WEATHERING AND EROSION

WEATHERING AND EROSIONWeathering - processes at or near Earth’s surface that cause rocks and minerals to break down

Erosion - process of removing Earth materials from their original sites through weathering and transport

WEATHERING AND EROSION

Weathering produces regolith (“rock blanket”) which is composed of small rock and mineral fragments. A loose layer of fragments that covers much of Earth’s surface.

When organic matter is mixed into this material it is called soil. The uppermost layer of regolith, which can support rooted plants.

Joints A fracture of rock , along

which no appreciable movement has occurred

Abrasion The gradual wearing

down of bed rock by the constant battering of loose particles transported by wind, water or ice

The jointing in these rocks has exposed new surface area which has broken and smoothed due to wind, water and ice.

WEATHERING-THE FIRST STEP IN THE ROCK CYCLE How rocks disintegrate

Weathering The chemical and physical

breakdown of rock exposed to air, moisture and living organisms

The rock in the photo has weathered in place with little erosion, forming soil

WEATHERINGMechanical Weathering - processes that break a rock or mineral into smaller pieces without altering its composition

Chemical Weathering - processes that change the chemical composition of rocks and minerals

PROCESSES AND AGENTS OF MECHANICAL WEATHERINGThese are actions or things that break down

Earth materials frost wedging thermal expansion and contraction mechanical exfoliation abrasion by wind, water or gravity plant growth

PROCESSES AND AGENTS OF MECHANICAL WEATHERINGFrost Wedging – cracking of rock mass by the expansion of water as it freezes in cracks

http://www.uwsp.edu/geo/faculty/ozsvath/images/frost%20wedging.jpg

FROST WEDGING (IN SOIL)

Ice crystals

PROCESSES AND AGENTS OF MECHANICAL WEATHERINGThermal expansion and contraction –

repeated heating and cooling of materials cause rigid substances to crack and separate

http://content.answers.com/main/content/wp/en-commons/thumb/d/dc/250px-Weathering_freeze_thaw_action_iceland.jpg

PROCESSES AND AGENTS OF MECHANICAL WEATHERINGExfoliation – As underlying rock layers are exposed, there is less pressure on them and they expand. This causes the rigid layers to crack and sections to slide off. The expanding layers often form a dome.

DOME EXFOLIATION

PROCESSES AND AGENTS OF MECHANICAL WEATHERINGAbrasion – Moving sediments or rock sections can break off pieces from a rock surface they strike. The sediments can be moved by wind or water and the large rock sections by gravity.

WIND ABRASION

http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/images/lithosphere/eolian/rock_wind_abrasion_p0772932441_NRCS.jpg

WIND AND WATER ABRASION

http://www.gsi.ie/Education/European+Landscapes/United+Kingdom.htm Photo Ref: P211442, "IPR/52-34CW BGS©NERC

PROCESSES AND AGENTS OF MECHANICAL WEATHERINGPlant Growth – As plants such as trees send out root systems, the fine roots find their way into cracks in the rocks. As the roots increase in size, they force the rock sections apart, increasing the separation and weathering.

PLANT WEDGING

PROCESSES OF CHEMICAL WEATHERINGdissolving (dissolution)oxidationhydrolysis

PROCESSES OF CHEMICAL WEATHERING

Dissolving (dissolution) Water, often containing acid from dissolved carbon dioxide, will dissolve minerals from a rock body leaving cavities in the rock. These cavities may generate sinkholes or cave features such as stalactites and stalagmites.

CHEMICAL WEATHERING

DissolutionThe separation of

materials into ions in a solution by a solvent, such as water or acid

Rainwater acts as weak solution of carbonic acid

Anthropogenic actions influence acidity of rainwater

The marble grave marker has been attacked by acidic rain because of the calcite composition. The grave marker on the right, while old, has not been dissolved because of its granite composition

CARBON DIOXIDE Carbon dioxide

dissolves in rain water and produces Carbonic acid.

This Carbonic acid easily weathers marble and Limestone.

PROCESSES OF CHEMICAL WEATHERING

Oxidation Minerals may combine with oxygen to form new minerals that are not as hard. For example, the iron-containing mineral pyrite forms a rusty-colored mineral called limonite.

PYRITE OXIDATION

http://www.windows.ucar.edu/earth/geology/images/pyrite_sm.jpg

http://www.dkimages.com/discover/previews/965/75014124.JPG

Pyrite

Limonite

OXYGENWater + Oxygen

+Iron = RUST

When water and oxygen mix with Iron it creates rust. This is called oxidation.

