Ralph Gust, age 10, of Lethbridge, Alberta, Canada, for his question:

What causes erosion?

Rains and melting snows seep through the soil and stream over the surface. All this moving water dissolves the earth's rocky minerals and carries countless tons of dirt from here to there. It wears away the high spots and dumps the debris in the low spots. It washes away crumbly soil and gouges gullies in the ground. Erosion of this kind is caused by moving water seeping down or running over the surface of the earth.

Winds also cause erosion, especially in dry desert regions. There they blow around clouds of sandy dust. They hurl the gritty grains at cliffs and hillsides and the hard solid rocks are scratched, scarred and gradually worn away. Frost and ice also cause erosion. Frost cracks rocks and makes it easier for water to wash away the broken fragments. Moving glaciers shift huge boulders and masses of loose gravel. They also claw scratches in the hardest rocks. The main forces of erosion are moving water, winds and ice.