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 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 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.