up of alternating layers of lava flows and pyroclastics.

(or)

a relatively long-lived volcano built up of both lava flows and pyroclastic material.

streak: the color of a mineral in powdered form. streak is normally determined by scraping a specimen across a surface of unglazed porcelain known as a "streak plate".

streak plate: a piece of unglazed porcelain that is used for determining the streak of a mineral specimen.

stream capture: a process of erosion where one stream erodes headward, diverting some of another stream’s drainage into its own channel. also called stream piracy.

stream order: a classification system that represents the relative position of streams in a drainage basin. the highest tributaries in the basin are first order streams. these converge to form second order streams, which have only first order streams as their tributaries. third order streams form by the confluence of two second order streams. the numbering system continues downstream resulting in higher stream orders.

stream terrace: one of a series of platforms in a stream valley, flanking and more or less parallel to the stream channel, originally formed near the level of the stream, and representing the dissected remnants of an abandoned flood plain, stream bed, or valley floor produced during a former state of erosion or depositon.

stress: a force acting upon or within a mass or rock, expressed in terms of unit weight per surface area such as tons per square inch.

striations: scratches or grooves on a rock or sediment surface caused by abrasive action of objects being transported above it by ice, water or wind.

strike: the geographic direction of a line created by the intersection of a plane and the horizontal. often used to describe the geographic "trend" of a fold or fault.

strike-slip fault: a fault with horizontal displacement, typically caused by shear stress.

stringer: a thin, discontinuous mineral vein or rock layer.

stromatolite: a mound-shaped fossil that forms from the repetitious layering of algal mat covered by trapped sediment particles.

subduction: process of one crustal plate sliding down and below another crustal plate as the two converge. the subduction zone is the area between the two plates, somewhat like a giant reverse fault.

subduction zone: an area at a convergent plate boundary where an oceanic plate is being forced down into the mantle beneath another plate. these can be identified by a zone of progressively deeper earthquakes.

sublimation: the process through which ice goes directly into a vapor without passing through the liquid state.

submarine canyon: an underwater canyon, carved into the continental shelf. these can be carved by turbidity currents or carved subaerially during a time when sea level was lower.

submarine fan: fan or cone-shaped accumulation of sedimentary debris--sand, gravel, mud--under the ocean along the edge of the land, either a continent or a volcanic arc. fans may be a few miles to a hundred or so miles across.

subsidence: a lowering of the land surface in response to subsurface weathering, collapse or slow settlement of underground mines, or the production of subsurface fluids such as ground water or oil.

summit: the topographically highest hillslope position of a hillslope profile and exhibiting a nearly level surface.

supercontinent: a large landmass that forms from the convergence of multiple continents.

superposed stream: a stream that cuts across resistant bedrock units. this can occur when the stream's course was determined at a previous time and on a previous landscape.

superposition: the concept that the oldest rock layers are at the bottom of a sequence with younger rock layers deposited on top of them. this can be considered a rule that applies in all situations, except where the rocks are extremely deformed.

supersaturated solution: a solution that contains more solute than its solubility allows. such a solution is unstable and precipitation can be triggered by a variety of events.

supraglacial: carried upon, deposited from, or pertaining to the top surface of a glacier or ice sheet.

surf: the breaking of waves as they enter shallow water.

surf zone: an area of breaking waves bounded by the point of first breakers, then landward to the maximum uprush of waves on the beach.

surface wave: a type of seismic wave that travels along earth's surface.

surficial deposit: any loose, unconsolidated sedimentary deposit lying on bedrock.

suspended load: small particles being carried by a stream and held in suspension by the movement of the water. (also see: load, dissolved load, bed load.)

suspension: transport of sediment by wind or water currents that are strong enough to keep the sediment particles continuously above the stream bottom or ground. (see traction and saltation for comparison.)

swale: a slight, open depression which lacks a defined channel that can funnel overland or subsurface flow into a drainageway.

swash: the rush of a breaking wave up the slope of a beach.

s-wave: secondary seismic waves. a seismic wave with a direction of vibration that is perpendicular to the direction of travel. s-waves are slower than p-waves and travel only through solids.

symbiosis: a relationship between two species who live in close association but do not compete with each other or prey on one another. at least one of the species derives benefit from this association.

syncline: a trough-shaped fold with youngest strata in the center.

system: a stratigraphic unit of major significance which was deposited during a specific time period, and which can be correlated worldwide on the basis of its fossil content.

tableland: an area of elevated land with a nearly level surface.

tar sand: a sandstone that contains asphalt within its pore spaces.

talc: magnesium silicate mineral, with water. commonly called soapstone. very soft and platy, like mica. can be easily carved with a knife. generally in very fine grained masses.

talus: an accumulation of angular rock debris at the base of a cliff or steep slope that was produced by physical weathering.

tarn: small lake left by the retreat of a glacier. may fill a basin formed by a moraine dam or eroded by the glacier into bedrock.

talus: pile of rock rubble below a cliff or slope.

