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SPHEROIDAL
WEATHERING
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• Spheroidal Weathering is a form of
chemical weathering. This occurswhen concentric shells of decayed
rock are successively loosened and
separated from a block of rock bywater penetrating the bounding
joints or other fractures and
attacking the block from all sides.
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• weathering occurs whenever amass of rock (most typically granitic
in composition), experiences a
drastic reduction in ambient heat
and pressure
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• eathering form in massive rocks,
granite, dolerite, basalt, and evensome sedimentary types such as
silicified sandstones, results in the
production of spheroidal boulders ofunweathered rock, around which
are to be observed layers or flakes
of disintegrated material
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• The process is closely related to
that of exfoliation, except thatthe latter refers usually to large!
scale sheeting on the scale of a
hill, or whole igneous dome,
while spheroidal weathering is
restricted to boulder si"edphenomena.
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GRANITES
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in granites, there are three mutually
perpendicular sets of joints that
develop when this overburden isremoved. Two things cause this in
granites#
• the quartz crystals expand about
5
• acidic !ater attac"s the #eldspar$inerals% turning the$ into clay&
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• $dges and especially corners
of an angular block weatherfaster than flatter surfaces.
The ultimate result of this
process is a rounded boulder
or a dome monolith
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'()E )(N(*IT+
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%n this basalt
outcrop,
groundwater
penetrates along
joints and fractures,
loosening and
decaying the rock
layer by layer. &sthe process
progresses into a
block, the surface of
weathering growsmore and more
rounded.
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'pheroidal
weathering
affects thismassive
mudstone in a
bluff above the
$el iver in
northern
alifornia. %t may
also be calledconcentric
weathering.
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FACTORS WHICH
CONTROL THE RATES OF
WEATHERING
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PROPERTIES OF THE PARENT ROCK
*. The mineralogy and structure
of a rock affects it+s susceptibilityto weathering.
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. -ifferent minerals weather at different rates.
afic silicates like olivine and pyroxene tend to
weather much faster than felsic minerals like/uart" and feldspar. -ifferent minerals show
different degrees of solubility in water in that
some minerals dissolve much more readily
than others. ater dissolves calcite more
readily than it does feldspar, so calcite is
considered to be more soluble than feldspar.
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01$2+' $&T%12 '$%$'
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3. & rock+s structure also affects its
susceptibility to weathering. assive
rocks like granite generally to not containplanes of weakness whereas layered
sedimentary rocks have bedding planes
that can be easily pulled apart andinfiltrated by water. eathering therefore
occurs more slowly in granite than in
layered sedimentary rocks.
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4%&T$
*. ainfall and temperature can
affect the rate in which rocks
weather. 5igh temperatures andgreater rainfall increase the rate
of chemical weathering.
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. ocks in tropical regions
exposed to abundant rainfall and
hot temperatures weather muchfaster than similar rocks residing
in cold, dry regions.
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'1%4
*. 'oils affect the rate in which a rock
weathers. 'oils retain rainwater so that rocks
covered by soil are subjected to chemical
reactions with water much longer than rocksnot covered by soil. 'oils are also host to a
variety of vegetation, bacteria and organisms
that produce an acidic environment which
also promotes chemical weathering.
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. inerals in a rock buried in
soil will therefore break down
more rapidly than minerals ina rock that is exposed to air.
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4$26T5 17 $891':$
*. The longer a rock is exposed to the
agents of weathering, the greater the
degree of alteration, dissolution and
physical breakup. 4ava flows that are/uickly buried by subse/uent lava flows
are less likely to be weathered than a flow
which remains exposed to the elementsfor long periods of time.