Download Mechanical Weathering

Weathering and Erosion
Weathering and Erosion
Weathering:
The disintegration or decomposition of rocks on the Earth's surface.
Two types: Mechanical and Chemical
Mechanical Weathering
Mechanical Weathering:
Breaks rocks by physical forces into smaller and smaller pieces, each retaining the
characteristics of the original piece.
Four important processes: Frost wedging, Unloading, Thermal expansion, and
Organic activity.
Increased surface area
More surfaces available for weathering
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Frost Wedging
Water expands 9% when frozen
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I-40
North
Carolina
Unloading
Sheeting or exfoliation
Reduction of overlying pressure causes fractures to
develop parallel to surface topography
Half Dome,
Yosemite
“Exfoliation Dome”
Table Rock, South Carolina
Thermal Expansion
Heating and cooling of rocks in very hot desert regions cause stress on the outer surface
of the rock. Mineral boundaries and stressed due to heating and cooling.
Eventually, the outer shell will crack and fall off.
Death Valley, California
Organic Activity
- Plant roots grow into rock fractures in search of water and mineral nutrients.
As roots grow, fractures widen.
- Burrowing animals move fresh material to the surface, allowing it to weather quicker
than it would undergound.
- Decaying organisms produce acids, which contribute to chemical weathering.
Chemical Weathering
- alters internal structure of minerals by removing/adding elements.
- original material changes into something stable in the surface environment.
- Water is the most important agent in chemical weathering.
- can oxidize (like rust on a fender) and make a material weak & friable.
- feldspar weathers into clay.
- combines w/CO2 to form carbonic acid: H2CO3
H2O+CO2=H2CO3
Acid Rain
>CO2= >Acidity
Acid Rain Forest Damage
Coal mines in Germany
Haze over northeastern US
Power plants &
automobiles
Phytoplankton bloom;
probably pollution
Smog layer over New York State
India
Ganges
River
Smog over China
Beijing
11oF warming in
winter months
during last 30 years
Iceland
2km retreat since 1973
All 40 of Iceland’s glaciers are retreating
Mt. Kilimanjaro, Africa
82% decrease since
1978
Ice cap will be
gone in 15 yrs.
Thinning Greenland ice cap
Thinning up to
3 feet (!) per year
Rocks rich in quartz are very resistant
Silica-poor rocks weather easily & quickly
quartz --> quartz
feldspars --> soft clay minerals
amphibole --> clay, hematite
olivine --> hematite
Green Sand Beach, Hawaii
Weathered olivine minerals,
not quartz sand
Bora Bora, Tahiti
Basalt; silica-poor
Knife-edged ridge
Monument Valley, Utah
Quartz-rich, highly-resistant
sandstone/quartzite layers
Soil Composition
Types of Soils
Soil Profile
Controls on soil formation:
1) Parent material
-source of the weathered material
- quartz-rich rocks will weather slower than an olivine-rich rock.
2) Time
- soils need time to evolve. The longer a soil has had time to form, the thicker it'll be. Soils need time to evolve before
they are washed away.
3) Climate
- most important control on the formation of soils.
- variations in temp and precip determine which process will dominate: mechanical or chemical weathering. And the rate
of weathering.
- Hot & Wet = thick layer of chemically weathered soil in same time as: Cold & Dry = thin mantle of mechanically
weathered debris.
4) Plants and Animals
- supply organic matter to soils
- bog soil is almost entirely organic, while desert soil lacks organics.
- plants supply acids which increase the weathering/soil forming process.
- microorganisms like fungi, bacteria also help
- end product of organic decay is called humus.
5) Slope
- steep slopes encourage washing away of soil, so they tend to be very thin.
- flats produce thick, dark, humus-rich soils due to the retaining of water and organic debris.
Humus
Decaying organic matter
In tropical regions, bacteria consume humus in
the soil, so the soil itself is nutrient poor.
Soil is slow to regenerate.
Central South America
9/19/2001
Tan = deforestation
Fires
Smoke
Deforestation in Brazil
Bolivia
Bolivia
Bolivia
1986
2002
South Africa
Mexico
Unprecedented fires
Global fires
African Dust
Exposed soil is prone to wind erosion
African Dust II
1991
1995
Urban Growth
Cairo
The population of the Cairo metropolitan area has increased
from less than 6 million in 1965 when the first picture was
taken, to more than 10 million in 1998 (United Nations
Population Division, World Urbanization Prospects, the 1999
revision). Population densities within the city are some of the
highest in the world and the urban area has doubled to more
than 400 square km during that period. Extraordinary rates of
population growth are expected to continue, with a predicted
population of around 14 million by 2015.
Iraq
Today, river flow into the Mesopotamian marshlands has
been cut by 20-50 percent, and the spring floods that
sustained the marshlands have been eliminated. The end
result is what was once a lush wetland environment roughly
the size of the state of New Jersey has been reduced by
about 85 percent in area to roughly the size of the small
island nation of the Bahamas. What was once a vast,
interconnected mosaic of densely-vegetated marshlands and
lakes, teeming with life, is now mostly lifeless desert and
salt-encrusted lakebeds and riverbeds.
US
Mexico
San Jose, CA
This pair of images illustrates the rapid growth that occurred
between 1973 and 1999 in San Jose, CA. From 1970 to 1996,
the population of the city of San Jose increased from 459,000
to 839,000. This is reflected by an increase in urban areas in
the above Landsat images, which show up as light blue.
Yellowstone
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The Paving Over of America
Water isn’t allowed to soak into the ground
Runoff
Winston-Salem, NC
Soil Erosion
Urban Flooding
More water, and more soil,
than the river can handle.