The Process of Weathering
By Trista L. Pollard
The rocks that you step on in your backyard and in front of
your house today were formed deep inside our planet. That's
right; these unimportant pebbles traveled a long way through
extreme temperature and pressure before they reached your
backyard. Now that they are on the Earth's surface, these
uplifted rocks will be exposed to the gases and water in the
atmosphere. These environmental agents change their
appearance and composition. Weathering is the process where
rocks are changed physically or chemically. Mechanical
weathering changes only the physical structure of rocks.
Chemical weathering, however, changes only the mineral composition of rock.
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Plants, animals, ice, gravity, running water, and wind are responsible for the
mechanical weathering of rocks. As rocks are changed physically, this may also
affect the process of mechanical weathering on underlying rock. When rocks are
weathered mechanically, their overlying layer is slowly removed, decreasing the
pressure on the underlying rock that forms deep beneath the surface. Once example
of this process occurs with granite. As the pressure decreases on granite, it expands
forming long curved cracks called joints. After these joints have developed on the
surface of the granite, the rock breaks into curved sheets. These curved sheets
exfoliate or peel away from the underlying rock. One area where you can see
granite exfoliation at work is Yosemite National Park.
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Mechanical weathering occurs in three major forms. Ice wedging usually
occurs in cold climates where water that seeps into rocks freezes. The freezing
water increases in volume, by about 10% causing an increase in pressure on the
surrounding rock. When the ice thaws and refreezes, the cracks within the rock
become wider and deeper. Over time, ice wedging causes the rock to split or break
apart. Rocks that are found in higher elevations also experience ice wedging. These
are the areas, along with colder climates, where the temperature regularly rises
above and falls below freezing. If you visit the northeastern United States, try to
look for evidence of ice wedging.
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As rocks and soil are moved by running water, gravity, and wind, they collide
with other rock materials. This causes the rocks to break into small pieces. The
process known as abrasion happens daily. It occurs as gravity sends loose soil and
rocks down the slopes of hills or mountains. The running water in rivers, streams,
and oceans drags particles of sand and rocks along causing them to collide with
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each other and stationary rocks. The moving and stationary rock materials become
weathered over time. The last agent of abrasion is wind. Small particles of soil,
rock, and sand are constantly lifted and carried by wind. As they are carried, the
wind hurls these particles against other rock surfaces. The wind-blown particles'
abrasive force is enough to wear away the surfaces of rocks. Although this process
occurs over time, abrasion is a powerful weathering force.
Plants, animals, and humans also cause mechanical weathering. Every time
people walk across rock paths and surfaces, the rock is slowly worn away. The
roots of plants and trees create pressure on rocks as they grow and expand. This
causes small cracks to form in the rock that will increase in size and eventually
break the rock apart. Burrowing animals cause weathering as they move soil
around when building their homes. Ground squirrels, prairie dogs, ants, and even
earthworms expose new rock surfaces as they move soil. These new rock surfaces
are now more susceptible to other agents of mechanical weathering. Over long
periods of time, these types of organic activity can change the physical appearance
of rocks dramatically. It also makes the rocks more vulnerable to chemical
weathering.
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During chemical weathering, the mineral composition of rocks is changed due
to its chemical interaction with the environment. This is also known as
decomposition. As rocks come into contact with water, carbon dioxide, oxygen,
and acids, chemical reactions act on the rocks' minerals. The hydronium ions in
water or H3O+ are the acids. Hydroxide ions or OH- are the bases in water. Once
the minerals react with either the acid or the base, the structure of the minerals
changes and new minerals may form. A rock that has gone through chemical
weathering not only has its composition changed, but its physical appearance also
changes. Rocks that have iron-bearing minerals are usually weathered through
oxidation. Oxidation occurs when elements like iron combine rapidly with
oxygen, O2, which was dissolved in water. A new material called iron oxide,
Fe2O3, or rust is formed. Soil and rocks that have a reddish color contain iron oxide
that was produced through oxidation. Reddish soil is usually found in the
southwestern United States.
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Water is the key ingredient in the chemical weathering of soil and rocks. One
type of chemical weathering called hydrolysis causes a rock's mineral composition
to change when it reacts with water. Feldspar is changed into common clay called
kaolin when it reacts with water. The potassium and calcium atoms in feldspar
crystals react with the hydronium ions in water. The clay is produced once the ions
displace the atoms in the crystal. The minerals in rocks and soil that go through
hydrolysis usually dissolve in the water. The minerals then go through the process
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of leaching. Leaching occurs when the dissolved minerals are carried by water to
lower layers of rock. Once these minerals settle in layers beneath the Earth's
surface, new deposits may form.
Carbonic acid, H2CO3, a weak acid, is produced when carbon dioxide, CO2,
from the air dissolves in water. When this carbonic acid combines with certain
rock minerals, new minerals are formed called carbonates. The process that
produces these new minerals is called carbonization. The hydrolysis process
occurs at a faster rate due to the higher concentration of hydronium ions in the
carbonic acid. As a result, the minerals are changed into carbonates that dissolve
quickly in water. Calcite, which is a major mineral found in limestone, is converted
into calcium bicarbonate when it reacts with carbonic acid. This new substance
dissolves easily in water and speeds up the weathering process of limestone.
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Other acids found naturally in living organisms also contribute to chemical
weathering. Mosses and lichens produce weak organic acids. As they grow on rock
surfaces, these weak acids erode the rock. Cracks in the rock are produced when
the acid seeps into the rock. Eventually, the rocks break apart.
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Acid precipitation is another form of chemical weathering. The small amount
of carbon dioxide in the air causes rain to be slightly acidic. However, with the
increased burning of fossil fuels like coal, higher amounts of nitrogen oxide and
sulfur dioxide are sent into the air. As the compounds combine with water in our
atmosphere, nitric acid, nitrous acid, or sulfuric acid are produced. Rain combines
with these acids and falls as acid precipitation. Rocks, especially those materials
used to build historic monuments and sculptures, are weathered more quickly.
Years of acid precipitation has caused damage to these monuments and sculptures.
In the United States, acid precipitation was a huge problem between 1940 and
1990. As a result, the Acid Rain Control Program was added to the Clean Air Act
of 1970 in 1990. This program included regulations that required power plants to
decrease their sulfur dioxide emissions within ten years. Power plants responded
by installing scrubbers that remove a majority of sulfur dioxide before it is released
into the atmosphere. Due to these scrubbers and government regulation, the
amount of acid precipitation from our atmosphere has been reduced.
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Copyright © 2013 edHelper
Name _____________________________
Date ___________________
The Process of Weathering
1. Effect: The addition of the Acid Rain 2. You are studying soil samples from a
Control Program to the Clean Air
Act of 1970 in 1990. What is the
cause?
3. There is an increased population of
ground squirrels in your town's park.
How will this event affect
mechanical weathering in the area?
5. What are the common agents of
mechanical weathering?
7. How would frequent rock slides
affect the weathering rate of
surrounding rocks?
particular area when you notice a
reddish color present in the soil.
Explain the weathering process that
contributed to the soil's change in
color.
4. Hydrolysis causes ______ when they
react with water.
Minerals to dissolve and move
into lower layers of soil
Minerals to combine with
carbonic acid to form carbonates
Minerals to combine with weak
organic acids and slowly wear away
rock
None of the above
6. True or False: Ice wedging occurs
when rocks carried by wind or water
collide with other rocks wearing
away their surfaces.
8. What is leaching?