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Chapter 4
Weathering and Soil Formation
Weathering breaks down rocks
into smaller pieces
1. Mechanical weathering
2. Chemical weathering
Mechanical weathering produces
physical changes in rocks:
• Ice wedging
• Pressure release-causes rock to expand,
cracking and leading to exfoliation
Exfoliation-process in which layers of exfoliation
rock gradually break off.
• Plant root growth
• Abrasion-wearing down by friction, the rubbing
of one object or surface against another
(example: running river)
Chemical weathering
• Breakdown of rocks by chemical reactions
that change the rocks composition (what it
is made of)
• Water (dissolving)
• Rusting
Weathering occurs at different rates
• Surface area-the more exposed area, the
faster the break down of rock (chemical)
• Rock composition ex: Granite compared
to limestone
• Climate:
• chemical weathering occurs in wet, hot
areas faster than cold, dry regions
• Mechanical weathering caused by freezing
and thawing
Weathering and organic processes
form soil
• Soil- a mixture of weathered rock particles
and other materials
• Humus-decayed organic (living things) in
the soil
Soil formation is affected by:
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Rock in an area
Climate: tropical, desert, temperate, artic
Landforms: mountains and valleys
Plant cover: provide organic matter
Living organisms: microorganisms and
animals
• Time
Soil composition
• Soil horizon-a layer of soil with properties
that are different than the layers above
and below it
• Soil profile- soil horizons in a specific area
Soil Properties
• Texture-determined by size of weathered rock
particles it contains
• Color-most comes from iron and humus
• Pore space-the spaces between the particles
• Chemistry-minerals and organic nutrients
(Does it react with water such as bubbling?)
Human activities
• Soil is a valuable resource for humans
1. Farming-soil loss and overgrazing results in
desertification- expansion of desert in areas
where natural plant cover has been destroyed
2. Construction and development
3. Mining
• Strip mining
• Open pit
Soil conservation
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Crop rotation
Conservation tillage
Terraces
Contour plowing
Windbreaks
Info for Soil lab
• Climate in Missouri- Temperate soils form
in region with moderate rainfall and
temperatures. Some temperate soils are
dark-colored, rich in organic matter and
minerals, and good for growing crops.
Soil Horizons
• The A horizon- is the upper layer of soil
and is commonly called topsoil.
• It contains the most organic matter out of
all the horizons due to the humus that it
contains gives it the dark color
• (Moderate pore space).
The B Horizon
• This layer lies just below the A horizon. It
has a little organic matter and is usually
brownish or reddish color and contains
clay (very little pore space for water)
and minerals.
The C horizon
• This is the deepest layer of soil. This layer
contains the largest and least-weathered
rock particles. It is usually light yellowish
brown (sand which has high amount of
pore space).