How Glaciers Move

Though the terminal end (lower end) of a glacier may be stationary, or even retreating uphill, the ice within the glacier continually moves downhill. The speed of this motion ranges from a few centimeters per day to more than 10 meters per day, but normal flow rates are generally nearer the lower figure.

If the ice at the base of a glacier is much below the temperature of freezing, as happens in winter, or perhaps even in summer at high altitude or in polar regions, the ice bonds tightly to the rock below. Then, the entire motion is within the glacier. The ice itself deforms as individual ice crystals warp and creep past each other.

Most of the deformational creep takes place in the deepest layer of the ice, near to the basal layer. It is here that the greatest stress is placed upon the ice because the stress is proportional to the thickness of ice above. Consequently, little deformation in the near-surface glacier ice occurs; the ice there is mostly carried along by the motions below.

Ice deforms most easily when it is warm. Especially in winter, the ice near the glacier's surface can be comparatively brittle, a fact that helps explain why crevasses form. These tensional cracks or other types of breakage occur where or when changes in stress are too abrupt or rapid for deformational creep to keep up with.

In summer, when the basal layer of the glacier ice warms up to near its melting point, a glacier begins to move by sliding on its bed. Thawing and refreezing take place, and water lubricates the rock surface. The pace quickens as both sliding and creep act in unison to move ice downhill.

Investigators are finding that the overall rate of glacier flow changes hour by hour and day by day, as well as with the seasons. The reasons for the variation are not understood, though availability of water in the basal layers is suspected to be a main cause. In some instances speedups have been observed after a few days of increased rainfall.