PROCESSES OF CHEMICAL WEATHERING

Hydrolysis Minerals may chemically combine with water to form new minerals. Again these are generally not as hard as the original material.

FELDSPAR HYDROLYSIS

http://www.mii.org/Minerals/Minpics1/Plagioclase%20feldspar.jpg http://www.uwm.edu/Course/422-100/Mineral_Rocks/kaolinite1.jpg

Feldspar Kaolinite (clay)

FACTORS IN CHEMICAL WEATHERING

Climate – wet and warm maximizes chemical reactions

Plants and animals – living organisms secrete substances that react with rock

Time – longer contact means greater change

Mineral composition – some minerals are more susceptible to change than others

EROSION TRANSPORT AGENTS OR FORCES Water rain

streams and riversocean dynamicsice in glaciers

Wind Gravity

STREAMSFlowing water will lift and carry small sediments such as silt and sand.

STREAM EROSION AND DEPOSITIONWhere water moves more swiftly there will be moreerosion.

Where the water slows down, sediments will bedeposited.

OCEAN DYNAMICS Tidal action and waves carry away

weathered materials.

http://www.dkimages.com/discover/previews/1000/50195183.JPG

http://edge.tamu.edu/waves2001/PC_tour/erosion_files/image002.jpg

GLACIERSGlaciers are large ice fields that slowly flow downhill over time.

http://images.encarta.msn.com/xrefmedia/sharemed/targets/images/pho/t628/T628797A.jpg

GLACIERSGlacial ice drags rocky material that scours the surface it flows over . The glacier deposits debris as it melts.

http://www.geology.um.maine.edu/user/Leigh_Stearns/teaching/kelley_island.jpg

WIND TRANSPORT OF SEDIMENTSWind will carry fine, dry sediments over long distances.

WIND TRANSPORT OF DUST

Photo shows Sahara Desert sand being transported overthe Atlantic Ocean.

TRANSPORT BY GRAVITY When sediments are weathered they

may be transported downward by gravity. The general term for this is mass wasting.

http://en.wikipedia.org/wiki/Mass_wasting

TRANSPORT BY GRAVITY When sediments are weathered they may be

transported downward by gravity as a slump.

Slump

http://new.filter.ac.uk/database/image.php?id=594

TRANSPORT BY GRAVITY Loose sediments transported by gravity are called scree.

Scree field

http://www.dave-stephens.com/scrambles/banff/aylmer/aylmer013.jpg

DEPOSITION FORMATIONTransported sediments are deposited inlayers and generate strata like those found in the Grand Canyon.

DEPOSITION FORMATION

FACTORS AFFECTING WEATHERING Tectonic setting

Young, rising mountains weather relatively rapidly

Mechanical weathering most common

FACTORS AFFECTING WEATHERING Rock

compositionMinerals

weather at different rates Calcite weathers

quickly through dissolution

Quartz is very resistant to chemical and mechanical weathering

Mafic rocks with ferromagnesian minerals weather more easily

FACTORS AFFECTING WEATHERING Rock structure

Distribution of joints influence rate of weathering Relatively close

joints weather faster

FACTORS AFFECTING WEATHERING Topography

Weathering occurs faster on steeper slopes Rockslides

FACTORS AFFECTING WEATHERING Vegetation

Contribute to mechanical and chemical weathering

Promotes weathering due to increased water retention

Vegetation removal increases soil loss

Vegetation can both hold waterAnd increase weathering. If removedRocks may also be vulnerable to abrasion

FACTORS AFFECTING WEATHERING Biologic activity

Presence of bacteria can increase breakdown of rock

FACTORS AFFECTING WEATHERING

ClimateChemical weathering

is more prevalent in warm, wet tropical climates Mechanical weathering

less important hereMechanical

weathering is more prevalent in cold, relatively dry regions Chemical weathering

occurs slowly hereNote: temperate regions such

as at the center of the chart undergo both chemical and mechanical weathering, i.e. New York area

FACTORS AFFECTING WEATHERING: COLOR DOTS ON MAP MATCH COLORS ON CHART

PRODUCTS OF WEATHERING Clay

Tiny mineral particles of any kind that have physical properties like those of the clay minerals

Clays are hydrous alumino-silicate minerals

PRODUCTS OF WEATHERING Sand

A sediment made of relatively coarse mineral grains

Soil Mixture of minerals with

different grain sizes, along with some materials of biologic origin

Humus Partially decayed organic

matter in soil

Landslides & Mass Wasting

Earth’s Surface is shaped by external processes…

Earth’s Surface is shaped by external processes…

In sculpting the Earth’s surface, the two most important agents

of erosion are : 1) Mass wasting2) Running water

PP.490-491

original artwork by Gary Hincks

There are a wide variety of manifestationsof the downslope movement of materials by gravity, some faster and some slower.

