Ch 5 ppt fall 2012

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Weathering, Soil, and Mass Movement

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What is Weathering?What is Weathering?The breaking down and changing of rocks near Earth’s surface

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Mechanical (Physical)Mechanical (Physical)Weathering?Weathering?

Physical forces break rock into smaller pieces without changing the rock’s composition.

Mechanical Weathering-Mechanical Weathering-In nature there are 3 main causes to mechanical weathering:Frost wedging, unloading, and biological activity

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Frost WedgingFrost WedgingExpansion of freezing

water in cracks and crevices

Eventually breaking rocks into pieces

• Sections of rock that are wedged loose may tumble into large piles called talus

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UnloadingUnloading• Large masses of

igneous rock may be exposed through uplift and erosion of overlying rocks

• Resulting in reduced pressure

Exfoliation—slabs of outer rock separate and break loose

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Biological ActivityBiological Activity• The activity of plants- roots

breaking rock

• burrowing animals-moving rocks to places where weathering more rapidly occurs, and humans

• Decaying organisms- producing acids

• Humans- because of deforestation, creation of roads, and blasting mineral

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Chemical Chemical Weathering?Weathering?

the transformation of rock into one or more new compounds

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Oxygen, water , and carbon dioxide are in chemical weather

Water: most important agent of weathering. It picks up CO2 and SO2 in the air which forms acid rain.

Or dissolves these compounds into cracks of rocks

Agents and Evidence of Chemical Agents and Evidence of Chemical WeatheringWeathering

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Rate of WeatheringRate of WeatheringHow fast the rocks

erodeTwo factors• Rock characteristics

• Mineral composition and solubility

• Climate• Temperature and

moisture• Favors high

temperature and abundant moisture

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SoilSoilpart of theregolith that supportsthe growth of plants.

Regolith: layer of rock and mineral fragments that covers Earth’s land surface.

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How is Soil Formed?How is Soil Formed?

By rock being eroded away.

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Characteristics of Characteristics of SoilSoil

Soil CompositionSoil has four major

components◦ A. Mineral matter

(broken-down rock) ◦ B. Humus (decayed

remains of organisms) ◦ C. Water◦ D. Air

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Characteristics of Characteristics of Soil Soil

Texture refers to the proportions of different particle sizes.

A. Sand (large size)

B. Silt – feels like flour

C. Clay (small size)

D. Loam (a mixture of all three sizes) is best suited for plant life.

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Characteristics of SoilCharacteristics of Soil

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Time, Climate, Time, Climate, Organisms and SlopeOrganisms and Slope

Time: The longer a soil has been forming, the thicker

it becomes.Climate Greatest effect on soil formationOrganisms Furnish organic matter to soilSlope Steep slopes often have poorly developed soils.

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Soil Types Location Contents

Pedalfer under forest vegetation

iron oxides and aluminum-rich clays in the B

horizon

Pedocal drier grasslands Calcium carbonate

Laterite Hot, wet, tropical climates

Intense chemical weathering to the top layers of soil

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How does How does water erode the water erode the

soil?soil?Rain uses gravity as

a force to move soil.Flash flooding and

runoff of water causes erosion

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Processes of Water Processes of Water ErosionErosion

1. Sheet erosion - 2. Rills (tiny streams)3. Gullies (trenches)4. Transports sediments for

deposits.

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What is Mass What is Mass Movement?Movement?

The transfer of rock and soil down-slope due to gravity

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Triggers of Mass MovementTriggers of Mass MovementWater Heavy rains and rapid melting

of snow

Oversteepened Slopes The steeper the slope the greater the chance for movement

Removal of vegetation

Roots keep the soil intact. Removing plants causes erosion

Earthquakes Aftershocks dislodges rocks

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Classification of Classification of Mass MovementsMass MovementsRockfalls occurs

when rocks or rocks fragments fall freely through the air.

Avalanchesextremely rapid

movement of earth material or snow

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SlidesA block of material moves

suddenly along a flat, inclined surface.Slides that include segments of bedrock are called rockslides.

Slumps The downward

movement of a block of material along a curved surface.

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Flows contain a large

amount of water. Mudflows move

quickly and carry a mixture of soil, rock, and water

Earthflows move relatively slowly and carryclay-rich sediment.

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Creep is the slow, downhill movement of soil and regolith.