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Chapter Three:   Weathering, Soil, and Mass Wasting

Chapter Three Learning Objectives:

Describe the processes of weathering, erosion, and mass wasting.

Explain the difference between mechanical and chemical weathering.

Discuss soil composition, texture, structure, formation, and classification

List and describe the various types of mass wasting.

Chapter Three Outline:

Earth's external processes include
Weathering - the disintegration and decomposition of material at or near the surface

Mass wasting- the transfer of rock material downslope under the influence of gravity

Erosion-the incorporation and transportation of material by a mobile agent, usually water, wind, or ice

Two kinds of weathering

Mechanical weathering

Breaking of rocks into smaller pieces

Four processes

Frost wedging

Unloading

Thermal expansion

Biological activity

Chemical weathering

Alters the internal structures of minerals by moving or adding elements

Most important agent is water

Oxygen dissolved in water oxidizes materials

Carbon dioxide dissolved in water forms carbonic acid and alters the material

Weathering of granite

Weathering of K-feldspar produces

Clay minerals

Potassium bicarbonate

Silica in solution

Weathering of silicate minerals produces

Soluble sodium, calcium, potassium and magnesium products

Insoluble iron oxides

Clay minerals

Rates of weathering
Advanced mechanical weathering aids chemical weathering by increasing the surface area

Rate of Weathering continued

Other important factors are

Mineral makeup

Marble (calcite) readily dissolves in weakly acidic solutions

Silicate minerals weather in the same order as their order of crystallization

Climate

Temperature and moisture are the most crucial factors

Chemical weathering is most effective in areas of warm temperatures and abundant moisture

Soil

Soil texture and structure

Texture

Refers to the proportions of different particle sizes

Sand (large size)

Silt

Clay (small size)

Loam is best suited for plant life

Structure

Soil particles clump together to give a soil its structure

Four basic soil structures

Platy

Prismatic

Blocky

Spheroidal

 


Controls of soil formation

Parent material

Residual soil-parent material is the bedrock

Transported soil-parent material has been carried from elsewhere and deposited

Time

Important in all geologic processes

Amount of time to evolve varies for different soils

Climate

Plants and animals

Organisms influence the soil's physical and chemical properties

Furnish organic matter to soil

Slope

Steep slope-often poor soils

Optimum is a flat-to-undulating upland surface

 

Soil Profile

Soil forming processes operate from the surface downward

Horizons - zones of layers of soil

Horizons in temperate regions

O - organic matter

A - organic and mineral matter

E - little organic matter

B - zone of accumulation

C - partially altered parent material

O and A together called topsoil

O, A, E, and B together called solum, or "true soil"

Soil Types

Hundreds of soil types worldwide

Three very generic types

Pedalfer

Accumulation of iron oxides and Al - rich clays in the B horizon

Best developed under forest vegetation

Pedocal

Accumulate calcium carbonate

Associated with drier grasslands

Laterite

Hot, wet, tropical climates

Intense chemical weathering

 


Soil Erosion

Recycling of Earth materials

Natural rates of erosion depend on

Soil characteristics

Climate

Slope

Type of vegetation

 

Weathering creates ore deposits

Process called secondary enrichment

Concentrates metals into economical deposits

Two ways of enrichment

Removing undesired material from the decomposing rock, leaving the desired elements behind

Desired elements are carried to lower zones and deposited

Examples

Bauxite, the principal ore of aluminum

Many copper and silver deposits

 

Mass Wasting

The downslope movement of rock, regolith, and soil under the direct influence of gravity

 

Gravity is the controlling force

Important triggering factors are

Saturation of the material with water

Destroys particle cohesion

Water adds weight

Oversteepening of slopes

Stable slope angle is different for various materials

Oversteepened slopes are unstable

Removal of anchoring vegetation

Ground vibrations from earthquakes

 

Types of mass wasting

Generally each type is defined by

The material involved

Debris

Mud

Earth

Rock

 


The movement of the material

Fall ( free-fall of pieces)

Slide (material moves along a surface)

Flow ( materials moves as a viscous fluid)

The velocity of the movement

Fast

Slow

 


Forms of mass wasting

Slump

Rapid

Movement along a curved surface

Along oversteepened slopes

Rockslide

Rapid

Blocks of bedrock move down a slope

Mudflow

Rapid

Flow of debris with water

Often confined to channels

Serious problem in dry areas with heavy rains

Mudflows on the slopes of volcanoes are called lahars

 

Forms of Mass Wasting

Earthflow

Rapid

On hillsides in humid regions

Water saturates the soil

Creep - the slow movements of soil and regolith downhill

Solifluction

Slow

In areas underlain by permafrost

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