Nancy Lyle, age 11, of St. Louis, Mo., for` her question:
What causes an avalanche?
Some time ago, we were all shocked at the news of an avalanche in the Andes of South America. Countless tons of rock and debris plunged down the slopes to bury whole villages in a few minutes. So fast was the fall that no one in its path escaped. If geologists had time to discover the exact make up of every mountain, they could figure the dangerous spots where avalanches could occur and give warning to prevent such disasters.
The word avalanche means a falling and it is used to describe the vast tons of rock and earth or snow and ice that tumble suddenly down the slope of a mountain. The bulky material falls because it is pulled down by the force of the earth's gravity. A high peak can perch up in the clouds only because it is supported by the massive shoulders of a mountain. When this support is weakened, material at the top of the mountain tumbles down the slopes.
The earths crust is forever poking up new mountain ridges and the force of gravity is forever trying to level them down. In this age long battle, the weather fights on the side of gravity to smooth down the mountains. Frosts and thaws, winds and running water weaken the lofty mountains with cracks.
The mountain is like a wrinkled sandwich of rocky layers. Some of these layers are less durable than others. Running water dissolves them, the wind crumbles and carries them away. ', .then deep layers in a mountain become weak,.the sturdy layers on top lose some of their support. The mountain is top heavy and, like a lopsided wall of bricks, sooner or later great chunks of rock and debris come tumbling down.
In winter, the lofty peaks of the Alps are covered with blankets of drifting snow. Spring brings thaws and Spring brings thaws and gushing streams of melting water seep down and biter out cracks into the massive snow banks. Bit by bit, the higher snows lose the support below them and down spills an avalanche. On asmall scale, this can happen to a heavy snowfall on the sloping roof of house. Heat from the house warms and melts the lowest layer of the snow, the rest of the white blanket slides down In a miniature avalanche.
Tunnels, caves and quarries can weaken the structure of a steep hill. This undermining of support can also cause the upper layers of a hill to avalanche down the slope. Gushing spring rains can eat away the support of a hill and the upper levels may slide down in la sea of mud.
An avalanche is a fast and dramatic event and countless tons of earth can be moved in a few minutes. But the same kind of earth shifting is going on all the time at a much slower rate, every stream that runs down a slope, every melting snowfall, every wind that whistles high in the hills loosens some of the rocks and debris and brings them sliding down to ground level.