EARTH SCIENCE
CHAPTER SEVEN
WEATHERING, SOIL, AND
EROSION
7.1 WEATHERING
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Weathering is the breakup of a rock due to
exposure to processes that occur at Earth’s
surface
Two main types:
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Mechanical weathering (disintegration) occurs when
rocks are broken down into smaller pieces but do not
change in composition.
Chemical weathering (decomposition) occurs when
the rock’s minerals are changed into different
substances
MECHANICAL WEATHERING
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Several processes:
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Frost wedging
Wetting and drying
Abrasion by rock materials
Plant and animal activity
Exfoliation due to upward expansion
FROST WEDGING
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Water occupies 10 % more space when it
freezes than it does as a liquid.
When water that is trapped inside the
cracks and pores rocks freezes, it can
break the rock as it expands.
This is called frost wedging or ice
wedging.
WETTING AND DRYING
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Liquid water also plays a role in
mechanical weathering.
Some rocks that are made heavily of clay
and silt (like shale), swell when they are
wet and shrink as they dry.
This repeated action can break the rock
over time.
ABRASION
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Occurs when moving sand, rocks, or
pebbles grind and scrape against one
another.
The sand we walk on at the beach was
once rock that has been repeatedly
broken down and carried by the waves.
PLANT AND ANIMAL ACTIVITY
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Lichens, plants, and mosses grown on
rocks and wedge into their pores—roots
can eventually split the rocks.
Even trees have been known to grow
through boulders.—pic pg 259
Many animals (ants, earthworms, rabbits,
etc) make holes in the ground that expose
rocks to agents like wind, ice, etc.
UPWARD EXPANSION
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Upward expansion occurs when granite beneath
the earth’s surface is pushed upward by forces
below.
This extreme force causes rocks on top of the
granite sheets to break apart and erode.
The granite sheets are then exposed and begin
to break and erode also.
This is called exfoliation-
The peeling away of surface layers of exposed
bedrock
CHEMICAL WEATHERING
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Can occur anywhere that water or water
vapor is present
Can occur by acids
Can occur by oxygen
WATER AND CHEMICAL
WEATHERING
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The reaction of water with other
substances is called hydrolysis.
Water readily reacts with some minerals
(like feldspar, hornblende, and augite) to
dissolve into ions that slowly react with
water to form different types of clay
minerals.
ACIDS AND CHEMICAL
WEATHERING
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Carbonic acid, H2CO3, is an acid commonly
found in water. It is formed when CO2 in the
atmosphere dissolves in rainwater.
This acidic mixture of carbonic acid and water
breaks down even more minerals to form clay.
Sulfur dioxide, nitrogen compounds, and carbon
dioxide from pollution forms acid rain.
This acid rain can very quickly decompose many
types of structures.
OXYGEN AND CHEMIMCAL
WEATHERING
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Oxidation is the chemical reaction of
oxygen with other substances.
Oxidation is very effective in weathering
rocks that contain iron. Oxidized iron is
called rust.
RATES OF WEATHERING
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Under average conditions, weathering is a slow process.
The process is influenced by three major factors:
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Surface exposure—all weathering happens at the surface, so
more surface area=faster process
Composition of the rock—certain types of rock are more
susceptible to oxidation and acid reactions, thus they would
weather faster than other rocks in the same area
Climate—in warmer, wetter climates—both mechanical and
chemical weathering play about the same role. However, in
dryer, colder climates, mechanical weathering plays a greater
role than chemical weathering
7.2 EROSION and DEPOSTION
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Erosion is the removal and transport of
materials by natural agents such as wind
and running water.
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Is affected by climate
Is affected by the composition of the rock
The overall effect depends on how the
process of erosion compares with the ongoing
process of uplift
Deposition is when the materials are
dropped in another location.
AGENTS OF EROSION
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Gravity
Running Water
Glaciers
Wind
Living things
To be discussed more in Chapter 8
7.3 SOIL
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Soil is made of loose, weathered rock and
organic material in which plants with roots
can grow
Without the weathering process we
studies in Section 1, there would be no
soil.
Without soil, what would we eat?
HOW SOIL FORMS—PARENT
MATERIAL
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The material from which soil forms is
called the parent material.
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If the parent material is the bedrock beneath
the soil, the soil is called a residual soil.
If the parent material is not from the
underlying bedrock but from weathered
particles that were carried there by wind,
waves, etc., the soil is called a transported
soil.
HOW SOIL FORMS—THE PROCESS
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Forms as parent material is weathered
away.
Rate varies by location.
Climate plays a role.
Organic materials (decaying plant and
animal remains) are added.
SOIL PROFILE
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A cross-section of the earth exposed by
digging
Allows us to study the “layers of soil”
Mature soils have three distinct zones or
soil horizons
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A Horizon
B Horizon
C Horizon
A-HORIZON
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Called topsoil
Generally gray to black in color
Contains humus—organic material that
forms from decayed plant and animal
materials
Tends to be sandy
B-HORIZON
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Called subsoil
Usually red or brown from oxides that
formed in topsoil and then got washed
down into the subsoil
Often contains clay and soluble minerals
like calcium and potassium
C-HORIZON
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Made of slightly weathered parent
material
Often has rock fragments
Lies just above the bedrock or parent
material
SOIL COMPOSITION
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Several factors affect soil composition:
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Parent material
Time
Plants and animals
Topography
Climate
SOIL AS A RESOURCE
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The world’s population is increasing at an
alarming rate.
All of these people need to eat!!!!!!
However, less than 25 % of the Earth’s land can
be used to grow crops.
Soil fertility is the ability of soil to grow plants.
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Certain types of soil are best suited for certain crops.
Many things currently threaten soil fertility.
SOIL DEPLETION
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Soil depletion occurs when the soil
gradually becomes so lacking in nutrients
that it can no longer be used to grow a
usable crop.
Farmers can do some things to prevent
this.
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Allow the soil to rest by rotating crops
SALINIZATION
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Irrigation is often used to make soils
fertile.
Irrigation water often brings in dissolved
minerals.
These dissolved minerals are often salts.
Over time, the buildup of salt in the soil is
called salinization.
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Often renders the soil completely useless
SOIL CONSERVATION
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Several methods exist:
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Windbreaks—also known as shelter belts—lines of trees planted
along the edges of fields
Contour farming—farmers plant crops parallel to land contours
to prevent water from running rapidly down a slope
Terraces—flattens a slope into separate terraces to slow the
speed of runoff
Strip cropping—alternate rows of crops to minimize soil erosion
No-till—method of farming where everything is done at once
(plowing, planting, fertilizing, weed control) and the soil is left
completely alone until harvest time