Download Ch.12, Sec.1 - Weathering

Learning Objectives
 I can describe how mechanical weathering breaks down rocks
 I can describe how chemical weathering breaks down rocks
 I can summarize three factors that affect weathering rates
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 ironrich 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
Exit Slip
What are the three factors that greatly
increase the rates of weathering on
Earth?