(or)

pile of rock rubble below a cliff or chute. talus slope is a common usage although it is redundant because the term "talus" actually includes the concept of a slope.

tectonically active: a term used to describe regions that are strongly affected by movement of earth's tectonic plates. earthquakes and volcanoes are common features in these regions.

tectonics: the study of processes that move and deform earth's crust.

tephra: general term for all sizes of particles ejected into the air during volcanic eruptions. includes particles as tiny as volcanic ash and as large as bombs and blocks (= pyroclastics).

terminal moraine: a mound of unsorted glacial till that marks the furthest advance of a glacier.

terrace: a long bench-like surface, often bordering a stream, lake, or sea.

(or)

level or near-level area of land, generally above a river or ocean and separated from it by a steeper slope. a river terrace is made by the river at some time in the past when the river flowed at a higher level. it a terrace may be made of river deposits such as gravel or sand, or it could be cut by the river on bedrock. a glacial terrace or outwash terrace is similar but is formed by a stream or river from a glacier upstream.

terrane: a rock formation or assemblage of rock formations that share a common geologic history. a geologic terrane is distinguished from neighboring terranes by its different history, either in its formation or in its subsequent deformation and/or metamorphism. terranes are separated by faults. an exotic terrane is one that has been transported into its present setting from some distance.

terrestrial planet: one of the four rocky planets closest to the sun, which include mars, venus, earth and mercury.

terrigenous sediment: sediment that is derived from the weathering of rocks which are exposed above sea level.

tertiary period: the earliest period of the cenozoic era, beginning about 66.4 million years ago and ending 1.6 million years ago.

texture: the visible characteristics of a rock which include its grain size, grain orientation, rounding, angularity or presence of vesicles.

thermal aureole: zone of rock around an igneous intrusion that has been altered or metamorphosed by heat from the hot magma. the rock in the zone is baked.

thermal pollution: water quality is not defined by chemistry alone. if natural waters are withdrawn for use they should be returned to the environment at approximately the same temperature. an increase or decrease in temperature can have an adverse effect upon plants, animals and chemical balances. returning water to a stream at a different temperature than it was withdrawn is known as thermal pollution. for example, coal-fired power plants use water in the production of steam that turns turbines. that water is then cooled in the large cooling towers before it is returned to the environment.

thrust fault: a reverse fault that has a dip of less than 45 degrees.

thrust plate: slab of rock, generally on the scale of a mountain or more, bounded by two thrust faults.

tidal current: currents of water that are produced in response to a rising or falling tide. these currents can flow into or out of a bay, delivering the rising water or removing the falling water.

tidal flat: a broad flat area, very close to sea level that is flooded and drained with each rise and fall of the tide.

tidal wave: a term that is incorrectly used in reference to a tsunami. tsunamis have nothing to do with the tides.

till: an unsorted sediment deposited directly by a glacier and not reworked by meltwater.

(or)

unsorted, unstratified rock rubble or debris carried on and/or deposited by the ice of a glacier .

(or)

dominantly unsorted and unstratified drift, generally unconsolidated deposited directly by and underneath a glacier without subsequent reworking by meltwater, and consisting of a hetergeneous mixture of clay, silt, sand, gravel, stones, and boulders.

till, ablation: a general term for loose, relatively permeable material deposited by the downwasting of nearly static glacier ice.

till, basal: unconsolidated material of mixed composition deposited at the base of a glacier. types of basal till include: lodgement, meltout, and flow till.

till, lodgement: a basal till commonly characterized by compact, fissile (platy) structure and containing rock fragments oriented with their long axes generally parallel to the direction of ice movement.

till, melt-out: till derived from slow melting of debris-rich stagnant ice buried beneath sufficient overburden to inhibit deformation under gravity, thus preserving structures derived from the parent ice.

till plain: an extensive flat to undulating surface underlain by till.

toeslope: the hillslope position that forms the gently inclined surface at the base of a hillslope.

tonalite: intrusive igneous rock made of plagioclase feldspar, quartz, and amphibole or biotite. may be similar to diorite but contains considerable quartz and is not as dark, and chemically has less calcium, iron and magnesium.

topographic map: a map that shows the change in elevation over a geographic area through the use of contour lines. the contour lines trace points of equal elevation across the map. see also: contour line and contour map.

topography: the shape of earth's surface or the geometry of landforms in a geographic area.

