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• Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s surface
• Erosion – the physical removal of material by agents such as water, wind, ice, or gravity
Weathering and ErosionFormation of Sedimentary Rocks
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Sediment: weathered material derived from pre-existing rocks
Sedimentary rock: consolidated sediment (compacted, cemented) plus fossils
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Clastic sediments seen during fieldtrip
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Clastic sedimentary rocks seen during fieldtrip
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insoluable
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basalt(Mg,Fe)2SiO4 (Mg,Fe)SiO3 pyroxine
H4SiO4 in solution
Mg2+ in solution
Fe (III) hydroxide (insoluble, rust)
CaAl2Si2O8 Ca-feldspar and NaAlSi3O8 Na-Feldspar
Ca+2 in solution
Na+1 in solution
Al2Si2O5(OH)4 (insoluble, “clay”)
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graniteSiO2 quartz
SiO2 (insoluble, “sand”)
CaAl2Si2O8 Ca-feldspar; NaAlSi3O8 Na-Feldspar KAlSi3O8 K-Feldspar
Ca+2 , Na+1, K+1 in solution
Al2Si2O5(OH)4 (insoluble, “clay”)
(Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH)2 amphibole (and also mica)
Mg+2, Ca+2 , Na+1 in solution
Al2Si2O5(OH)4 (insoluble, “clay”)
Fe (III) hydroxide (insoluble, rust)
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in quartz sand
River sediments are consistent with the composition of the continental crust
in clay
in rust
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Effect of surface area
on weathering
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Climateand
Weathering
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Hot and wet favors chemical
weathering
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Cold and snowy favors
mechanial weathering
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Why erosion is important
• Life in the sea depends on it to supply critical nutrients.
• It is responsible for the salt content of the sea.
• It is the source of the basic materials to form sedimentary rocks.
• It continually reduces and shapes the surface of the land.
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Sea water
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Where’s the Cl come from?
Why no silica?
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Mechanisms of Erosion
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Mass Wasting
The often catastrophic (geologically rapid)
movement of material on the Earth’s surface is
referred to as mass wasting.
Such features constitute the most widespread of the natural geological
hazards.
the Slumgullion earthflow/landslide, Hinsdale County, CO
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It began high in the mountains as an
earthquake-induced avalanche of snow
and ice, but picked up glacial sediment on
its way.
It hit the towns of Yungay and
Ranrahirca, 18 km away, at around 150
km/hr. The former town was completely
buried.
An astounding 66,000 people were
estimated dead as a result of this massive
debris avalanche.
Nevado Huascaran, Peru, Nevado Huascaran, Peru, 19701970
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Mudslides: Costa Rica, June, 2000
The June 27 slide killed 10. Six were killed in the same place in 1993 mudslides.
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landslides
This is a typical landslide.
Note that materials hold
together in more-or-less
singular blocks.
As is common, when the slide
blocks reach a lower slope,
they break up (becoming a
debris flow in this case).
The La Conchita landslide, near Santa Barbara, CA, Spring, 1995.
