Weathering and erosion

At the foot of a mountain or cliff you often see heaps of sand, soil and stones. These are sediments. Where did they come from?

Big rocks are slowly broken into smaller pieces by physical changes:

• temperature changes: hot days and cold nights
• growing roots: a seed in a crack grows roots that slowly force the rock apart (you may have seen a plant growing out of an old wall)
• water freezing in cracks: ice takes up more room than water and pushes the crack wider

Rocks also change chemically. Basalt is a black rock that contains iron. After a long time in water, or in moist air, its surface turns red: the iron has become iron oxide, the same as rust.

These physical and chemical changes in rocks together are called weathering, and over a long time weathering makes soil.

Erosion is when rock pieces, soil and sand are carried away by wind and flowing water. A landslide is erosion, and a physical change. River pebbles are smooth because water has rubbed them for years. Where the water or wind slows down, as in a lake or the sea, the material settles at the bottom, and over time it hardens into new rock.

Most of these changes take thousands of years, and they cannot be reversed.