(or)

the shape of the land surface. see relief.

(or)

the relative position and elevations of the natural or manmade features of an area that describe the configuration of its surface.

trace element: an element that is present in very small quantities.

traction: transport of sediment by wind or water in which the sediment remains in contact with the ground or bed of the stream, moving by rolling or sliding. (see suspension and saltation for comparison.)

transform fault: a strike-slip fault that connects offsets in a mid-ocean ridge.

transgression: an advance of the sea over land areas. possible causes include a rise in sea level or subsidence.

transpiration: a process of plants removing water from the soil and releasing it into the atmosphere through their leaves.

transverse dunes: sand dunes that are oriented at right angles to the direction of the prevailing wind. these form where vegetation is sparse and the sand supply is abundant.

(or)

a very asymmetric sand dune elongated perpendicular to the prevailing wind direction, having a gentle windward slope and a steep leeward slope standing at or near the angle of repose of sand.

trap: a sedimentary or tectonic structure where oil and/or natural gas has accumulated. these are structural highs where a porous rock unit is capped by an impermeable rock unit. oil and gas trapped within the porous rock unit migrate to a high point in the structure because of their low density.

travertine: calcium carbonate deposits which form in caves and around hot springs where carbonate-bearing waters are exposed to the air. the water evaporates, leaving a small deposit of calcium carbonate.

tread: the flat or gently sloping surface of natural step-like landforms, commonly one of a series, such as successive stream terraces.

trellis drainage: a drainage pattern in which streams intersect at right angles. this forms in areas of long parallel valleys such as in folded mountain belts. rivers occupy the valleys and tributary streams join them at right angles.

trench: a long, narrow, deep depression in the ocean floor that parallels a convergent boundary involving at least one oceanic plate.

(or)

trenches are deep, linear zones that form where an oceanic plate sinks (subducts) beneath another plate.

triple junction: a point where three lithospheric plates meet. triple junctions can be areas of unusual tectonic activity due to the differential motions of the three intersecting plates.

tsunami: a large sea wave normally produced by sudden movement of the ocean floor caused by an earthquake or volcanic eruption. these waves can travel at high speeds across an ocean basin and cause great destruction when they reach land.

tuff: a rock composed of pyroclastic materials that have been ejected from a volcano. in many instances these fragments are still hot when they land, producing a "welded" rock mass.

(or)

volcanic rock made up of rock and mineral fragments in a volcanic ash matrix. tuffs commonly are composed of much shattered volcanic rock glass--chilled magma blown into the air and then deposited. if volcanic particles fall to the ground at a very high temperature, they may fuse together, forming a welded tuff.

turbidite: a vertical sequence of sediments deposited by a turbidity current. because the largest particles of the current settle first a turbidite will be graded deposits with coarsest grain sizes at the bottom and finer grain sizes going upwards.

turbidity current: a mixture of sediment particles and water that flows down the continental slope. these high density currents can reach great speeds and generally erode loose sediments from the seafloor beneath them. see "density current".

turbulent flow: an irregular state of fluid flow in which the particle paths cross one another and may even travel in opposing directions. (compare with laminar flow.)

ultrabasic rock: an igneous rock with a very low silica content and rich in minerals such as hypersthene, augite and olivine. these rocks are also known as ultramafic rocks.

ultramafic rock: an intrusive igneous rock very rich in iron and magnesium and with much less silicon and aluminum than most crustal rocks. most come from the earth’s mantle.

(or)

see "ultrabasic rock".

unconformity: a contact between two rock units of significantly different ages. an unconformity is a gap in the time record for that location.

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the contact between older rocks and younger sedimentary rocks in which at least some erosion has removed some of the older rocks before deposition of the younger. an angular unconformity shows that the older rocks have been deformed and eroded before the younger sedimentary rocks were deposited; there is an angle between the beds of the older and the younger.

unconsolidated: a term used when referring to sediment that has not been lithified into a rock. uncemented.

