Weathering and Erosion

Objective:2.1.3 I can explain how natural actions such as weathering, erosion (wind, water and gravity), and soil formation affect Earth’s surface.

What is Weathering?

• Is the breaking down and changing of rocks near Earth’s surface

• Two Types– Mechanical– Chemical

What is Mechanical Weathering?

• When physical forces break rock into smaller pieces

• Does not change the rock’s composition

• Three Ways– Frost Wedging– Unloading– Biological Activity

Mechanical Weathering: Frost Wedging

• Water enters cracks and crevices in rocks

• Water freezes expanding the cracks

• Eventually breaking rocks into pieces

Mechanical Weathering: Unloading

• The uplift and weathering of rocks overlying igneous rocks

• Why?– Pressure on igneous

rocks is reduced causing uplift

• Exfoliation: slabs of outer rock separate and break loose

Mechanical Weathering: Biological Activity

• Activities of living organisms

• Example:–Plant roots

wedge into rocks, breaking them apart

What is Chemical Weathering?

• Is the transfer of rock into one or more new compounds

Agents of Chemical Weathering• Water

– Most important agent– Picks up gases from the

atmosphere• Oxygen

– Causes oxidation of metal minerals

– Ex: rusting• Carbon Dioxide

– Combines with water in the atmosphere

– Causes acids to form like in acid rain

Chemical Weathering in Action

• Granite– Minerals turns into clay– Washed away with water

• Spheroidal Weathering– Causes the corners and

edges of rock to be rounded

Rate of WeatheringWhat affects it?

• Rock Characteristics– Mineral composition– Mineral solubility

• Climate– Temperature and

moisture– Favors high

temperatures and abundant moisture

What is Soil?

• Part of the regolith that supports the growth of plants

• Regolith– Layer of rock and

mineral fragments that cover most of Earth’s land surface

How is Soil Formed?

• Weathering of rocks that is carried away

• Factors– Parent Material– Time– Climate– Organisms– Slope

Soil Formation Factors

• Parent Material– Source of the mineral

matter in the soil

• Time– Important in all

geologic processes– The longer a soil has

been forming, the thicker it becomes

• Climate– Greatest effect on

soil formation– Influences of

temperature and precipitation affect rate, depth and type of weathering

• Organisms– Furnish organic

matter in soil

• Slope– Steep slopes often

have poorly developed soils

Characteristic of Soil: Soil Composition

• 45% mineral matter• 25% air• 25% water• 5% humus

– Decayed remains of organisms

Characteristic of Soil:Soil Texture

• Refers to the proportions of different particle sizes– Sand (large size)– Silt (feels like flour)– Clay (small size)– Loam (mix of all

three; best for plants)

Soil Texture Triangle

Percent Clay

Percent Silt

Percent Sand

Texture Type

60 20 20

20 40 40

10 20 70

Characteristic of Soil: Soil Structure

• Clumping together of the particles

Soil Types

• Pedalfer– Forest areas

• Pedocal– Drier

grasslands• Laterite

– Hot, wet, tropical climates

What is Erosion?

•Removal and transport of weathered material from one location to another

Agents of Erosion

• Running Water• Wind• Glaciers• Ocean currents

and Waves• Biological

Organisms

What happens to the Material?

• Deposition–Materials are

dropped in another location

–Final stage of erosion

How does Water Erode Soil?

•Rain uses gravity as a force to move soil down a slope

Process of Water Erosion

1. Sheet Erosion– Thin surface of water

moving soil particles2. Rills

– Tiny streams3. Gullies

– Trenches

• Each move the soil a small distance

What is a Glacier?

• Large, moving mass of ice that forms near Earth’s poles and in mountainous regions at high elevations

How do Glaciers Erode Soil?

• Scrape and gouge out large sections of landscape

• Can carry huge rocks and piles of debris over great distance

Landforms Created by Glaciers

• Glaciated Valley– U-shaped valley

• Cirques– Bowl-shaped depression

at the head of a glaciated valley, that is surrounded on three sided by steep rock walls

• Arêtes– Snaking ridge

• Horns– Pyramid-like peaks

How does Wind Erode Soil?

• Strong in areas of little vegetation cover

• Picks up and carry sediment

• These particle batter other structures

Landscapes Shaped by Wind

• Sand Dunes– Mounds or ridges of

sand– Wind also can cause

them to move• Loess

– Windblown silt that blankets the landscape

How can humans control erosion?

• Planting rows of trees called windbreakers

• Terracing hillsides• Plowing along the

contours of hills• Rotating crops

•Objective:2.1.4 I can explain the probability of and preparation for geohazards such as landslides, earthquakes, tsunamis, sinkholes, groundwater pollution, and flooding.

What is Mass Movement?

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

Triggers of Mass Movement

• Water– Heavy rain and rapid

snow melting saturate the surface

– Particles slid past one another easier

• Over-steepened Slopes– The steeper the slope,

the greater the chance for movement

• Removal of Vegetation– Roots keep soil intact– Removing plants

increase chances of movement and erosion

• Earthquakes– Dislodge rocks and

minerals

How to Classify Mass Movement?

• Classified by– Kind of material

the move– How it Moves– Speed of

movement

Types of Mass Movement

• Rock falls• Slides

– Rockslides– Landslides

• Slumps• Flows

– Mudflow– Earthflow

• Avalanches• Creep

Rock falls

•When rock or rock fragments fall freely through the air

Slides• A block of material

moves suddenly along a flat, inclined surface

• Rockslides– Include segments of

bedrock• Landslides

– Movement of relatively thin block of loose soil, rock and debris

Slumps

• Downward movement of block of material along a curve surface

Flows• Containing a large amount

of water which move downslope as a thick fluid

• Mudflow– Swiftly moving mixture of

mud and water• Earthflow

– Moves relatively slow– Carry clay-rich sediment

Avalanches

•Extremely rapid movement of Earth material or snow

Creep

• Slow, downhill movement of soil

• Only noticeable over long period time

• Slowest type of mass movement

Preventive Actions

• Dig series of tranches to divert running water

• Constructing protective fences on highways

• Retaining walls for weak slopes

• Don’t build on steep slopes