Mark Jung, age 12, of Des Moines, Iowa, for his question:
HOW CAN GLACIERS MOVE?
Many glaciers are perched between lofty mountain peaks. It is easy to imagine how and why they manage to slide down the slopes. But flat glaciers also move all the time. The great Antarctic icecap is a mile thick and big enough to cover half of North America. It is hard to imagine that this enormous slab of ice manages to move.
As a rule, the earth's major projects are performed by zillions of small units. In the case of moving glaciers, most geologists suspect that the teamwork depends on mini fragments of slippery ice. The operation begins when the polar atmosphere builds feathery snowflakes from tiny ice crystals arranged around tiny pockets of air. In polar regions and on lofty mountains, winter snowfalls accumulate on the ground from season to season.
Through the years new snows are added, and the lower levels are crushed and impacted to form dense glacial ice. More ice forms as surface layers melt and refreeze. Yet the whole mass of solid, rigid ice is made from mini crystals of slippery, slithery ice. Because of this particular crystal structure, the whole glacial mass is unstable. Unlike most rocky minerals, ice is classed as a fragile mineral, too weak to support its own weight.
When the weight of the ice reaches a certain limit, its interior crystal structure of slippery ice fragments begins to slip and slide. As a rule, the limit occurs when a glacier reaches a thickness of 200 to 300 feet. Other factors decide where and how fast it must move.
Valley glaciers tend to move fastest because the earth's gravity helps them to flow down the slopes. They inch down like frozen rivers, which is what they are. Flat glaciers and icecaps move outward from the heavy center, pushing toward the edges. After thousands of years, this winter's polar snowfalls finally will be pushed out to meet the Antarctic seas in walls of solid ice.
Slowpoke glaciers move just a few inches, and even the speeders move only 20 to 30 feet per day. The speed and direction are determined by the size, weight and location of the glacier. But the whole operation depends upon the mini ice fragments that form its fragile crystal structure.