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Name: _____________________________________Date: __________________Period: ______
Unit 9: Earth’s Destructive Forces
Introduction to Weathering
A. Kinds of Weathering
Objectives:
Distinguish between two major processes that change the Earth surface.
Identify two types of weathering.
Two major processes are causing the earth to continue change billions of years after its formation. The
tectonic processes that we previously discussed are constructional forces that build up the earth’s crust.
Destructional forces wear down the earth’s surface. These are weathering and erosion processes that
are part of the rock cycle. In most instances, these processes are noticeable only after long periods of
time.
Weathering is the process by which the rock and minerals of the earth’s crust are broken down by
mechanical and chemical means.
Weathering is often used interchangeably with the term erosion, but they are NOT the same thing. In
weathering, materials are not transported from one place to another.
B. Mechanical Weathering
Objectives:
Describe the climate most effective for mechanical weathering.
Identify and describe the five agents of mechanical weathering.
Mechanical Weathering, also known as physical weathering, consists of the breaking up of rocks
without changing their chemical composition. During mechanical weathering, only the size and/or
shape of the rock changes.
The different factors that cause mechanical weathering are known as agents of mechanical weathering.
Temperature changes, freezing and thawing (frost action), exfoliation (peeling of rocks in layers like
onion skins peel), and the action of plants and animals (biological factors) are important agents of
weathering. Mechanical weathering is most effective in regions where the climate is dry and the daily
seasonal temperature ranges vary greatly.
Rocks are composed of mixtures of different minerals. Each has a different rate of expansion and
contraction. The effects of daily and seasonal temperature changes cause the individual minerals in a
rock to expand and contract and work themselves loose. Eventually, the minerals fall away from the
rock, causing the rock to crumble or fragment. Intrusive igneous rocks, which have large mineral
crystals, are most effectively weathered by temperature changes.
Test Yourself: Circle the right choice.
1. During the day rocks (expand, contract).
2. At night, the rocks (expand, contract).
3. Expansion and contraction can strain rocks. Strain can (cool, crack) rocks.
4. Changes in temperature can cause (mechanical, chemical) weathering.
Frost action, also known as ice or frost wedging, is a very effective agent of mechanical weathering.
When water freezes, it expands by almost 10% as water molecules organize themselves.
When water freezes and expands, it exerts a great deal of pressure on all
sides. Rainwater running into cracks in a rock may collect and then turn
to ice as temperatures go below freezing. The expanding ice exerts
enough pressure to increase the size of the crack. With repeated freezing
and thawing, the crack can get big enough to cause a piece of the rock to
break off. Areas where bedrock is exposed on the surface are effectively
weathered by frost action.
Test Yourself: Circle the right choice.
1. Water (does, does not) seep into cracks easily.
2. Water (expands, contracts) when it freezes.
3. Ice presses against the cracks in rocks. Pressure can (contract, crack) the rock.
4. Frost action is an example of (chemical, mechanical) weathering.
Exfoliation is the peeling or scaling of the outer layers of rocks. This may occur as a result of changes
in temperature or pressure. Rocks will expand and contract with temperature changes. Only a thin
outer layer of the rock is greatly affected by the heat of the sun. When the temperature drops, the rock
outer layer cools and contracts. Repeated expansion and contraction weakens the layer and eventually
the surface layer peels off from the rock.
Exfoliation may also occur where the pressure of overlying rock is decreases sufficiently. An intrusive
rock that has had the overlying layers of rock and sediment eroded experiences a reduction in pressure.
The intrusive rock may expand enough to loosen the outer layer. This layer eventually breaks away.
Abrasion or scraping occurs when rocks bump against each other. Rocks moving in water chip away
at other rocks in the riverbed. Sand carried by wind can wear away the rocks in its path (like
sandblasting).
Biological agents play a role in mechanical weathering. Sometimes tree roots or other plants will
wedge themselves into rocks. As the plants grow, so do the roots, and the increase in pressure from this
growth can split the rock (root action). Animals can also burrow, providing passages for water and air.
