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WEATHERING
Weathering, Soil, and Erosion
2011-2012
WEATHERING DEFINITIONS
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Weathering – the breakup of rock due to exposure to
processes that occur at Earth’s surface.
Mechanical weathering
Also called disintegration.
 Takes place when rock is split or broken into smaller pieces
of the same material without changing its composition.
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Chemical weathering
Also called decomposition.
 Takes place when the rock’s minerals are changed into
different substances.
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MECHANICAL WEATHERING – ICE AND WATER
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Water occupies 10% more space when it freezes
 puts a lot of pressure on the container it’s in.
Frost wedging – water freezes in the cracks in
rocks and pushes the rock apart.
Occurs in areas where the temperature varies
from below freezing (0°C) to above freezing.
MECHANICAL WEATHERING – ICE AND WATER
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Repeated wetting and drying can break up rocks
that contain clay (i.e. shale).
Clay absorbs water easily and swells when wet.
 Clay shrinks as it dries.
 Repeated swelling and shrinking causes the rocks to
break apart.
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MECHANICAL WEATHERING – ABRASION
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Rocks are moved by water, wind, and ice.
Abrasion occurs when moving rocks grind and
scrape against each other, wearing the rock
materials away.
The sand on the beach is a result of abrasion.
MECHANICAL WEATHERING – PLANTS AND
ANIMALS
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Plant roots can wedge into pores and crevices in
rocks.
As the roots grow larger, they push out on the
rocks and cause them to split.
Burrowing animals and insects dig holes in the
soil, allowing air and water to reach bedrock and
weather it.
MECHANICAL WEATHERING – UPWARD
EXPANSION
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Exfoliation – the peeling of
surface layers from exposed
bedrock.
Example: Granite is lifted
up and the rocks above it
are worn away. The rocks
form sheets and break off as
seen in the photo.
http://0.tqn.com/d/geology/1/0/H/L/exfoliation.jpg
CHEMICAL WEATHERING – WATER
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Hydrolysis
The chemical weathering by reaction of water with
other substances.
 When some minerals are exposed to water, they
dissolve into ions.
 These ions slowly react with the water and form clay
minerals.
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CHEMICAL WEATHERING – WATER
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Carbonic acid
Weak acid that is formed when carbon dioxide
dissolves in rainwater.
 Seeps into the ground and reacts chemically with
many common minerals.
 Example: Carbonic acid reacts with calcite, causing it
to dissolve completely. This dissolving has formed
underground caverns.
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CHEMICAL WEATHERING – WATER
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Acid rain
Sulfur dioxide, nitrogen compounds, and carbon
dioxide react with water in the air.
 Causes structures made of concrete, stone, and metal
to wear out more quickly.
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CHEMICAL WEATHERING – OXYGEN
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Oxidation
The chemical reaction of oxygen with other
substances.
 Very effective at weathering minerals that have iron
in their chemical formulas.
 The formation of different types of rust (iron oxides).
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RATES OF WEATHERING – SURFACE EXPOSURE
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The rate at which a rock weathers is affected by
the amount of surface that is exposed to chemical
weathering processes.
The more of a rock that is exposed, the faster it
will weather.
RATES OF WEATHERING – COMPOSITION OF
ROCK
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How fast or how slow a rock will weather
depends on what it is made of.
Rocks with layers of different compositions will
wear at different rates.
RATES OF WEATHERING – CLIMATE
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Wet climates are conducive to both chemical and
mechanical weathering processes.
Regions with cold or dry climates are more
conducive to mechanical weathering.