GEO 101 Lecture Notes - Lecture 21: Royal Aircraft Factory F.E.2, Humus, Hematite
24 Jun 2018
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Processes of Mechanical Weathering
Ice. The formation of ice in the myriad of tiny cracks and joints in a rock's surface slowly
pries it apart over thousands of years. Frost wedgingresults when the formation of ice
widens and deepens the cracks, breaking off pieces and slabs. Frost wedging is most
effective in those climates that have many cycles of freezing and thawing. Frost
heaving is the process by which rocks are lifted vertically from soil by the formation of
ice. Water freezes first under rock fragments and boulders in the soil; the repeated
freezing and thawing of ice gradually pushes the rocks to the surface.
Exfoliation. If a large intrusion is brought to the surface through tectonic uplift and the
erosion of overlying rocks, the confining pressure above the intrusion has been
released, but the pressure underneath is still being exerted, forcing the rock to expand.
This process is called unloading. Because the outer layers expand the most, cracks,
or sheet joints, develop that parallel the curved outer surface of the rock. Sheet joints
become surfaces along which curved pieces of rock break loose, exposing a new
surface. This process is called exfoliation; large rounded landforms (usually intrusive
rocks) that result from this process are called exfoliation domes.Examples of
exfoliation domes are Stone Mountain, Georgia, and Half Dome in Yosemite National
Park.
Friction and impact. Rocks are also broken up by friction and repeated impact with
other rock fragments during transportation. For example, a rock fragment carried along
in a river's current continuously bounces against other fragments and the river bottom
and eventually is broken into smaller pieces. This process occurs also during
transportation by wind and glacial ice.
Other processes. Less important agents of mechanical weathering include
the burrowing of animals, plant roots that grow in surface cracks, and the digestion of
certain minerals, such as metal sulfides, by bacteria. Daily temperature changes,
especially in those regions where temperatures can vary by 30 degrees centigrade,
result in the expansion and contraction of minerals, which weaken rocks.
Extreme temperature changes, such as those produced by forest fires, can force rocks
to shatter.
Processes of Chemical Weathering
When a rock is brought to the surface millions or billions of years after it has formed, the
original minerals that were crystallized deep in the crust under high pressures and
temperatures are unstable in the surface environment and eventually break down. The
primary agents in chemical weathering are water, oxygen, and acids. These react with
surface rocks to form new minerals that are stable in, or in equilibrium with, the
physical and chemical conditions present at the earth's surface. Any excess ions left
over from the chemical reactions are carried away in the acidic water. For example,
feldspar minerals will weather to clay minerals, releasing silica, potassium, hydrogen,
sodium, and calcium. These elements remain in solution and are commonly found in
surface water and groundwater. Newly deposited sediments are often cemented by
calcite or quartz that is precipitated between the sediment grains from calcium and ‐
silicabearing water, respectively.
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