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Physical weathering represents the disintegration of rocks and
minerals into smaller pieces. Physical weathering can be further
subdivided into pressure release and ice wedging.
- Pressure Release
Rocks below the earth surface support the
weight of the overlying column of rock. Erosion strips away this overlying
rock and decreases pressure on buried rocks. All rocks are slightly
elastic, so the buried rocks respond to the reduction of pressure by
expanding upwards. This results in the formation of pressure release
fractures (cracks) that form parallel to the surface. With continued
erosion, these rocks are exposed on the surface and slabs of rock break
off along the pressure release fractures. This weathering creates bare
rock surfaces that may be more resistant than surrounding rocks. These
features are termed exfoliation domes; the slabs of rock that
break off are termed exfoliation sheets.
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Independence
Rock, Wyoming; an example of an exfoliation dome. Note bare rock
and rounded surfaces. Image courtesy of Dr. Annabelle Foos.
Click here to see another example of an exfoliation dome, Half
Dome, Yosemite National Park, California.
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- Ice Wedging
Ice wedging occurs as the result of the
expansion (9%) of water as it is converted to ice. Cracks filled with
water are forced further apart when it freezes. This process requires a
range of temperatures to generate alternating freeze - thaw activity.
Both pressure release and ice wedging break up rocks and minerals,
creating more surface area for chemical weathering. |
