Chapter Weathering, Soil, andMass Movements

5.1 Weathering

Mechanical weathering occurswhen physical forces break rockinto smaller and smaller pieces

Mechanical Weathering

into smaller and smaller pieceswithout changing the rock’smineral composition.

In nature three physical processare especially important causes ofweathering: frost wedging,unloading, and biological activity.

Increase in Surface Area byMechanical Weathering

5.1 Weathering

1. Frost wedgingMechanical Weathering

•The mechanical breakup ofrock caused by the

•Sections of rock that are wedgedloose may tumble into large pilescalled talus, which typically form atthe base of steep, rocky cliffs.

rock caused by theexpansion of freezing waterin cracks and crevices

Frost Wedging

5.1 Weathering

2. UnloadingMechanical Weathering

•Reduced pressure on igneous rockcauses it to expand and allowsslabs of outer rock to break off inslabs of outer rock to break off inlayers in a process calledexfoliation.

Unloading and Exfoliationof Igneous Rocks

5.1 Weathering

Mechanical Weathering

3. Biological activity

•The activity of organisms, includingplants, burrowing animals, andhumans, can also causemechanical weathering.

Weathering andBiological Activity

5.1 Weathering

ChemicalWeathering

Chemical weathering is thetransformation of rock into one ormore new compounds.

5.1 Weathering

• Water• Water is the most importantagent of chemical weathering. Itpromotes chemical weathering by

Chemical Weathering

agent of chemical weathering. Itpromotes chemical weathering byabsorbing gases from theatmosphere and the ground.These dissolved substances thenchemically react with variousminerals.

5.1 Weathering

Chemical Weathering of Granite

• Weathering of potassium feldsparproduces clay minerals, solubleproduces clay minerals, solublesalt (potassium bicarbonate), andsilica in solution.

• Quartz remains substantiallyunaltered.

5.1 Weathering

Weathering of Silicate Minerals

• Produces insoluble iron oxides andclay mineralsclay minerals

Spheroidal Weathering• Causes the corners and edges ofrock to be more rounded

SpheroidalWeathering

5.1 Weathering

Two other factorsaffecting the rate of

Rate of Weathering

affecting the rate ofweathering are rockcharacteristics andclimate.

5.1 Weathering

1. Rock characteristics• Mineral composition and solubility

• Physical features such as cracks

2. Climate• Temperature and moisture are the

most crucial factors.• Chemical weathering is most

effective in areas with hightemperatures and abundant moisture.

• Physical features such as cracks

5.1 Weathering

Differential Weathering

• Caused by variations in mineralcomposition and variation in the

Rate of Weathering

• Creates unusual and spectacularrock formations and landforms

composition and variation in thenumber and spacing of cracks indifferent parts of a rock mass.

5.2 Soil

Soil is part of the regolith thatsupports the growth of plants.

Characteristics of Soil

supports the growth of plants.

• Regolith is the layer of rock andmineral fragments that covers mostof Earth’s land surface.

5.2 Soil Soil Composition

• Soil has four major components:

1. Mineral matter - broken-down rock2. Humus - the decayed remains of

organisms2. Humus - the decayed remains of

organisms3. Water4. Air.

Composition by Volume ofGood-Quality Soil

5.2 Soil

Soil Texture• Texture refers to the proportions

of different particle sizes.- Sand (large size)

Characteristics of Soil

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

of different particle sizes.- Sand (large size)- Silt- Clay (small size)

SoilTexture

5.2 Soil

Soil Structure• Soil particles clump together togive a soil its structure.

Characteristics of Soil

give a soil its structure.•Soil structure determines howeasily a soil can be cultivated andhow susceptible it is to erosion.

5.2 Soil

The most important factorsin soil formation are parentmaterial, time, climate,

Soil Formation

material, time, climate,organisms, and slope. Allwork together in soilformation.

5.2 Soil

1. Parent material

• Residual soil—parent material isthe bedrock

Soil Formation

• Transported soil—parent materialhas been carried from elsewhereand deposited

the bedrock

Parent Material and Soils

5.2 Soil

2. Time

• The longer a soil has been forming,the thicker it becomes.

• Important in all geologic processes

Soil Formation

the thicker it becomes.

