Ronny Johnson, age 12, of Houston, Texas, for his question:

Why do glaciers move?

Ice is a mineral, in the same class with the minerals of the earths crust. However, it is the weakest of earthts minerals, far weaker than any rock. It is so weak that when it reaches a certain mass, it collapses under its own weight. This collapse of ice under its own weight is what causes the movement of glaciers.

When a glacier becomes 200 to 300 feet thick, it begins to move under its own weight. If it is a mountain glacier, it flows slowly down hill. If it is a flat ice field, it begins to spread out from the center to the edge Some glaciar mova less than one inch a year, some move 20 feet or more.