This process aids in chemical weathering.
Fill in the blanks:
1. Mechanical weathering is most effective in a ___________________________ climate.
2. Exfoliation occurs when there is a sudden ___________________ in pressure on a rock.
3. River rocks are often rounded because of the __________________ that occurs as they scrap
against the riverbed.
4. The roots of trees can cause weathering by creating ___________________ on the rocks.
C. Chemical Weathering:
Objectives:
Describe the climate most effective for chemical weathering.
Identify and describe the four agents of chemical weathering.
Chemical weathering, also known as decomposition, is the breakdown of rocks and minerals by
altering their chemical composition. Oxidation, hydrolysis, carbonation, and acid action are important
agents of chemical weathering. Chemical weathering is more effective in warm, humid or rainy
regions.
Oxidation is the process by which oxygen in the air combines with other materials to form new
compounds. An example would be oxygen combining with iron in a rock to form rust. The rock’s color
would change as a result.
Hydrolysis occurs when water reacts with a mineral to form a new substance. An example is water
reacting with the mineral feldspar to form clay, another mineral.
Dissolution: Some minerals in rocks can dissolve in water.
Acids from plant roots and decaying plant matter can also break down rocks.
Carbonation is the dissolving of carbon dioxide in water, producing a weak acid known as carbonic
acid. Your soda is carbonated. Carbon dioxide in the air mixes with water in the atmosphere making
rain slightly acidic. When this water interacts with rocks that contain the mineral calcite, such as
limestone and marble, the rocks can dissolve.
Carbonation makes normal rainwater acidic with a pH of 5.6. When the pH of rainwater or other forms
of precipitation are lower than this, scientists call it acid precipitation. Many areas throughout the
world are experiencing acid precipitation as a result of the burning of fossil fuels. Of the fossil fuels,
the use of coal is the most important because burning it produces the gases SO2 and NOx. These gases
mix with rainwater, like carbon dioxide does, to produce acids that lower the pH of rainwater further.
Acid precipitation is a major environmental concern.
Fill in the blanks:
1. Chemical weathering is most effective in a _______________________________ region.
2. When _____________________________ is dissolved in water, carbonic acid is formed.
3. Oxidation occurs when _________________ interacts with minerals in the rock.
4. Hydrolysis occurs when _________________ reacts with a mineral to form a new substance.
5. The burning of _________________ causes precipitation to have a lower _______ than normal.
In the space provided, write TRUE if the statement is true. If the statement is false, change
the underlined word to make the statement correct and write the correct word on the line.
______________________1. Volcanoes and mountain building are destructive forces.
______________________ 2. The breaking up of a large rock into smaller pieces is an example
of chemical weathering.
______________________ 3. The breaking down of minerals by altering their chemical
composition is an example of chemical weathering.
______________________ 4. Carbonation is the peeling of the outer layers of rock.
______________________ 5. The chemical process in which oxygen combines with other
materials to form new compounds is rust.
______________________ 6. Exfoliation is a process of chemical weathering.
______________________ 7. Weathering and erosion are destructive forces.
______________________ 8. Frost action usually occurs in desert regions.
______________________ 9. The chemical weathering where water reacts with feldspar to
produce clay is hydrolysis.
______________________ 10. The burning of fossil fuels raises the pH of rainwater.
Use the information in the reading to fill in the chart below.
DESTRUCTIVE FORCES OF WEATHERING
Type of
Weathering
Agents
1. _______________
Action/Process (describe what
happens)
Results
1.
Forms
boulders,
pebbles, and
sand
2. _______________
2.
________________ 3. _______________
3.
4. _______________
4.
5. _______________
5.
1. _______________
1.
Fine-textured
soil
2. _______________
2.
Feldspar
crumbles
3. _______________
3.
_______________
4. _______________
4.
5. _______________
5.
Limestone
caves and
caverns