Physical Weathering

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Physical Weathering
Physical weathering produces regolith from
massive rock by the action of forces strong
enough to fracture rock. Some physical weathering
processes include frost action, salt-crystal
growth, unloading, expansion-contraction, and
wedging by plant roots.
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Physical Weathering
Joint-block separation and granular
disintegration are two common forms of bedrock
disintegration.
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Physical Weathering
Frost-shattered blocks Ice crystal growth within
the joint planes of rock can cause the rock to
split apart. This split boulder, on the high
country of the Sierra Nevada of California, is an
example.
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Niche Formation
Zones of rock lying close to the base of a cliff
are susceptible to erosion by the growth of salt
crystals. Where water evaporates to leave salt,
the crystals exert a force to break rocks apart.
In niche formation, it is the seepage of
groundwater and the subsequent growth of salt
crystals that create a niche which identifies a
change in lithologies.
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Niche Formation
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Chemical Weathering
Chemical weathering involves the reaction of
minerals within rock to water in the environment.
For example, water becomes acidic as carbon
dioxide dissolves in it to create a weak carbonic
acid. Carbonate rocks such as limestone react
with carbonic acid, to create a weaker form that
is removed by solution.
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Chemical Weathering
These weathered tombstones are from a burying
ground in Boston, Massachusetts. The marker on
the left, carved in marble, has been strongly
weathered, weakening the lettering. The marker on
the right, made of slate, is much more resistant
to erosion.
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Unloading
As a rock mass is revealed as overlying material
is removed, the pressures on underlying layers of
rock lessen. Consequently, rocks exposed at the
surface may break apart through the weathering
process known as unloading.
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Unloading
This granite outcrop in Yosemite National Park,
California, displays sheetlike joints, giving a
stepped appearance to the mountain slope.
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Biological Weathering
Organisms also have a role to play in the breakup
of surface materials. Burrowing animals and plant
roots are just examples of where animals and
plants can exert forces to break rocks apart.
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Biological Weathering
A Ponderosa pine tree that began growing in a
crack on this outcrop of bedrock has caused a
large flake of rock to break away, exposing the
tree's expanding root system. The rock is granite.
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Slopes and Mass Wasting
Mass wasting is the spontaneous downhill movement
of soil, rock, and regolith under the influence
of gravity. Slopes are mantled with regolith and
which downwards towards bedrock. Soil may also
develop on the regolith and support vegetation.
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Slopes and Mass Wasting
Alluvium, a form of transported regolith, lies in
the floor of an adjacent stream valley.
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Mass Wasting
Mass wasting is the spontaneous downhill movement
of soil, rock, and regolith under the influence
of gravity. Examples of mass wasting include soil
creep, mudflows, landslides debris falls and
slumps.
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Mass Wasting
Examples of slurry flows and granular flows.
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Mass Wasting
Examples of slope failures.
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Soil Creep
Alternate wetting and drying of the soil and the
formation of needle ice produces tiny
displacement of particles which eventually result
in their movement downslope. Consequently, soil
creep is an extremely slow and gradual movement
of slope materials. It causes the gradual tilting
of objects such as fence posts and monuments.
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Soil Creep
Indicators of soil creep.
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Soil Creep
Needle ice growth.
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Earth flow
Slope materials may retain their stability as
long as no changes are made to the mass of the
slope or alters the cohesion of the slope
materials. However, if an earthflow begins to
form, a scarp will indicate where slope materials
are beginning to move downslope. Materials flow
down as well as forward to create a rotational
slump zone. Materials flow outward at the
flowage zone. This debris then protrudes at the
base of the earth flow to create a toe. See
animation on earth flow in the geodiscoveries
section of your texts website.
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Periglacial Landscapes
Periglacial landscapes account for 25 of the
land surface, and form over areas of permafrost.
Permafrost is permanently frozen ground, ranging
from the deep, continuous permafrost of very cold
regions, to the thinner, discontinuous permafrost
of less cold regions. Above the permafrost is the
active layer which thaws in spring and summer to
create a mobile and water logged volume of
surface materials.
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Periglacial Landscapes
Distribution of permafrost in the northern
hemisphere.
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Periglacial Landscapes
Diagrammatic transect across northeastern Siberia
showing distribution and thickness of permafrost
and thickness of the active layer.
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Periglacial Ice Wedges
In periglacial regions, the upper layer of the
ground, or active layer, is strongly influenced
by extremely low temperatures. Where surface
materials are fine, as the ground freezes and
contracts, it creates a vertical crack. See
animation on periglacial ice wedges in the
geodiscoveries section of your texts website.
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