3. Climate• Greatest effect on soil formation•Variations in temperature andprecipitation influence the rate,depth, and type of weathering.

5.2 Soil

4. Organisms

• Furnish organic matter to soil

• Organisms influence the soil'sphysical and chemical properties.

• Furnish organic matter to soil5. Slope

• Angle- Steep slopes often have poorlydeveloped soils.- Optimum slope is a flat-to-undulating upland surface.

5.2 Soil

5. Slope

• Orientation, or direction the

Soil Formation

• Orientation, or direction theslope is facing, influences soilformation.

- Soil temperature

- Moisture

5.2 Soil

Soil varies in composition,texture, structure, and color atdifferent depths. Soil horizons

The Soil Profile

different depths. Soil horizonsare zones or layers of soil. Asoil profile is a verticalsection through all the soilhorizons.

5.2 Soil

• The A horizon is commonly know astopsoil.

• The B horizon is subsoil and

The Soil Profile

• The B horizon is subsoil andcontains clay particles washed outfrom the A horizon.

• The C horizon is between B horizonand unaltered parent material.

Soil Profile

A Soil Profile ShowingDifferent Horizons

5.2 Soil

Three common types of soil arepedalfer, pedocal, and laterite.

1. Pedalfer

Soil Types

1. Pedalfer• Best developed under forestvegetation

• Accumulation of iron oxidesand aluminum-rich clays in the Bhorizon giving it a brown to red-brown color.

5.2 Soil

2. Pedocal

• Accumulatescalcium carbonatecalcium carbonate

• Associated withdrier grasslands

•Less clay thanpedalfer

•Light gray-brownin color.

5.2 Soil

3. Laterite

• Hot, wet, tropical climates• Intense chemical weathering• Intense chemical weathering•Deeper than others but nutrientpoor

•Water filters out calcite and silicaleaving iron oxide which makesthe soils orange or red.

5.2 Soil

• Human activities that remove naturalvegetation, such as farming, logging,and construction, have greatly

Water erodes soil. Rates of Erosion

Soil Erosion

and construction, have greatlyaccelerated erosion.

• Wind erodes soil much more slowly thanwater except in times of drought - DustBowl of the 1930’s.

• Rate of soil erosion depends on soilcharacteristics and on factors such asclimate, slope, and type of vegetation.

5.2 Soil

Sediment Deposition• Reservoirs fill with sediment.• Sediments are contaminated bypesticides and fertilizers.

Soil Erosion

pesticides and fertilizers.•Excessive nutrients in lakes andponds can lead to eutrophication(water pollution caused byexcessive plant nutrients) anddeath of the lake or pond

5.2 Soil

• Planting rows of trees calledwindbreaks

Controlling Erosion

Soil Erosion

windbreaks•Terracing hillsides

•Plowing along the contours of hills

•Rotating crops

5.3 Mass Movements

The transfer of rock and soildownslope due to gravity is calledmass movement.

Triggers of Mass Movements

Among the factors that commonlytrigger mass movements aresaturation of surface materialswith water, oversteepening ofslopes, removal of vegetation, andearthquakes.

5.3 Mass Movements

Geologists classify massmovements based on the kind ofmaterial that moves, how it moves,and the speed of movement.

Types of Mass Movements

and the speed of movement.

Rockfalls• A rockfall occurs when rocks orrocks fragments fall freely throughthe air.

5.3 Mass Movements

Slides

• In a slide, ablock of

• Slides that include segments ofbedrock are called rockslides.

block ofmaterial movessuddenly alonga flat, inclinedsurface.

5.3 Mass Movements

Slumps

• A slump is the downwardmovement of a block of materialalong a curved surface.along a curved surface.

Heavy Rains Can Trigger Slumps

5.3 Mass Movements

Flows

• Flows are massmovements ofmovements ofmaterialcontaining alarge amount ofwater.

5.3 Mass Movements

• Mudflows move quickly and carrya mixture of soil, rock, and waterthat has a consistency ofwet concrete.wet concrete.

• Earthflows move relatively slowlyand carry clay-rich sediment.

5.3 Mass Movements

Creep• Creep is the slow,downhill movementof soil and regolith.of soil and regolith.

Creep