Weathering, Erosion & Soils
Weathering is the the
breakdown of solid rock at
or near the Earth's surface.
Weathering may be
mechanical or chemical
• Mechanical weathering is the physical
abrasion due to the action of:
– Water (Streams, Rivers and Surf)
– Ice (Frost, Snow, Glaciers)
– Wind
• Chemical weathering is the chemical
reaction of minerals with the water
and oxygen of our atmosphere.
Mechanical Weathering
• Mechanical Action of Streams and Rivers
Mechanical Weathering
• By the Action of Ice
Mechanical Weathering
• By the action of Wind
Chemical
weathering
• By oxidation,
hydration, or
biological
activity
Chemical Weathering:
Exfoliation
Chemical Weathering
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Increases with Temperature
Increases with Moisture (Rainfall)
Increases with Acidity
Decreases with Silica
Polymerization (Bowen’s Reaction
Series)
Chemical Weathering of
Igneous Rock Minerals
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Oxides
>
Hydroxides
Ferromags > Mg-Fe Clay
Feldspar > Al-Clay + Na+ + K+ + Ca++
Mica > Al-Fe Clay + Na+ + K+ + Ca++
Quartz
>
Quartz Sand
Erosion and Transport
• Erosion is the reduction of exposed
landforms.
• Transport is the movement of eroded
material down slope.
• Transport by water sorts the particles by
size.
• Because different minerals dominate
different size fractions, the deposited
material differs from the parent rock in
mineralogy and chemistry.
Weathering Products Are
Sorted by Size
• Coarse particles require moving
water or wind
– Rock grains:
– Quartz:
Pebbles, Cobbles,
Boulders
Pebbles and Sand
• Fine particles require standing
water.
– Clays
Very fine (<10 mm)
Depositional Environments
Soils
• A regolith is any accumulation of fine
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rock material on a planetary surface.
A soil is the accumulation of weathered
rock material together with organic
matter.
A pedocal is a dry-climate soil containing
soluble calcium minerals (calcite).
A pedalfer is a humid-climate soil rich in
Al and Fe.
A laterite is an extreme pedalfer in
tropical climates.
Soil Types
Laterites are Tropical Soils
Soil Changes in US
Acid Rain
• Results from burning of sulfur-rich
fossil fuels (mainly coal).
• Is mainly a problem in humid
temperate climates (NE US,
Europe).
Acid Mine Drainage
• Results from oxidation of sulfide
minerals (pyrite) in unsaturated rock.
• 2FeS2 +7H2O + 2O2 > 2Fe(OH)3 +
4H2SO4
• Pyrite + water + oxygen > limonite +
sulfuric acid
• Big Problem in CO
Acid Mine Drainage
Weathering Terms
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Chemical weathering
Mechanical weathering
Spheroidal weathering
Pedocal
Pedalfer
Laterite
Bauxite
Hydration
Oxidation
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Exfoliation
Erosion
Soil
Regolith
Humus
Acid mine
drainage