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(mechanical) weatheringGlaciers
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Freezing action
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Freezing action
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Frost wedging
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Joint-controlled weathering
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Roots (mechanical) weathering
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Root Wedging
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Chemical Weathering
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Chemical Weathering
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Differential Weathering and Erosion
creates topography
Slowly weathered and eroded - high
(Morningside Heights, Palisades, Ramapo Mountains)
Quickly weathered and eroded - low
(sediments beneath Hudson River and west of Palisades)
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Differential Weathering
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Differential Weathering
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Differential Weathering
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Differential Weathering
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Differential Weathering
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Resistant cap rock
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Clastic Sediments
from “clast” … little piece
Can be associated with
rivers, glaciers, wind
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Clastic Sediments and Clastic Sedimentary Rocks
A. Sediments
B. Sedimentary Rocks
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Energy and Depositional Environment
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Coarse-Grained Sediments
Breccias Conglomerates
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Tillite
Brian J. Skinner
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Worldwide sediment yield of major drainage basins
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Where erosion occurs
…elevationa factor
…highermore
erosion
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Meaning of rate100 tons per sq km per year
100 tons
Rock density about 2.5 tons per cubic meter
so 100 tons is about 40 cubic meters (a cube 3.4 m on edge)
1 sq km is 1,000,000 sq meters
Spread 40 cu meters over 1,000,000 sq meters and get a layer 40/1,000,000 = 0.00004 meters = 0.04 millimeters thick
So in 100 years, wear away 4 mm1000 meters
1000 m
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Rivers and Sediments
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Migrationof meanders
leads tocross-bedding
crossbed from fieldtrip
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Deltas
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Cross-section of Deltanote that delta grows (progrades) towards sea
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Hjulstrom Curve
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Hjulstrom Curve
Pebbles and cobbles
Pebbles and cobbles: hard to get moving, an hard to keep moving
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Hjulstrom Curve
SandSand: easy to get moving, a fairly easy to keep moving
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Hjulstrom Curve
Silt and Clay
Silt and Clay: hard to get moving, but very easy to keep moving
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Human Influence
• Human beings move more sediment through mining and building than rivers do.
• Agricultural practices also increase erosion in rivers. Rates thousands of years ago were less than today.
• Empounded water from lakes behind dams also prevent sediment from reaching the sea.
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Existence of Lakes
• Largely due to glaciation disrupting drainage networks.
• North American lakes mostly at latitudes greater than 45°, where glaciers were during Pleistocene.
• Will slowly fill up with sediment and disappear
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Removal from sea water • Particles settle and are deposited as sediment.
– Sedimentary rocks consisting of fine-grained particles (< .06 mm) are called shale.
– Sedimentary rocks made of medium-grained particles (.06-2 mm) are called sandstone.
– Sedimentary rocks consisting of coarse-grained particles (> 2 mm) are called. conglomerates.
• Calcium and bicarbonate are removed from seawater by organisms to form shells made of CaCO3 (limestone).
• SiO4 is removed from solution by organisms to form silica “ooze” which lithifies to chert (arrowheads).
• Mg++ and SO4-- are removed from ocean water as that water is heated and pumped through mid-oceanic ridges.
• Na+, K+, and Cl- are removed from seawater sporadically in evaporite deposits.
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Formation of sedimentary rocksCementation of grains by CaCO3 or SiO2.
• Names of clastic rocks depend on size of grains (big to small). – Conglomerates - tend to be poorly sorted. – Sandstones - often well-sorted. – Shales - made of clay minerals.
• Non-clastic rocks: from dissolved load– Limestones - CaCO3, usually removed from water by a biological
process (e.g. corals and sea-shells). – Evaporites - NaCl and CaSO4 from evaporation of seawater in enclosed
basins (e.g. Utah's Great Salt Lake and the Mid-East's Dead Sea).
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Shale Formation
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Bioclastic Limestone
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Fine-Grained Limestone
Deep sea limestones will not
have visible fossils, because
they are made up of the shells of
little dead bugs like these:
foraminifera.
This specimen
(viewed by scanning electron
microscope) is about
100 microns long
(0.1 millimeter).
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Coccoliths – another type of organisms with a CaCO3 test
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Chert
Some microorganisms secrete
silica shells.
When these pile up on the deep
ocean floor, they lithify to
become a micro-crystalline
quartz rock called chert (the
same stuff as the substance
flint).
diatoms and radiolaria
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Diatoms – organisms with silica tests
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Radiolaria – another silica test
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Chert arrowhead
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Chert beds, now deformed
Near Golden Gate Bridge
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Evaporites
Owens Valley, CA
Calcite, halite and gypsum are
common precipitates from
dried lakes in arid
environments.
the white is halite, the red are bacteria that love salt
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Martin G. Miller
Evaporites in Death Valley
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