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The Epic Interactive Encyclopedia 1998
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Epic Interactive Encyclopedia, The - 1998 Edition (1998)(Epic Marketing).iso
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Weathering
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INFOTEXT
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1992-09-02
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Process by which exposed rocks are broken
down by the action of rain, frost, wind, and
other elements of the weather. Two types of
weathering are recognized: physical and
chemical. They usually occur together.
Physical weathering involves such effects as:
frost wedging, in which water trapped in a
crack in a rock expands on freezing and
splits the rock; sand blasting, in which
exposed rock faces are worn away by sand
particles blown by the wind; and soil creep,
in which soil particles gradually move
downhill under the influence of gravity.
Chemical weathering is a process by which
carbon dioxide in the atmosphere combines
with rainwater to produce weak carbonic acid,
which may then react with certain minerals in
the rocks and break them down. Examples are
the solution of caverns in limestone
terrains, and the breakdown of feldspars in
granite to form china clay or kaolin, thus
loosening the other minerals present - quartz
and mica - which are washed away as sand.
Although physical and chemical weathering
normally occur together, in some instances it
is difficult to determine which type is
involved. For example, onion-skin weathering,
which produces rounded inselbergs in arid
regions, such as Ayers Rock in central
Australia, may be caused by the daily
physical expansion and contraction of the
surface layers of the rock in the heat of the
Sun, or by the chemical reaction of the
minerals just beneath the surface during the
infrequent rains of these areas. Weathering
Physical weathering
temperature changes - weakening rocks by
expansion and contraction frost - wedging
rocks apart by the expansion of water on
freezing rain - making loose slopes unstable
wind - wearing away rocks by sandblasting,
and moving sand dunes al ong
unloading - the loosening of rock layers by
release of pressure aft er the erosion and
removal of those layers above Chemical
weathering
carbonation - the breakdown of calcite by
reaction with carbonic ac id in rainwater
hydrolysis - the breakdown of feldspar into
china clay by reaction with carbonic acid in
rainwater
oxidation - the breakdown of iron-rich
minerals due to rusting hydration - the
expansion of certain minerals due to the
uptake of water
Gravity
soil creep - the slow downslope movement of
surface material landslide - the rapid
downward movement of solid material avalanche
- scouring by ice, snow, and accumulated
debris Rivers
abrasion - wearing away stream beds and banks
by trundling boulders along
corrasion - the wear on the boulders
themselves as they are carried along
Glaciers deepening of valleys by the weight
of ice scouring of rock surfaces by embedded
rocky debris Sea hydroulic effect -
expansion of air pocketsin rocks and cliffs
by constant hammering by waves abrasion and
corrasion - see Rivers