Post on 10-Jul-2020
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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