Overview
Surface processes such as weathering and erosion transform mountains and other landforms. Rocks and minerals disintegrate into sediment, and it is transported into other areas, building up new landforms.
Mechanical Weathering
At the surface, rocks and minerals crumble into smaller pieces from a variety of forces. Many of these forces, known as mechanical weathering, do not change their chemical composition. For example, when water freezes in a crack in a boulder, ice expands to widen the crack. When the ice thaws, the boulder fractures into smaller rocks. Rocks can also be ground into smaller pieces by friction when other rocks rub against them, driven by wind or water.
Chemical Weathering
Chemical weathering changes the chemical composition of the rocks and minerals themselves. Corrosive chemicals in water as a result of pollution can eat away at rock. Although natural rainfall is slightly acidic, sulfur dioxide and nitrogen oxides combine with water as acid rain. Oxygen in the air rusts metals. Feldspar in granite turns to clay when it combines with water. Clay takes up more space as it expands, resulting in mechanical weathering of the remaining rock, similar to that caused by ice.
Erosion
Weathered rock is carried from place to place by erosion. For example, glaciers carry rocks down mountain slopes as they flow and melt. Rocks are ground into smaller pieces, and the sediment is transported as debris. Water in rivers and streams bring sediment long distances, depositing it to form river deltas. Streams and rivers carve canyons through cliffs of sedimentary rock over time. Wind blows sand across rock, sandblasting patterns into cliff faces in desert areas.
Mass Wasting
The greatest destruction from erosion occurs when steep slopes give way, called mass wasting. Most of the time, rock cliffs or soil are fairly stable, held in place by gravity. If the area is undermined by flowing streams or saturated by heavy rainfall, however; sudden landslides careen down unstable slopes. Avalanches of rocks, snow, and other debris devastate the steepest slopes regularly. Earthquakes and volcanic eruptions can trigger huge landslides, as the tilted faults become unstable.
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