• 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

Sediment: weathered material derived from pre-existing rocks

Sedimentary rock: consolidated sediment (compacted, cemented) plus fossils

Clastic sediments seen during fieldtrip

Clastic sedimentary rocks seen during fieldtrip

insoluable

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”)

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)

in quartz sand

River sediments are consistent with the composition of the continental crust

in clay

in rust

Effect of surface area

on weathering

Climateand

Weathering

Hot and wet favors chemical

weathering

Cold and snowy favors

mechanial weathering

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.

Sea water

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Where’s the Cl come from?

Why no silica?

Mechanisms of Erosion

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

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

Mudslides: Costa Rica, June, 2000

The June 27 slide killed 10. Six were killed in the same place in 1993 mudslides.

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.

(mechanical) weatheringGlaciers

Freezing action

Freezing action

Frost wedging

Joint-controlled weathering

Roots (mechanical) weathering

Root Wedging

Chemical Weathering

Chemical Weathering

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)

Differential Weathering

Differential Weathering

Differential Weathering

Differential Weathering

Differential Weathering

Resistant cap rock

Clastic Sediments

from “clast” … little piece

Can be associated with

rivers, glaciers, wind

Clastic Sediments and Clastic Sedimentary Rocks

A. Sediments

B. Sedimentary Rocks

Energy and Depositional Environment

Coarse-Grained Sediments

Breccias Conglomerates

Tillite

Brian J. Skinner

Worldwide sediment yield of major drainage basins

Where erosion occurs

…elevationa factor

…highermore

erosion

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

Rivers and Sediments

Migrationof meanders

leads tocross-bedding

crossbed from fieldtrip

Deltas

Cross-section of Deltanote that delta grows (progrades) towards sea

Hjulstrom Curve

Hjulstrom Curve

Pebbles and cobbles

Pebbles and cobbles: hard to get moving, an hard to keep moving

Hjulstrom Curve

SandSand: easy to get moving, a fairly easy to keep moving

Hjulstrom Curve

Silt and Clay

Silt and Clay: hard to get moving, but very easy to keep moving

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.

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

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.

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).

Shale Formation

Bioclastic Limestone

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).

Coccoliths – another type of organisms with a CaCO3 test

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

Diatoms – organisms with silica tests

Radiolaria – another silica test

Chert arrowhead

Chert beds, now deformed

Near Golden Gate Bridge

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

Martin G. Miller

Evaporites in Death Valley