The process of weathering breaks down rocks exposed to the elements into smaller particles that can be carried away by wind and water erosion. Weathering is divided into three broad categories: mechanical or physical weathering, chemical weathering, and biological weathering. Mechanical or physical weathering is further divided by its causes into four different categories; the causes are mechanical exfoliation or unloading, thermal expansion, frost wedging, and abrasion.
Mechanical Exfoliation or Unloading
Exfoliation weathering is the process whereby curved layers or plates of rock break away from the rock beneath them leaving behind characteristic dome shaped monoliths such as those found in Yosemite National Park and in parts of the Idaho Batholith, a composite mass of granite rock in central Idaho. The cause of exfoliation is the expansion of the rock. When pressure on the rock is removed or reduced, as when a mass resting upon it is removed by erosion or other means, the rock expands, causing the plates to separate and fall away.
Thermal expansion weathering is caused by changes in temperature of the rock. The temperature of exposed rocks is alternately heated and cooled every day it is exposed. When it is heated it expands and when it cools it contracts, and since its constituent minerals expand and contract at different rates, stresses along the boundaries of the minerals cause the rock to crack and break apart.
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Frost wedging weathering has two causes. One cause is the expansion of freezing water inside the rock itself. Rock is porous and absorbs liquid water when it is exposed to it. As the temperature drops and the water inside the rock freezes, it expands and causes pieces of the rock to break away.
Alternatively, rocks that are repeatedly wet and dried can experience frost wedging weathering without freezing temperatures. This is true because water dissolves salt crystals from the wet rock that grow when the rock dries. This crystal growth puts the same sort of pressure on the rock as the expansion of freezing water.
Abrasion weathering is caused by the abrading or grinding together of rock particles. As rocks weather into smaller pieces, they are moved around by wind and water and rub against each other, grinding and abrading off more pieces of rock. For instance, small wind-blown rock particles have the same effect on rocks they come in contact with as sand blasting has. Similarly, small rocks pushed along by a stream of water grind against each other, chipping and scraping away bits of rock.