(or)

loose sediment; lacking cohesion or cement.

uniformitarianism: a basic geologic principle. processes that act upon the earth today are the same processes that have acted upon it in the past. the present is the key to the past.

unit cell: the smallest sample of a substance that has a complete representation of its atomic structure. a crystal structure is formed by repetition of the unit cell in three dimensions.

upland: a general term for the higher ground of a region, in contrast with valley, plain, or other lower lying adjacent land.

uplift: a structurally high area in earth's crust. formed by movements that bend the crust into a structure such as a dome or an arch.

upwelling: movement of cold water from the floor of a lake or ocean up into a shallow area.

u-shaped valley: a deep valley with a flat floor and very steep walls. shaped in cross-section like the letter "u". valleys with this geometry are frequently cut by a glacier.

vadose water: water that exists in the pore spaces of a rock or soil, between the ground surface and the water table.

valence electrons: electrons in the outermost shell of an atom. the electrons that are typically involved in making chemical bonds.

valley glacier: a glacier that occurs in a mountainous region and occupies a valley. also known as an alpine glacier.

valley train: a long narrow body of outwash confined within avalley beyond a glacier.

van der waals bond: a weak chemical bond in which atoms are held together by weak electrostatic attraction.

varve: a thin layer of fine-grained sediment deposited in the still waters of a lake. varves are frequently associated with glaciation and represent a yearly sedimentation cycle - a silty, light-colored layer deposited in summer and a darker, organic-rich clay layer deposited during winter.

(or)

a sedimentary layer, lamina, or sequence of laminae, deposited in a body of still water within a period of 1 year; specifically, a thin pair of graded glaciolacustrine layers seasonally deposited, usually in glacial lakes or meltwater streams.

vein: a fracture that has been filled with mineral material.

(or)

a mineral-filled fracture or fault in a rock.

(or)

tabular rock or mineral filling of a generally small crack such as a quartz vein. a product of chemical precipitation from a watery solution, in contrast to a dike crystallized from magma, although gradations exist.

veneer: a thin, widespread layer of sediment covering an older surface.

ventifact: a rock that has been shaped or polished by the sandblasting effect of wind-blown sand.

vertical exaggeration: in making sketches of landscapes and cross-sections the vertical dimension is frequently exaggerated to show detail. vertical exaggeration is a number that represents the magnitude of this exaggeration. it is a proportion between the vertical scale and the horizontal scale. for example, a cross section with a vertical exaggeration of 4 has a vertical scale that is four times the horizontal scale (in this example the vertical scale could be 1:25 while the horizontal scale is 1:100).

vesicle: spherical or elongated cavities in an igneous rock that are created when a melt crystallizes with bubbles of gas trapped inside.

(or)

a small bubble formed in volcanic rock during solidification.

viscosity: the resistance of a fluid to flow. fluids with a high viscosity resist flow. fluids with a low viscosity flow freely.

vitreous: glassy.

vitrophyre: a volcanic rock with larger crystals (phenocrysts) embedded in a glassy groundmass.

volcanic arc: arcuate chain of volcanoes formed above a subducting plate.

(or)

arcuate chain of volcanoes formed above a subducting plate. the arc forms where the downgoing descending plate becomes hot enough to release water and gases that rise into the overlying mantle and cause it to melt. arc rocks are mostly volcanic rocks from the volcanoes and sedimentary rocks made up of eroded debris from the volcanoes. melted rock in the deeper plumbing of the arc which may crystallizes at depth to become an arc root plutons.

volcanic ash: sand-sized particles of igneous rock that form when a spray of liquid magma is blown from a volcanic vent by escaping gas.

volcanic ash fall: an accumulation of volcanic ash produced by an eruption. these can be very thick near the vent and decrease to a light dusting in a downwind direction.

volcanic bomb: a projectile of hot magma or rock that is blown from the vent during a volcanic eruption. these solidify in flight and frequently form an elongated rock of streamlined shape.

volcanic breccia: a rock made up of pyroclastic fragments that are at least 64 millimeters in diameter.

volcanic cone: a cone-shaped hill or mountain composed of pyroclastic debris and/or lava which builds up around a volcanic vent during eruptions.

volcanic dome: a steep-sided extrusion of very viscous lava that is squeezed from a volcanic vent without major eruption. these are frequently rhyolitic in composition and produce a rounded mound above the vent.

volcanic neck: a vertical intrusion with the geometry of a volcanic pipe. an erosional remant of a volcanic pipe.

