Download Visualizing Earth Science Chapter Overview

2/23/2009
Visualizing Earth Science
By Z. Merali and B. F. Skinner
Chapter 4 – Weathering, Soils, and Mass Wasting
Chapter Overview
• Weathering
• Soil
• 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 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 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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Soil
• Produced by weathering
– Composed of variety of solid
matter
• End products of weathering
– Sand, silt and clay
• Hummus
– Decaying organic matter
– Air and water
– Soil is unique to Earth
• A complex mixture
– plays an important part in
ensuring bio-diversity
Soil
Soil Profiles
• Consists of a sequence of soil
horizons or layers
– With distinct characteristics
• Biologic
– Hummus rich O and A
Horizons
chemical leaching
• Physical
– B horizon has deposits from
above – zone of accumulation
– C horizon – weathered parent
rock
• Chemical
– A and E horizons
horizons, undergo
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Soil Formation
•
Depends on five important factors
– Parent Material
• Moisture and temperature have
important consequences
• Residual regolith
– From “native” weathered rock
– Develops slowly
– In the biologic, chemical and
physical development of soil
profile
• Transported regolith
– From “foreign” sources
– More rapid development of soil
profiles
– Climate
Soil Formation
Soil Formation
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Soil Formation:
Important Factors
• Living organisms
• Time
– Soil micro-organisms break down
organic matter
– Soil formation slow
• From biological remains
• Soil maturation a gradual process
dependent on many factors
– Burrowing animals aerate soils
• 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 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
p failure – occurs as a fall,,
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
– 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
stream
– 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
p
and horizons
– 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
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