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Processes of Mechanical
Weathering
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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 wedging results 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 calledunloading. 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 impactwith
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 theburrowing
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. Extremetemperature changes, such as
those produced by forest fires, can force rocks to shatter.
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