Bill Wallace, age 12, of Concord, N.H., for his question

WHAT CAUSES AN AVALANCHE?

An avalanche is a sudden flow of a large mass of snow or ice down a slope or cliff, sometimes at speeds exceeding 100 miles per hour.

Avalanches are set off by a combination of factors, including temperature, shearing of creeping snow masses and sudden vibrations, including loud noises.

Snow patrols in mountain areas reduce the hazard of avalanches by detonating strategically placed explosives that cause smaller, less destructive flows.

Avalanches are most .common on slopes exceeding 30 degrees, frequently when a deep snow falls suddenly and does not have a chance to cohere, or when a,thaw undercuts a blanket of older snow. Pelletlike snow, called graupnel, is also more prone to avalanche than a fall of ordinary snowflakes. Flows of wind packed slabs of snow can be especially hazardous.

Avalanches can be destructive of life and property.

Similar massive movements of rock and soil are called landslides.