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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.
@ Alchemy - You described the movement of glaciers across the bedrock, but glacial movement can also be examined as a natural force much like the ebb and flow of the tides. In this context glacial movement is referred to in terms of accumulation and ablation.
Accumulation is the addition of snow that turns to ice on a glacier; resulting in the glacier flowing down slope. The distance the glacier expands every year is the accumulation. Ablation is just the opposite. When glaciers actually melt, and retract this is referred to as ablation.
This natural cycle of accumulation and ablation has been thrown completely out of sync in the past century due to climate change. Some of the climate change is natural, but some is also the result of human influences on the environment. In many glaciers ablation is occurring at a faster rate than accumulation; resulting in the overall decrease in ice cover.
Glaciers have two main types of movement. The Basal sliding that the article describes is one way that glaciers move; although, this type of movement is more common at the toe of a glacier. This type of movement leaves behind characteristic grooves and valleys running parallel to the glaciers flow (Cirque glaciers actually leave behind bowls). Glaciers also leave their mark on the terminal landscape of the glacier. These can range from alluvial and till deposits to moraines, eskers, and kettles.
The other type of glacial movement is internal flow. The article did not touch on this type of glacial movement, but it is very significant. At the head of the glacier, friction holds the glacier to the bedrock, but internal stress causes the glacier to creep along its internal planes. This causes the glacier to move from its interior, with the oldest ice remaining at the depths of the glaciers highest elevation.