Spheroidal Weathering

8/9/2019 Spheroidal Weathering

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SPHEROIDAL

WEATHERING



• 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.



• weathering occurs whenever amass of rock (most typically granitic

in composition), experiences a

drastic reduction in ambient heat

and pressure



• 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





• 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.



GRANITES



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&



• $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



'()E )(N(*IT+



%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.



'pheroidal

weathering

affects thismassive

mudstone in a

bluff above the

$el iver in

northern

alifornia. %t may

also be calledconcentric

weathering.



FACTORS WHICH

CONTROL THE RATES OF

WEATHERING



PROPERTIES OF THE PARENT ROCK

*. The mineralogy and structure

of a rock affects it+s susceptibilityto weathering.



. -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.



01$2+' $&T%12 '$%$'



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.



4%&T$

*. ainfall and temperature can

affect the rate in which rocks

weather. 5igh temperatures andgreater rainfall increase the rate

of chemical weathering.



. ocks in tropical regions

exposed to abundant rainfall and

hot temperatures weather muchfaster than similar rocks residing

in cold, dry regions.



'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.



. inerals in a rock buried in

soil will therefore break down

more rapidly than minerals ina rock that is exposed to air.



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.

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Spheroidal Weathering