volcanic pipe: a vertical or near vertical tunnel which connects a magma reservoir to the surface. magma and gas travel up this tube to produce the eruption. after the eruption the tube can be filled with a cooling magma which preserves its shape as an intrusive body.

volcanic rock: igneous rock that cools and solidifies at or very near the earth’s surface. volcanoes produce volcanic rock.

volcano: a vent in earth's surface through which molten rock and gases escape. the term also refers to deposits of ash and lava which accumulate around this vent.

v-shaped valley: a valley with a narrow bottom and a cross section shaped like the letter "v". valleys of this shape are almost always cut by stream erosion.

wadi: a stream valley in an arid region that is dry except during the rainy season.

weathering: weathering includes two surface or near-surface processes that work in concert to decompose rocks. both processes occur in place. no movement is involved in weathering. chemical weathering involves a chemical change in at least

some of the minerals within a rock. mechanical weathering involves physically breaking rocks into fragments without changing the chemical make-up of the minerals within it. mechanical weathering includes processes such as water in cracks freezing and expanding, or changes in temperature that expand and shrink individual minerals enough to break them apart.

warping: a slight bend, uplift or subsidence of earth's crust on a regional scale.

wash: a normally dry stream bed that ocassionally fills with water.

water cycle: the movement of water between the atmosphere, ground and surface water bodies through the processes of evaporation, precipitation, infiltration, percolation, transpiration and runoff. also known as the "hydrologic cycle".

water quality: an assessment of the physical, chemical and biological characteristics of water, especially how they relate to the suitability of that water for a particular use.

watershed: the geographic area that contributes runoff to a stream. it can be outlined on a topographic map by tracing the points of highest elevation (usually ridge crests) between two adjacent stream valleys. the watershed of a large river usually contains the watersheds of many smaller streams. also referred to as a "drainage basin".

water table: a level beneath the earth's surface, below which all pore spaces are filled with water and above which the pore spaces are filled with air. the top of the zone of saturation in a subsurface rock, soil or sediment unit.

wave-cut terrace: a long, level surface formed by wave erosion during a time when sea level was higher.

wavelength: an interval of repetition in a wave-like disturbance. the distance between two successive crests or two successive troughs.

well-sorted: refers to a sedimentary deposit or rock with grains of the same approximate size.

withdrawal: a removal of water from a surface or ground water source for use.

xenolith: a preexisting rock that has been incorporated into magma without melting. when the magma crystallizes the preexisting rock fragment is known as a xenolith.

(or)

a piece of foriegn rock enclosed within an igneous rock. the foriegn rock is usually picked up from the walls surrounding the igneous rock and is frozen in place before it has a chance to melt. (=inclusion)

x-ray diffraction: a technique used to identify minerals by bombarding them with x-rays. planes of repetition within the atomic structure of the mineral diffract the x-rays. the pattern of diffraction is unique for each mineral structure and can be used for identification.

yazoo stream: a tributary that parallels the main channel for a considerable distance. joining of these streams is normally blocked by a natural levee along the larger stream.

yellow ground: oxidized kimberlite. a yellow soil that is characteristic of the area above a kimberlite diamond pipe.

yield: the quantity of water, coal, gold or other resource that can be produced from a deposit.

youth: the earliest stage in the development of a landscape. during this stage streams are actively downcutting and flowing straight for long distances with frequent waterfalls and rapids. the valleys are typically steep sided and v-shaped.

zeolite: a group of hydrous aluminosilicates that are similar to the feldspars. they easily lose and regain their water of hydration and they fuse and swell when heated. zeolites are frequently used in water softening, ion exchange and absorbent applications

zircon: mineral of zirconium, silicon, and oxygen (zirconium silicate). generally glassy-looking, microscopic, four-sided prisms. most commonly formed in igneous rocks.

zoned crystal: a crystal that grew while temperatures were changing or while the composition of the parent solution was changing. crystals of these minerals can have a range of compositions, with a certain chemistry in the center reflecting the early growth conditions and the outer chemistry reflecting the later growth conditions. minerals such as olivine or plagioclase which have a solid solution series frequently form such crystals.

zone of aeration: a zone between the land surface and the water table where pore spaces are filled mainly with air. water that exists in the pore space in this zone is referred to as "soil moisture".

zone of saturation: the zone beneath the water table where all pore spaces are completely filled with water. water that exists within this zone is known as "ground water".