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Chapter 14
Weathering and Erosion
Weathering Processes
Weathering - the natural process by which
atmospheric and environmental agents, such
as wind, rain, and temperature changes,
disintegrate and decompose rock at or near
the surface.
There are two main types of weathering
processes—mechanical weathering and
chemical weathering.
Each type of weathering has different effects
on rock.
Mechanical Weathering
Mechanical – produces smaller pieces,
adds to effectiveness of chemical
weathering, leads to development of
talus slopes, does not change a
mineral’s composition.
Mechanical Weathering
Mechanical weathering is strictly a physical
process and does not change the
composition of the rock.
Common agents of mechanical weathering
are ice, plants and animals, gravity, running
water, and wind.
Physical changes within the rock itself
affect mechanical weathering.
Types of Mechanical Weathering
Ice wedging – water penetrates into cracks
in rock and freezes.
When the water freezes, its volume
increases by about 10% and creates pressure
on the surrounding rock.
This process eventually splits the rock apart.
Must have:
• Adequate moisture
• Cracks in rocks
• Freeze/thaw cycles
Ice Wedging
Thermal expansion
Repeated daily heating and cooling of
rock;
Heat causes expansion; cooling causes
contraction.
Expanding and contracting causes
cracks.
Abrasion
Is the grinding and wearing away of rock
surfaces through the mechanical action of
other rock or sand particles
Abrasion is caused by gravity, running
water, and wind.
Organic Activity
Plants and animals are important agents of
mechanical weathering.
Biological wedging – plant roots penetrate
into cracks causing cracks to widen.
Must have:
• Climate hospitable for plants
• Adequate moisture and
temperature
Biological
Wedging
Root wedging
widens fractures
in rock.
Organic Activity
Digging and burrowing activities.
Earthworms and other animals that move
soil expose new rock surfaces to both
mechanical and chemical weathering.
Chemical Weathering
Chemical alteration of minerals.
Chemical reactions commonly occur
between rock, water, carbon dioxide,
oxygen, and acids.
Most effective in a warm, humid climate.
Chemical weathering changes both the
composition and physical appearance of the
rock.
Types of Chemical Weathering
Ion Exchange – H+ replaces other cations.
Oxidation - reaction in which elements gain
or lose electrons (example: rust).
Oxidation commonly occurs in rock that
has iron-bearing minerals, such as
hematite and magnetite.
OXIDATION
ROCKS / MINERALS
CHEMICALLY REACT
WITH THE OXYGEN IN
THE ATMOSPHERE,
CAUSING A
DECOMPOSITION ON
THE MATERIALS
Types of Chemical Weathering
Hydrolysis - any reaction in which
water participates.
Water plays a crucial role in chemical
weathering.
Minerals that are affected by hydrolysis
often dissolve in water.
Water can then carry the dissolved minerals
to lower layers of rock in a process called
leaching.
Dissolution - mineral completely
dissolves, leaving only ions in solution.
Chemical Weathering
The image below shows how water plays a
crucial role in chemical weathering.
Carbonation
Is the conversion of a compound into a
carbonate
When carbon dioxide, CO2, from the air
dissolves in water, H2O, a weak acid called
carbonic acid, H2CO3, forms.
H2O + CO2  H2CO3
Acid Precipitation
Such as rain, sleet, or snow, that contains a
high concentration of acids, often because
of the pollution of the atmosphere
Acid precipitation weathers rock faster than
ordinary precipitation does.
Rainwater is slightly acidic because it
combines with small amounts of carbon
dioxide.
Factors influencing Weathering Rates
The processes of mechanical and
chemical weathering generally work
very slowly.
Rock Structures – chemical/mineral
composition, Physical features
Topography
Climate
Spheroidal Weathering
Spheroidal
Weathering
Differential Weathering
Rocks weather at different rates.
Softer rocks are less resistant to weathering.
Amount of Exposure
Surface Area
Both chemical and mechanical weathering may
split rock into a number of smaller rocks.
The part of a rock that is exposed to air, water, and
other agents of weathering is called the rock’s
surface area.
As a rock breaks into smaller pieces, the surface
area that is exposed increases.
Greater exposure on smaller pieces
Climate
Climates with alternating periods of hot and
cold weather allow the fastest rates of
weathering.
In warm, humid climates, chemical
weathering is also fairly rapid.
The slowest rates of weathering occur in
hot, dry climates.
Weathering is also slow in very cold, dry
climates.
Topography
Ice wedging is more common at high
elevations than at low elevations.
On steep slopes weathered rock fragments
are pulled downhill by gravity and washed
out by heavy rains.
New surfaces of the mountain are
continually exposed to weathering.
Soil Horizons
Horizon O
Bedrock
Erosion
erosion a process in which the materials of Earth’s
surface are loosened, dissolved, or worn away and
transported from one place to another by a natural
agent, such as wind, water, ice, or gravity
When rock weathers, the resulting rock particles
do not always stay near the parent rock.
Various forces may move weathered fragments of
rock away from where the weathering occurred.
Soil Erosion
Ordinarily, new soil forms about as fast as
existing soil erodes.
Some farming and ranching practices
increase soil erosion.
Soil erosion is considered by some
scientists to be the greatest environmental
problem that faces the world today.
Gullying and Sheet Erosion
Gullying is caused by furrow plowing.
Eventually land that is plowed in this way can
become covered with deep gullies.
Sheet erosion is the process by which water flows
over a layer of soil and removes the topsoil
Wind also can cause sheet erosion during
unusually dry periods.
Results of Soil Erosion
Constant erosion reduces the fertility of the soil by
removing the A horizon, which contains the fertile
humus.
The B horizon, which does not contain much
organic matter, is difficult to farm because it is
much less fertile than the A horizon.
Without plants, the B horizon has nothing to
protect it from further erosion.
All the soil layers could be removed within a few
years by continuous erosion.
Soil Conservation
Certain farming techniques can increase the
rate of erosion.
Land clearing accelerates topsoil erosions.
Soil erosion can be prevented by soil
conservation methods
Soil Conservation
Contour Plowing: soil is plowed in curved bands
that follow the contour, or shape of the land.
Strip-Cropping: crops are planted in alternating
bands slowing the runoff of rainwater.
Terracing: construction of steplike ridges that
follow the contours of a slope
Crop Rotation: farmers plant one type of crop
one year and a different type of crop the next.
An earthflow on a
newly formed slope
Some visible effects of creep
End of Chapter 14