Erosion

Weathering and erosion

Sandstone cliffs in the Sahara DesertSandstone cliffs in the Sahara Desert Over the millions of years of geological time, great mountain ranges have been forced up—then disappeared. Colliding tectonic plates, faulting and other earth movements have created these mountains. What has happened to them? Most have been ground down by the processes of weathering (attack by rainwater, frost and temperature change) and erosion (the removal of rock fragments by water, ice or wind).

The weathered slopes of Bryce Canyon, Utah, USAThe weathered slopes of Bryce Canyon, Utah, USA
Frost-wedgingFrost-wedging

Weathering

Attack by rainwater, frost, temperature change, plant roots and micro-organisms breaks down rocks in a process called weathering. Rocks heat up and expand in the sun, then cool and contract at night. The temperature changes, combined with chemical attack by water, crack the rock surface, causing small pieces to flake off. Rainwater seeps into crevices and, as it freezes into wedges of ice, expands with great force and splits off pieces of rock. This is known as frost-wedging. On slopes or cliffs, rockfalls and rockslides are the frequent result, with piles of scree accumulating at the bottom.

Deposition

Deforestation and overgrazing can speed up the forces of erosion and strip the land of soils.

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