All of these processes have destructive effects…

Mass Wasting: Downslope, mass movement of Earth materials

Driven by: The pervasive background force of …GRAVITY…

Contributing factors: Saturation of sediments by water

Over-steepened slopes

Removal of vegetation

Earthquakes

Water fills pore spaces between sediment grains,reduces internal resistance, adds weight.

Plants add slope stability byprotection against erosion.

Strong ground vibrations.

Slopes become unstable once they reach the angle of repose = The steepest angle a slope can attain without slumping.

Stability againstgravity depends on the strength of a material, which can berepresented

by its angle ofrepose…

In sediments, thisangle depends on grain and sorting.

In sediments, the angle of repose depends on grain size and sorting of materials…

Mass Wasting

Types of materials:

Types of movement:

Rates of movement:

Soil/regolith -or- Rock/bedrock

Rock Falls - Free-fall of materialRock/Debris Slides - Coherent slabs

slide along fracture surfacesMudflows - Soil and rock mixes with water

and becomes fluidized.Earth or Debris Flows - Materials

move as a viscous mass.

Fastest - Rock falls & avalanches. Avalanches “float” on

entrapped air.Slowest - Creep (cm/year).

Talus slopes

FIG. 16.12

W. W. Norton

Types of mass wasting processes arrayed by typical velocity of movement….

FIG. 16.12

W. W. Norton

Rock Fall/Debris Fall

Rock/Debris FallsMASSWASTING

Blocks of bedrock break free, and fall from a steep cliff face.

Contributing factors:- Steep slopes.- Rocks loosened along joint fractures…

…by expansion of water on freezing,…by thermal expansion/contraction,…by biological activity (e.g. root growth)

- Ground shaking during earthquakes.

FIG. 16.14

Stephen Marshak

FIG. 16.15

W. W. Norton

FIG. 16.22

W. W. Norton

FIG. 16.27 HI

W. W. Norton

Steps to mediate…

W. W. Norton

Mediation…

FIG. 16.08

Stephen Marshak

Mediation by terracing

FIG. 16.12

W. W. Norton

Avalanches

Peruvian Valley Rock Avalanche, May 1970

Before After

FIG. 16.12

W. W. Norton

Rock/debris slides

Rock Slides…

Beds dip downslope.

Rock SlideMASSWASTING

Blocks of bedrock break free, and slide down slope along a fracture surface.Often occurs where strata are inclined, with slip occurring along bedding planes of weak units,like shales.Other important contributing factors:

- Slopes become undercut by stream or wave erosion.- Rain or melting snow seeps into deposits andlubricates a slip surface.

Often deadly! If materials are unconsolidated called a “debris slide”.

FIG. 16.18

W. W. Norton

Common triggering mechanism: Saturation (water) of a weak,expansive, clay-rich shale unit.

FIG. 16.20 A

W. W. Norton

Common triggering mechanism…undercutting of slopes by streams or waves…

Rock slides candevelop in any type of rock

where there is are preferredplanes of weakness dipping downslope…

Sedimentary Metamorphic Igneous

Jointing may facilitate process.

Mediation…

FIG. 16.12

W. W. Norton

Mudflow/Debris Flow

Mudflow / Debris Flow:Common in high rainfall areas

where fine materials mobilized by abundant water…

FIG. 16.12

W. W. Norton

Slumps

Slumps: Rotational TypeMASS WASTING

Mass of material slides downward alonga curved surface (slump surface)

Speed is usually intermediateand material doesn’t travel very far.

Slumping often involves several massesthat move separately (along diferentslump planes).

Common in weak, water saturated sediments that are over-steepened.

Common in coastal areas where sea cliffs are constantly removed by wave erosion.

FIG. 16.04 B

Morphology of a Rotational Slump

Rotational Slump: Headwall shows evidence of backward rotation.

