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Visualizing Earth Science

By Z. Merali and B. F. Skinner

Chapter 4 – Weathering, Soils, and Mass Wasting

• Weathering

• Soil

Chapter Overview

• Erosion and Mass Wasting

• Resources formed by Weathering and Erosion

Weathering and Erosion• Weathering: The

breakdown of rock– By physical or chemical

means– Through exposure to air,

moisture and living thingsmoisture and living things

• Erosion– Weathering of bedrock and

its transport

• Weathering and erosion produce regolith and soil– Soil supports rooted plants

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Weathering and Erosion

Mechanical Weathering

• Breakdown of rocks– Through physical process

• Forces of stress and strain– Without chemical changes

• Proceeds throughProceeds through– Joint formation

• Cracking of rock• Widening of cracks by the

environment– Penetration by plant roots– Abrasion

• Friction caused by particles in water and wind

Mechanical Weathering

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Chemical weathering• Rocks break down

– Through chemical processes

• Dissolution– Due to rainwater

acidity• Ion Exchange

– Heavier ions replaced by hydrogen ions in acidic water

• Oxidation– Changes in minerals

in the presence of Oxygen

Chemical weathering

Chemical weathering

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Factors affecting rate of weathering • Climate

– Moisture and temperature extremes have strong effects

• Topography and Physical Setting• Organic processes• Rock Composition

Factors affecting rate of weathering

Factors affecting rate of weathering

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• Produced by weathering– Composed of variety of solid

matter• End products of weathering

– Sand, silt and clay• Hummus

Soil

Hummus– Decaying organic matter

– Air and water– Soil is unique to Earth

• A complex mixture– plays an important part in

ensuring bio-diversity

Soil

• Consists of a sequence of soil horizons or layers– With distinct characteristics

• Biologic– Hummus rich O and A

Horizons• Chemical

A and E horizons undergo

chemical leaching• Physical

– B horizon has deposits from above – zone of accumulation

– C horizon – weathered parent rock

Soil Profiles

– A and E horizons, undergo

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Soil Formation• Depends on five important factors

– Parent Material• Residual regolith

– From “native” weathered rock– Develops slowly

• Transported regolith– From “foreign” sources

• Moisture and temperature have important consequences

– In the biologic, chemical and physical development of soil profile

– More rapid development of soilprofiles

– Climate

Soil Formation

Soil Formation

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Soil Formation:Important Factors

• Living organisms– Soil micro-organisms break down

organic matter• From biological remains

– Burrowing animals aerate soils

• Topography

• Time– Soil formation slow

• Soil maturation a gradual process dependent on many factors

Topography– Slope of the land

• Influences water retention in the soil• Influences soil erosion

Erosion• Erosion processes

– Transport the products of weathering– Via flowing matter

• Erosion by water– In streams

• Rock particles are further broken down and transported

– As bed load• Moved along the stream bed

– Via saltation• In arcs buoyed and propelled

by water flow– As suspended load

• Fine particles supported in suspension by flow velocity

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Erosion

Erosion• Erosion by wind

– Wind generally moves fine particles• Sand is transported via saltation• A smaller fraction consists of

– Extremely fine particles suspended for long durations

• Erosion by iceErosion by ice– Via glacial action

• Scrape, smooth and transport rock

Erosion

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Gravity and Mass Wasting• The constant pull of gravity

– Slowly shapes land directly and indirectly

– Mass Wasting is the downslope loss of soil and rock

• Slope failure – occurs as a fall, p ,slide or slump

• Falls are sudden near vertical drops of rocks and debris

• Slides are rapid, straight downslope movements on a steep slippery surface

• Slumps are rolling movement of soil and debris – often along a rounded slope

Gravity and Mass Wasting

Gravity and Mass Wasting• Flows of regolith

– Vary between wet and dry; and fast and slow– Wet flows of saturated regolith are slurry flows

• These can be rapid or slow– Flows in regolith which is not water saturated are granular flows

• Slow granular flow are called creeps• Slow granular flow are called creeps– Most common form of mass wasting

• Rapid granular flows produce debris avalanches– Often triggered by earthquakes and volcanoes

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Gravity and Mass Wasting

Tectonics and Mass Wasting

• Mass wasting at tectonic plate boundaries– Occurs due to the effects of plate

motion• Volcanoes and mountain building

– Steep slopes facilitate slides and flows

• Earthquakes– Shaking dislodges debris

• Earth geologic activity– Ensures mass wasting does not

flatten the surface– Tectonic forces push up new regolith

to keep Earth’s surface uneven

Resources Formed by Weathering• Air, water and living organisms contribute to chemical

weathering of minerals– Chemical weathering is most effective under warm, wet tropical

conditions– Minerals broken down often flow away in solution– This action concentrates the non-soluble minerals– Clay minerals formed by chemical weathering of feldspar

• Sometimes contain high concentrations of aluminum and manganese• Chemical weathering is the chief source of insoluble laterites

– An ore containing these metals

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Resources Formed by Erosion

• Flowing water separates particles by size and density– Smaller, lighter particles are

carried longer and further by streamstream

– Heavier particles are not moved unless the flow is rapid enough

• Flowing water acts as a sieve• Concentrations of high

density particles due to water flow are called placer deposits

• Gemstones and gold are often found in placer deposits

Chapter Summary• Weathering: The Earth System at Work

– Weathering: Mechanical and Chemical– Factors affecting the rate of weathering

• Soil: The most Important Product of Weathering– Soil components and horizonsp– Soil Formation: Governing factors

• Erosion and Mass Wasting: Gravity at Work– Erosion: action of environmental factors– Mass wasting: Loss induced by gravity

• Resources Formed by Weathering and Erosion– Can be mined but at the cost of depletion and environmental degradation– Is in danger of difficult to detect contamination due to human activity– Forms geological landforms: karst topology of caves and sinkholes