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Chapter 23 - Weathering and Erosion
Weathering, erosion, and deposition act together in a cycle that wears down and builds up the Earth’s
surface.
Weathering – wears down the Earth’s surface.
Erosion – moves the Earth’s surface (acts as a bridge between the weathering and deposition.
Deposition – builds up the Earth’s surface.
Weathering
 The breaking down and/or wearing away of Earth materials (rocks) into smaller pieces.
 Must occur before erosion can happen.
 No movement is involved.
 May occur quickly or happen slowly.
Physical/Mechanical weathering…
The breaking down of rocks into smaller pieces (wind, flowing water, ice, gravity, animals/plants,
temperature changes.) There is no change to the chemical properties of the rock.
 Example - The steep walls of the Grand Canyon were carved out by moving water in the
Colorado River.
Mechanical weathering can be caused by freezing and thawing, release of pressure, growth of plants,
and actions of animals (including humans.)
a) Freezing and Thawing
 Example – water seeps into a crack in a rock. When the water freezes, it expands. The
force of the water expanding causes the rock to crack more. This is called ice wedging.
b) Plant Growth
 Example - Roots of trees and other plants enter into cracks of rocks. As the plants get
bigger, so do the roots. The growing roots force the cracks further apart.
c) Abrasion
 Example – Abrasion is the grinding away of rock by other rock particles. These particles
scrap away at other rocks like sandpaper on wood as they rub or roll against the surface.
d) Exfoliation – warming and cooling of rocks over time causes the rocks to peel into sheets, which
is also an example of physical weathering.
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Chemical weathering…
Occurs when chemical reactions dissolve the minerals in rocks or changes them into different minerals
(rusting, acid breakdown, etc.)
 Example – caves are typically formed by slightly acidic water dissolving the limestone.
Five things that cause chemical weathering: water, oxygen, carbon dioxide, living organisms (like
lichen), and acid rain.
Chemical weathering occurs more rapidly in warm, wet climates.
Chemical Change
Chemical weathering causes the rock to experience a chemical change (changes its makeup).
 Water – dissolves rocks over time.
 Oxygen – causes oxidation (rusting).
 Acid rain – reacts with marble and limestone.
 Living organisms – plant roots produce weak acids to dissolve rock around the roots.
Naturally formed acids also weather rocks chemically. When water mixed with carbon dioxide gas in
the air or soil it forms carbonic acid. Carbonic acid wears down limestone to form caves like Natural
Bridge Caverns near Fredericksburg, Texas.
Climate can affect the rate of weathering in different parts of the world.
Climate is the pattern of weather that occurs in a particular area over many years. In cold climates,
where freezing and thawing are frequent, mechanical weathering rapidly breaks down rocks through ice
wedging.
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Chemical weathering is more rapid in warm, wet climates. Lack of moisture in deserts and and low
temperatures in polar regions slow down chemical weathering.
Erosion
The movement of weathered earth materials by moving water, wind, gravity, or ice.
 Examples: waves causing beach erosion, sand storm winds, rain causing a mudslide, landslides,
rain carrying soil and gravel down a street or driveway.
When rock and soil are moved from one place to another by natural forces, this is called erosion.
Things that aid movement of soil and rock:
 Water – (rain water runoff, streams, rivers, groundwater, and waves).
 Ice (Glaciers)
 Wind
 Gravity (landslides, mudslides, avalanches, etc.)
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Sediment is the material moved by erosion. It is usually small, solid particles of rocks or organisms.
Why care about erosion?
Human activities such as construction increase the rate of erosion 200, even 2,000 times that amount.
When we remove vegetation or other objects that hold soil in place, we expose it to the action of
wind/water and increase its chances of eroding.
The loss of soil from a construction site results in loss of topsoil, minerals and nutrients, and it causes
ugly cuts and gullies in the landscape. Surface runoff and the materials it carries with it clog our
culverts, flood channels and streams. Sometimes it destroys wildlife and damages recreational areas
such as lakes and reservoirs.
Bare slopes vs. vegetation stabilized slopes
Bare slopes allow for…
 Mudslide danger
 Loss of topsoil
 Clogged storm drains
 Flooding problems
 Expensive clean-up
 Eroded or buried house
foundations
Vegetation stabilized slopes
 Soil in place
 Less clean-up
 Minimum erosion
 Protection for house foundations
Causes and effects of soil erosion
Soil is eroded when it is moved from the place where it formed. Erosion occurs when water flows along
the Earth’s surface or when wind picks up and transports sediments.
Erosion is more common on steep slopes than on gentle slopes. It is also more common in areas with
little vegetation.
Several man-made activities contribute to increased erosion.
 Agricultural cultivation – the Earth’s population is approximately 7 billion people. The
population is increasing rapidly at the rate of 95 million every year. This leads to an
increase in farmable land and increased soil erosion. It also reduces soil quality over
time.
 Forest harvesting – the removal of forests causes exposed soil and an increase in
erosion.
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
Overgrazing – the overgrazing of land increases soil erosion. In some regions of the
world, sheep and cattle graze on grasses until almost no ground cover remains to protect
the soil and slow erosion.
Landslides
A landslide is a geological phenomenon which includes a wide range of ground movement, such as rock
falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore
environments.
Deposition
Deposition is the process of eroded Earth materials (sediments) being deposited in a new location.
 Example – delta – where sediment is deposited where a river flows into an ocean or lake
builds up a landform.
When the sediment being carried is dropped (or deposited) in a new area, it is called deposition.
Depositions can create new landforms like beaches and sand dunes.
Formation of Soil
Soil is a mixture of weathered rock, decayed organic matter, mineral fragments, water, and air.
It can take thousands of years for soil to form and can be from a few centimeters thick to upwards of 60
meters thick in some locations.
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There are five primary factors that affect soil formation:
A. Climate
B. Slope of the land
C. Types of rock
D. Types of vegetation
E. Amount of time rock has been weathering
Composition of soil
The rock and mineral fragments found in soil come from rocks that have been weathered. Most are
fragments of sediment like clay, silt, and sand.
Most organic matter in soil comes from plants. Leaves, stems, and roots all contributed organic matter
to the soil.
Animals and microorganisms provide additional organic matter when they die. After plant and animal
material gets into the soil, fungi and bacteria cause it to decay.
The decayed organic matter turned into a dark-colored material called humus. This serves as a source
of nutrients for plans.
Worms, insects, and some rodents burrow through the soil mixing the humus with the fragments of rock.
Good quality surface soil has about the same amount of humus as weathered rock material.
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