FIG. 16.27 F

W. W. Norton

Approaches to mediation…

Earthquake-triggered slumps, Alaska EQ 1964

Earth/Debris Flow

Slumps: Earth/Debris Flow TypeMASSWASTING

Common in high rainfallareas.

Occur on hillsides.Develop in rock units

rich in clay/silt. Slow rate of movement. Stabilized by “toe” and

by “dewatering”. Destructive!

Robert L. Schuster/U.S. Geological Survey

W. W. Norton

Stabilization of slumpswith plant cover…

FIG. 16.27 D

W. W. Norton

Stabilization by terracing…

FIG. 16.27 C

W. W. Norton

Stabilization by lowering water table…

FIG. 16.27 B

W. W. Norton

Reduce slope angle…

FIG. 16.12

W. W. Norton

Creep

Soil/Regolith CreepMASSWASTING Creep

- Slow (cm/year) downhill movement of material.- Driven by alternate expansion/contraction of material during freeze/thaw or cycles of wetting/drying.

Gravitational force acts onrocks/soil to move them downslope…

FIG. 16.02 B

W. W. Norton

FIG. 16.02 A

W. W. Norton

Effect of cycles of freeze-thawon soil/regolith creep…

Soil/regolith creep…

Soil/Regolith Creep

Slow!

Assisted by:

“Frost heaving”(expansion of iceupon freezing).

SoilRegolith

Evidence of soil/regolith creep…

FIG. 16.02 C

W. W. Norton

Tell-tell signs of soil/regolith creep…

Signs of soil/regolith creep…

FIG. 16.02 D

(c) Martin Miller

Signs of soil/regolith creep…

FIG. 16.03 A

(c) Martin Miller

Solifluction: Soil creep in high latitude, cold climate areas where freeze-thaw is

active…

Reducing soil creep…

THE SOIL PROFILE

THE 6 SOIL ROLESA Soil’s role includes: Serving as a foundation Emitting and absorbing gases Providing habitat Interacting with water Recycling nutrients Supporting human settlements

THE 5 FACTORS OF FORMATION

Soil is formed by… Parent Material: the original “Mom & Pop” soil transported

from elsewhere, usually by wind or water, at different speeds Climate: the amount, intensity, timing, and kind of

precipitation that breaks down parts of ecosystem (i.e. rocks, trees) into soil

Topography: Slope and Aspect affect the angle of the land and position toward/away from the sun that soil will be exposed to

Biological: Plants, animals, microscopic organisms, and humans interact with soil in different ways

Time: the amount of time it takes for the four factors (above) to interact with each other

WHAT IS A SOIL PROFILE? A Soil Profile is a vertical cross-section of layers of

soil found in a given area. Below are two examples of soil profiles.

WHAT IS A SOIL HORIZON? Soil horizons are the layers in a soil profile used to

classify soil types. Horizons based on color, texture, roots,

structure, rock fragments, and any unique characteristic worth noting.

Master Soil Horizons are depicted by a capital letter in the order (from top down): O, A, B, C, and R

O-HORIZONThe “Organic Matter”

Horizon Surface-layer, at depths of 0-2

feet Dark in color, soft in texture Humus - rich organic material

of plant and animal origin in a stage of decomposition

Leaf litter – leaves, needles, twigs, moss, lichens that are not decomposing

Several O-layers can occur in some soils, consisting only of O-horizons

A-HORIZON“Topsoil” or “Biomantle”

Horizon Topmost layer of mineral soil,

at depths of 2-10 feet Some humus present, darker in

color than layers below Biomantle - most biological

productive layer; earthworms, fungi, and bacteria live this layer

Smallest and finest soil particles

B-HORIZONThe “Subsoil” Horizon At depths of 10-30 feet Rich in clay and minerals like

Fe & Al Some organic material may

reach here through leaching Plant roots can extend into

this layer Red/brown in color due to

oxides of Fe & clay

C-HORIZONThe “Regolith”

Horizon At depths of 30-48 feet Made up of large rocks or

lumps of partially broken bedrock

Least affected by weathering and have changed the least since their origin

Devoid of organic matter due to it being so far down in the soil profile

R-HORIZONThe “Bedrock”

Horizon At depths of 48+ feet Deepest soil horizon in the

soil profile No rocks or boulders, only a

continuous mass of bedrock Colors are those of the

original rock of the area