Transcript lec12_Weathering_and_Soils

Weathering and Soils
The Rotting Earth
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Physical weathering processes
Chemical weathering processes
Factors affecting weathering
Soil: The residue of weathering
• Why isn’t the Earth
covered with impact
craters like the Moon?
• Both planetary bodies
have experienced the
same frequency of
impact events over
geologic time
The Moon
Earth has evolved a hydrosphere
and the water cycle
Continually induces degradation
of its surface
Hydrosphere + Plate
tectonics erases evidence
Acid rain disfigured
limestone statue
Meteor Crater, AZ
50,000 years ago
Importance of Weathering:
• produces soil
• produces some important mineral deposits (Al, Fe)
• Essential part of the rock cycle
• Weathering of silicate minerals removes CO2 from
atmosphere and influences global climate
Bauxite (Al) deposit
Soil profile, Cape Cod, Ma
Essential Part of Rock Cycle
weathering
Small rock
fragments
Na+, Cl-, ions
Transport
Wind, water
Ice, gravity
Deposition of
sediment
Burial & Lithification
Sedimentary Rock
Weathering
The mechanical breakdown and chemical
alteration of rock and sediment when
exposed to the atmosphere
Weathering
The mechanical breakdown and chemical
alteration of rock and sediment when
exposed to the atmosphere
There are two (interrelated) main types of
weathering
Physical weathering
Chemical weathering
Physical Weathering
Breakdown of rock
– by physical processes
– involves no change in chemical composition
– rock is simply broken down into small
fragments
Physical Weathering
Breakdown of rock
– by physical processes
– involves no change in chemical composition
– rock is simply broken down into small
fragments
Both Physical and Chemical Weathering are
facilitated by JOINTS, FRACTURES, and
CRACKS in rocks
Joints- fractures in rocks along which no
movement has taken place
Rocks break at weak spots when they are twisted,
squeezed, or stretched by tectonic forces.
Such forces form joints.
Joints- fractures in rocks along which no
movement has taken place
Rocks adjust to removal of overlying rock by
expanding upward
cracks
Removal of the weight of overlying rocks releases
stress on the buried rock
causes joints to open, allowing water, air, and
microscopic life to enter.
Columnar Joints
Basalt
Sheet Joints in massive
granite, Yosemite National Park
Physical Weathering
• Frost Wedging
Water in crack
Freezing water expands in
cracks and wedges the
rock apart
Ice expands by 9%
increases pressure on
walls of the crack
Frost Wedging- produced Granite
blocks from bedrock (Mt. Whitney)
Physical Weathering
Crystal Growth
• Water moving slowly through fractured rocks
contains ions, which may precipitate out of
solution to form salts.
• force exerted by salt crystals growing can be
very large  break-up of rocks.
Weathering in Antarctica by Crystal Growth
Effective
in desert
regions,
both hot
and cold
Physical Weathering
Effect of Heat
Heat Spalling
Root Wedging
Physical Weathering: Small effects
acting over long geologic time
Chemical Weathering
Pressure
Magma
Igneous rock
Temperature
Chemical Weathering
Pressure
Magma
Igneous rock
Low T
Low P
High H2O
High O2
High P
High T
Surface
Condition
Temperature
Chemical Weathering
Pressure
Magma
Igneous rock
Low T
Low P
High H2O
High O2
Surface
Condition
High P
High T
Formation of new
minerals stable at
Earth’s Surface
Temperature
Chemical Weathering
Pressure
Igneous Rocks + Acids 
Sedimentary Rocks + Salty
Oceans
Low T
Low P
High H2O
High O2
Magma
Igneous rock
High P
High T
Surface
Condition
Temperature
Chemical Weathering
Main Agent
water solutions that behave as weak acids
Main Chemical Processes
Hydrolysis, oxidation, dissolution, leaching
Main Products
New minerals, ions in solution
Hydrolysis of Feldspar
Carbon Dioxide + Rain
Becomes Acid (H2CO3)
Dissolves Feldspar (H+)
Leaves ----Clay
Carries away---Ions
Silica, K+
Other Common Chemical
Weathering Reactions
Oxidation of Fe silicatesFe-oxides
Monument Valley
Other Common Chemical
Weathering Reactions
• Dissolution
All of mineral is completely dissolved
Limestone (CaCO3)+ Carbonic Acid Ca
ions +bicarbonate
Other Common Chemical
Weathering Reactions
• Oxidation
Fe-oxides
• Dissolution
All of mineral is completely dissolved
Limestone (CaCO3)+ Carbonic Acid Ca ions
+bicarbonate
• Leaching
Ions are removed by dissolution in water
K-feldspar+waterK ions
Exfoliation and Spheroidal Weathering
• concentric shells of rock may break from the
outside of an outcrop or a boulder, a process
known as exfoliation.
• Exfoliation is caused by differential stresses
within a rock that result mainly from chemical
weathering.
Spheroidal weathering produces, by
such progressive decomposition,
rounded boulders.
The effects of surface area
The effects of surface area
The effects of surface area
• r
1 cm3 of rock (surface area= 6cm2)
 clay mineral, 40 million cm2
The effects of surface area
• r
Physical/Chemical weathering  increases surface area
More physical/chemical weathering
Factors that Influence Weathering
• Rock type (composition)
High quartz content resists chemical
weathering
Think of beach sand
Granite, high quartz
Marble, calcite
Tombstones, same location, different rock types
Factors that Influence Weathering
• Texture/Structure
– Massive or jointed
rock
– Differences in
weathering rates
Differential Weathering
Mudstone layers weather fast
Factors that Influence Weathering
• Role of Time
Marble tombstones
Same location, 1970, 1870
Factors that Influence Weathering
• Climate
Moisture and heat promote chemical
reactions
Warm/moist climates  chemical weathering is more intense
Cold/dry climates
 chemical weathering is slow
The
effects of
climate
• A Tale of Two
Egyptian Obelisks
Granite, 1500 BC
One stayed in Egypt
One went to NYC
in 1880
Wetter climate
Importance of
climate
Climate and
Weathering
Products of Weathering
Soils are one of the most important
natural resources.
• Soils support plants
• Basis for the
terrestrial food chain
• Soils store organic
carbon
Rates of Soil Formation
• Typical (to make 1 m thick soil layer)
Midwest
~10,000 years
Tropics
~100’s -1000 years
Human perspective
non renewable resource
Soil Erosion
• Natural process ACCELERATED by
human activity
• Soil erosion rates ~ few years
• Global topsoil depletion ~ 7% per decade
Leading Causes of Soil Erosion
Deforestation
Overgrazing
Farming practices
urbanization
Final Thought
Global food production
25 billion tons of topsoil
is lost per year
90 million new people per
year
Soil erosion from deforestation