Download Chapter 4 Notes: Weathering and Soil

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Big Idea: Natural forces break rocks
apart and form soil, which supports life.
• Weathering is the process by
which natural forces break down
rocks.
• There are 2 types of weathering:
1. Mechanical weathering
2. Chemical weathering
• Mechanical weathering is the
breaking up of rocks by physical
forces.
• There are 4 main types:
1. Ice wedging
2. Exfoliation
3. Plant root growth
4. Abrasion
• When water freezes, it expands.
When water freezes in the cracks
and pores of rocks, the force of its
expansion is strong enough to
split the rocks apart.
• The process in which layers or
sheets of rock gradually break off
due to the expansion of the rock
caused by a decrease in pressure.
• Trees, bushes, and
other plants may take
root in cracks of rocks.
As the roots of these
plants grow, they
wedge open the
cracks.
• The process of wearing down
by friction, the rubbing of one
object or surface against
another.
• The breakdown of rocks by
chemical reactions that change
the rocks’ makeup, or
composition.
• There are 2 main types:
1. Dissolving
2. Rusting
• Water is the main cause
of chemical weathering.
• Many more minerals
dissolve in water that is
slightly acidic – like
lemonade.
• The oxygen in air is also
involved in chemical
weathering.
• Many common minerals
contain iron. When these
minerals dissolve in water,
oxygen in the air and the
water combines to produce
rust.
• Most weathering occurs over long
periods of time – hundreds,
thousands, or even millions of
years.
• There are 3 Influential factors:
1. Surface area
2. Rock composition
3. Climate
• The more of a rock’s surface that is
exposed to air and water, the faster
the rock will break down.
• Different kinds of rock break down
at different rates.
• Chemical weathering occurs faster
in hot, wet regions than it does in
cold, dry regions.
• Mechanical weathering caused by
freezing and thawing occurs more
in cold regions than in hot regions.
Big Idea: Natural forces break rocks
apart and form soil, which supports life.
• Soil is a surface mixture of
rocks, minerals, organic
matter, water, and air.
• The rocks and minerals
come from weathering.
• The organic matter is
called humus, which
comes from the decay of
different life forms in or on
the soil.
• Water is from precipitation
• Soil air is from the
atmosphere.
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•
•
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•
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Type of rock
Climate and overall weather
Landforms – mountains &
valleys
Plant cover
Organisms
Time
• Rocks, minerals, and organic
matter form soil particles
(~ 50% of a soil by volume).
• The spaces between soil
particles are called soil pores
(~ 50% of a soil by volume).
• Water and air fill the pores.
• Over time, soils develop
horizons, distinct layers of
soil identified by color,
texture, and particle
structure.
O: Decaying organic matter
on soil surface (fallen leafs).
A: Loose dark surface soil
good for food production
(topsoil).
B: Dense light-colored subsurface soil not good for food
production (contains rocks).
C: Mostly partially weathered
rocks and minerals
(bedrock).
• Texture: determined by the size of
the weathered rock particles.
• Color: indicates the content of the
soil and how well it drains.
• Pore space: the spaces between
soil particles.
• Chemistry: determines how well
nutrients dissolve in water.
Big Idea: Natural forces break rocks apart and
form soil, which supports life.
• Supports the growth of plants, which
in turn supply us with many things,
such as food, clothing, medicines,
lumber, and even oxygen.
• Purifies water as it drains through
the ground and keeps water systems
clean.
• Nutrient recycling by decomposers in
soil that enable plants to grow.
• Home to a variety of living things.
Farming: use of fertilizers & practices
• farmers lose 5 metric tons of soil per 1
metric ton of grain
• microorganisms have difficulty
producing nutrients naturally.
• pollutes water systems.
• reduce plant cover and increases soil
loss due to wind and water.
• overgrazing by livestock.
Construction & Development: removal
of vegetation to build
• soil is blown or washed away.
• makes water systems muddy and can
harm the organisms living there.
• causes flooding.
Mining: removal of soil
• speeds up rates of chemical
weathering by exposing rocks &
minerals to rainwater and air.
• contaminates soil (sulfuric acid).
• Crop rotation:
The practice of planting different crops
on the same field in different years or
growing seasons.
• Conservation tillage:
Reduces the number of times that
fields are tilled in a year; therefore
reducing the amount of erosion.
• Terraces:
Flat, step-like areas built on a hillside
to hold rainwater and prevent it from
running downhill.
• Contour plowing:
Plowing along the curves of a slope
which helps channel rainwater
resulting in less erosion.
• Windbreaks:
Rows of trees planted between fields
to reduce the force of winds that can
carry off soil.