GEO 303 Chapter Notes - Chapter 7: Basalt, Metamorphic Rock, Laterite

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10 May 2016
Geo Chapter 7 Notes:
Weathering: A transitional stage
oWeathered material doesn’t qualify as sediment because it hasn’t been
transported yet
oWeathering of Granite:
Not only temperature, but also pressure, the chemical
composition of the rock, and the presence of H2O or other
fluids must be considered
Chemical reactions may proceed in opposite directions in the
depths of the earth and at the surface
Moderate to high density minerals with closely packed crystal
structures are stable under high pressure
Low density minerals with open crystal structures are
favored at the earth’s surface
Water tends to be driven out at high temperature and
During weathering, the large crystals of granite are
reduced to ever smaller size
oAt first t he rock falls apart into a mass of loose
fitting coarse grains called grus
oBut with continued attack many of these grains
decompose chemically, then re form as very tiny
crystals of minerals, which incorporate H20
supplied from the atmosphere.
Clay minerals are a product of the weathering of
feldspar, the most abundant mineral in granite
oMechanical Weathering
Simply breaks down larger pieces of rock into smaller pieces.
Whereas chemical weathering causes the original minerals in a
rock to be transformed into a new and different set of minerals
Granite magma solidifies underground, when exposed, it
expands and develops fractures, or joints
As weathering continues, these joints pop apart along
sweeping curved sheets, which separate, crumble, and slide
downhill to join a pile of rubble at the base. This process of
exfoliation, the granite may be exposed as bald exfoliation
Another way that weathering occurs is through the sun, the
heat causes different minerals to heat up and expand, with
differing rates within the rocks, which will cause internal
fractures to occur
oChemical weathering
Every 10 celcius rise in temperature, the speed of chemical
reactions doubles
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