Mechanical weathering breaks down rocks.
Mechanical weathering, according to the United States Geological Survey, is the process by which rocks break down into smaller pieces. This happens a number of ways, including landslides and tree root growth. It is one of the key aspects of the rock cycle, which involves the processes that change rocks and the earth's surface.
Impact from Other Rocks
One type of mechanical weathering is the breaking up of rock through impact from other falling rocks. During a landslide, rocks tumbling down a cliff face or a hill come apart, leaving behind smaller pieces. They also fall onto other rocks and break them apart. With enough impact over time, these rocks break down into smaller grains that can be picked up by the wind or washed away by rain.
Freeze and Thaw
When water seeps down into cracks in the rock and starts to freeze, its volume increases. As the water begins to form into ice crystals, it pushes with enough force to begin to fracture the rock. The ice then thaws during the warmth of the day, allowing the water to seep deeper into the crack. The next night, as it freezes again, the ice expands and pushes on the rock and finally begins to break it apart.
Abrasion
This aspect of mechanical weathering has a slightly different effect on the rocks on the Earth's surface. Rocks that have been broken into very small pieces, such as sand grains, can be easily moved by water or wind. Grains picked up by the wind come into contact with larger rocks, and scour the surface and begin to wear it down. The same can happen with larger rock fragments that are moved along with ice from a glacier. These fragments strike rocks below the water, breaking off pieces and at the same time damaging themselves.
Tree Root Growth
As tree roots grow and push down into the soil, they often come into contact with rocks. Many times the roots will actually grow into a crack in the rock, and as the root gets bigger it exerts a force on the rock. Over time the fracture in the rock will be pushed to the point that it can actually break the rock apart.
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