Glaciers are gigantic, solid “rivers” of ice. They have been around for much of earth’s history, and are responsible for large geographic features on its surface, including the Great Lakes. Glaciers travel miles from their point of origin and deposit debris in wide swaths of land.
The question is, how does a solid like ice move like that? There are rockslides, but they are sudden and caused merely by erosion. Rockslides do not flow for miles in the way that glaciers do. So what is the cause of this glacial motion?
There are several mechanisms at play. The primary one has to do with the relationship between temperature and pressure. The melting point of most substances increases as the pressure increases – atoms pushed more closely together become more stable. This is not the case with ice. For ice, the melting point drops as pressure increases.
The ice at the bottom of glaciers is under enormous pressure. Some glaciers are over a mile deep. Through a combination of these extreme pressures and latent heat coming from the earth itself, some of the ice melts and gives the glacier above it a slick surface to slide down.
However, this melting process is not reliable. It varies depending on pressure and temperature variations. Therefore, glaciers only move slowly, between an inch and a couple of feet per day. The large variance in glaciers' flow speeds is due to the equally large variance in pressures and temperatures within the glacier.
Another mechanism is motions of the ice crystals within the glacier itself. A glacier is faster at its center, where there is the least friction with surrounding rock. Little ice particles, even in solid form, move tiny millionths of an inch in response to slight pressure changes and small inclines. The aggregate influence of all these little motions adds up to a significant global effect that propels the glacier forward.
Sometimes, glaciers move forward at an unprecedented pace, called a surge. For instance, in 1953, the Himalayan Kutiah Glacier moved seven miles in three months. Scientists are still not entirely sure of the cause of these surges, but they may occur when delicate structural arrangements within the glacier reach a “tipping point” and cause a cascade of collapses and a corresponding flow.