Canyons form over millions of years of drastic physical weathering.
Mechanical weathering is the disintegration of rocks or rock surfaces by physical action. Some weathering occurs rapidly, especially affected by the flow of water or the wind, and some occurs very slowly, caused by natural stresses on rocks. Mechanical weathering processes are distinct from chemical weathering processes, but rocks usually are affected by both.
Constant wave action shapes steep sea cliffs like these at Big Sur, California.
Hydraulic weathering occurs when incessant and sometimes powerful water currents abrade rocks. This type of weathering works constantly on sea cliffs, for example, with pounding waves breaking up sheer rock formations. Some dramatic geological formations, like Arizona’s Grand Canyon, are the result of intense hydraulic action, in this case the cutting action of a river, over a very long time.
Desert sand dunes are formed by constant action of the wind.
In desert environments, temperatures range from extremely hot during the day to very cold at night. This extreme change in temperature, combined with the action of high desert winds, causes the top layer of desert rock formations to shear off minutely due to the endless stress. A great deal of desert sand is created in this way.
Ice is a powerful mechanical force that eventually breaks the largest rocks down to sand, carried off by the meltwater.
Winter freezes force water into minute cracks and fissures in rock formations. When this water accumulates and freezes, it expands, putting pressure on the fissure and weakening the rock. This allows even more water and ice to accumulate, leading to further expansion. Eventually rocks may split from this action, an early step in the gradual breakdown of this rock by other weathering forces into smaller and smaller pieces, and finally into sand and silt particles.
Glacial action, one form of mechanical weathering, shaped Yosemite valley. Exfoliation and other mechanical weathering forces continue to shape the rocks.
When igneous rock such as granite is exposed to the surface by weathering of surrounding rock, it often weathers in a process called exfoliation. Layers break off of the exposed granite, often forming it into rounded boulders. This happens because the formerly molten rock cooled under pressure far below the surface of the earth. When it reaches the surface and is exposed, the pressure under which it formed is released, and the rock tends to break apart in slabs along the lines of least pressure.