Rocks, Weathering, and
Soil Information
The Effects of Weathering
• All exposed rock is in the
path of weathering and
erosion.
• Weathering is the process
that breaks down rock and
other substances at the
Earth’s surface.
• Heat, cold, water, ice,
carbon dioxide, oxygen etc..
all contribute to weathering.
Weathering and Erosion
• Weathering examples:
– Repeated freezing and thawing.
– Rainwater dissolving minerals.
• Erosion is the movement of rock
particles by wind water, ice, or
gravity.
• Weathering and erosion work
together to carry away the rocks
at the surface.
• There are two types of
weathering:
– Mechanical Weathering
– Chemical Weathering
Mechanical Weathering
• Mechanical Weathering: rock is physically
broken into smaller pieces that have the same
chemical composition as the rock they came
from.
– Examples:
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freezing and thawing,
release of pressure,
growth of plants,
actions of animals, and
abrasion (grinding away of rock by
particles in the wind and water).
Chemical Weathering
• Chemical Weathering: the process that
breaks down rock through chemical
changes and produces rock particles
that have a different mineral makeup
from the rock they came from.
– Examples:
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Hot or soft spots in rock
Water
Oxygen
Carbon Dioxide
Living organisms
Acid Rain
Rate of Weathering
• The most important factors that determine the rate
at which weathering occurs are the type of rock and
climate .
• Type of Rock
– Minerals will determine the rate.
– Permeable rocks (full of tiny air spaces) weather faster.
• Climate:
– Weathering occurs faster in wet climates and higher
temperatures.
Soil Formation
• Over many years, weathering
and erosion will cause the
formation of soil.
• Soil is the loose, weathered
material on the Earth’s surface
in which plants can grow.
• Soil forms as rock is broken
down by weathering and mixes
with other materials on the
surface.
Soil Composition
• Soil is constantly being
formed wherever bedrock is
exposed.
– Bedrock is the solid layer of
rock beneath the soil.
• Soil is a mixture of rock
particles, minerals, decayed
organic material, air, and
water.
Soil Composition and Texture
• The decayed organic
material in the soil is
humus.
– Humus is a dark-colored
substance that forms as
plant and animal remains
decay.
• Soil texture depends on the
size of individual soil
particles.
– Soil texture is important for
plant growth.
Soil Horizons
• A soil horizon is a layer of soil
that differs in color and texture
from the layers above or below it.
– Topsoil (A horizon) is a crumbly,
dark brown soil that is a mixture of
humus, clay, and other materials.
– Subsoil (B horizon) usually consists
of clay, and other particles washed
down from the topsoil, but little
humus.
– C horizon contains only partly
weathered rock.
The rate at which soil forms
depends on the climate and
type of rock.
Life in
Soil
• Some organisms mix the soil and
make spaces in it for air and water.
• Other soil organisms make humus,
the material that makes soil fertile.
– Humus forms through decomposition.
– Fertile soil is rich in nutrients that plants
need.
• As plants shed leaves, they form a
loose layer called litter.
With and Without Worms 
Soil Types in the U.S.
• Tundra Soils
• Northern Forest
Soils
• Prairie Soils
• Mountain Soils
• Southern Forest
Soils
• Desert Soils
• Tropical Soils
Soil Conservation
• Soil is one of Earth’s most
valuable resources because
everything that lives on the
land depends directly on
indirectly on soil.
• Fertile soil is valuable
because there is a limited
supply.
– Less than 1/8th of the land on
Earth has soils well suited for
farming.
Soil Damage and Loss
• Soil can become
exhausted, or lose its
fertility.
• Soil can also become lost
to erosion by water and
wind.
– Water erosion can occur
wherever soil is not
protected by plant cover.
– Wind erosion caused the
Great Dust Bowl!!
The Dust Bowl
• In the 1930s plowing removed the grass form
the Great Plains and exposed the soil. In
times of drought, the topsoil quickly dried out,
turned to dust, and blew away.
• This event helped people appreciate the
value of soil.
• Check It Out!!
Soil Lost To The Seas!!
Soil Conservation
• Soil Conservation is the
management of soil to
prevent its destruction.
• Two ways that soil can be
conserved include contour
plowing and conservation
plowing.
Contour Plowing
• Contour plowing is the
practice of plowing
fields along the
contours of a slope.
• This helps slow the
runoff of excess
rainfall and prevents it
from washing the soil
away.
Conservation Plowing
• Conservation Plowing disturbs the soil and its
plant cover as little as possible.
• Dead weeds and stalks of the previous year’s
crop are left in the ground to help return soil
nutrients.