Ch.12, Sec.1 - Weathering
 Weathering
- weathering is the breakup of rock due to exposure to processes that occur at
Earth’s surface
- mechanical weathering, or disintegration,
takes place when rock is split or broken
into smaller pieces of the same material
without changing its identity
- mechanical weathering is also called
“physical weathering”
Ch.12, Sec.1 - Weathering
- chemical weathering, or decomposition,
takes place when rock’s minerals are
changed into different substances
- mechanical & chemical weathering
almost always act together
Ch.12, Sec.1 - Weathering
 Mechanical Weathering
- water occupies 10% more space when it freezes, so more
pressure is applied to surrounding walls
- when water freezes in the cracks of rocks, it wedges the
rock apart
- frost wedging only occurs in areas where the temperature
fluctuates above and below freezing point
Ch.12, Sec.1 - Weathering
- frost wedging occurs on mountaintops,
in porous rocks, and also on our
highways (potholes)
Ch.12, Sec.1 - Weathering
Ch.12, Sec.1 - Weathering
Ch.12, Sec.1 - Weathering
- mechanical weathering can also
create abrasion, which is the grinding of
rocks, pebbles, and boulders against
one another
- sand is a product of abrasion as
pebbles get ground down into particles
of sand as they are carried by rivers,
streams, and ocean waves
Ch.12, Sec.1 - Weathering
- the growth of plants and the activities
of animals also contribute to the
mechanical weathering of rock
- the roots of trees, shrubs, & mosses
help to split rocks as they grow
- ants, earthworms, rabbits, woodchucks,
and other animals dig holes in soil
allowing air & water to weather bedrock
Ch.12, Sec.1 - Weathering
- the upward expansion of rocks that are formed deep underground may result in a
mechanical weathering process called exfoliation
- reduced pressure on the surface of granite can create curved joints that are parallel
to the surface – eventually they get exposed and break down over time
Half Dome (exfoliation dome) at Yosemite National Park, California
Yosemite National Park
Ch.12, Sec.1 - Weathering
Ch.12, Sec.1 - Weathering
Stone Mountain (exfoliation dome) in Georgia
Stonewall Jackson, Jefferson Davis, & Robert E. Lee
(Civil War Heroes for the Confederacy)
Ch.12, Sec.1 - Weathering
Sugarloaf Mountain (exfoliation dome) in Brazil
Sugarloaf Mountain in Rio de Janeiro, Brazil
Ch.12, Sec.1 - Weathering
 Chemical Weathering
- chemical weathering occurs almost
everywhere because water or water vapor is
found almost everywhere
- all chemical weathering involves at least one
of the following: water, water vapor, oxygen,
and acids
- the chemical weathering by reaction of water
with other substances is called hydrolysis
Ch.12, Sec.1 - Weathering
- water’s chemical effect on minerals is
increased by the presence of acids that
are dissolved in the water: carbonic
acid & sulfuric acid
- carbonic acid is created from excess
carbon dioxide (vehicles) & sulfuric acid
is created from sulfur burn-off in
manufacturing plants
- these acids mix with rain in the
atmosphere and cause acid rain, which
further increases chemical weathering
Ch.12, Sec.1 - Weathering
- carbonic acid has a dissolving effect
on limestone and can create caverns
once it mixes with water and seeps
into the ground
Perry’s Cave at Put-in-Bay, Ohio
Mammoth Cave in Kentucky
Ch.12, Sec.1 - Weathering
- the brown or red color of some exposed rocks may be the result of a process called
oxidation
- oxidation is a chemical reaction of
oxygen with other substances, especially
iron-rich rocks
Zion National Park, Springdale, Utah
Cathedral Rocks, Sedona, Arizona
Ch.12, Sec.1 - Weathering
 Rates of Weathering
- under average conditions,
weathering is a slow process
- the three factors that
increase weathering are:
surface area, composition of
the rock, and climate
- more surface area exposed
to the elements means faster
weathering of the rock
more surface area = faster mechanical/chemical weathering
Ch.12, Sec.1 - Weathering
- granite, quartz, and marble are not easily weathered, whereas gypsum, feldspar,
and limestone weather at faster rates
- warm, wet climates are conducive to
faster mechanical/chemical weathering
because of the excessive moisture,
changes in pressure, and changes in
temperature
- cold, dry climates are conducive to faster
mechanical weathering because of the
excessive winds
McMurdo Dry Valley Desert, Antarctica