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Weathering and Erosion
Weathering
Weathering is the process that
breaks down rock and other
substances at Earth’s surface.It is
a set of physical, chemical and
biological processes that change
the physical and chemical
properties of rocks and soil at or
near the earth's surface.
More about Weathering
• Definition – the breakdown of rocks and other substances at
Earth’s surface to form sediment
**Sediment is very small pieces of rock.
– Weathering happens to rocks that are NOT MOVING
– Weathering is part of the Rock Cycle
Weathering and Erosion
• The forces of weathering break rocks into
smaller and smaller pieces. The forces of
erosion carry the pieces away.
• Erosion is the removal of rock particles
by wind, water, ice or gravity.
• Weathering and erosion work
together continuously to wear
down and carry away the rocks at
Earth’s surface.
Uniformitarianism
• The weathering and erosion that geologists
observe today also shaped Earth’s surface
millions of years ago.
• How do geologists know this?
• Geologists make inferences based on the
principle of uniformitarianism. This principle
states that the same processes that operate
today operate in the past.
There are three types of
weathering
• Mechanical [sometimes called
physical]
• Chemical
• Biological
Mechanical Weathering
• Mechanical weathering is the type of
weathering in which rock is physically broken
down into smaller pieces.
• These smaller pieces of rock have the same
composition as the rock they came from.
• The cause of mechanical weathering include
freezing and thawing, release of pressure, plant
growth, actions of animals of abrasion.
– Abrasion refers to the grinding away of rock by rock
particles carried by water, ice, wind, or gravity.
Types of Mechanical
Weathering
– Frost heaving and Frost wedging
– Plant roots
– Friction and impact
– Burrowing of animals
– Temperature changes
Ice Wedging
Water seeps into the cracks of rocks.
When the temperature decreases,
the water freezes and expands. The
ice then acts as a wedge and forces
things apart. Wedges of ice in rocks
widen and deepen cracks.
Ice Heaving
Plant Roots
Friction and Repeated Impact
Burrowing of Animals
Temperature Changes
Chemical Weathering
• The process that breaks down rock
through chemical changes.
• The agents of chemical weathering
– Water
– Oxygen
– Carbon dioxide
– Living organisms
– Acid rain
Water
• Water weathers rock by dissolving it.
Oxygen
• Iron combines with
oxygen in the
presence of water in
a processes called
oxidation.
• The product of
oxidation is rust.
Carbon Dioxide
• CO2 dissolves in rain water and creates
carbonic acid.
• Carbonic acid easily weathers rocks such
as limestone and marble.
Living Organisms
• Lichens that grow on rocks produce weak
acids that chemically weather rock
Acid Rain
• Compounds from burning coal, oil and gas
react chemically with water forming acids.
• Acid rain causes very rapid chemical
weathering.
Biological weathering is the breakdown of
rock caused by the action of living
organisms, including plants, burrowing
animals, and lichens.
A lichen is a combination of fungus and
algae, living together in a symbiotic
relationship.
• Lichens can live on bare rock, and they
break down rocks by secreting acids and
other chemicals.
Rate of Weathering
• The most important factors that determine the rate at which
weathering occurs are the type of rock and climate.
• The minerals that make up the rock determine how fast it
weathers. Rock made of minerals that dissolve easily in water
weathers faster.
• Some rock weathers more easily because it is permeable.
Permeable means that a material is full of tiny connect air
spaces that allow water to seep through it.
• Climate refers to the average weather conditions in an area.
Both chemical and mechanical weathering occur faster in wet
climates. Rainfall provides the water needed for chemical
changes as well as for freezing and thawing.