Why Does the Ice Move?Ice is water frozen solid. However, when ice is close to its melting point, it is relatively weak (think of how ice becomes thinner and weaker as it melts over time in a glass of warm liquid). The force of gravity acting on a large mass like an ice sheet is strong enough to cause the ice to slowly flow—we can also say the ice deforms.
To give some perspective, a typical glacier is approximately 100 meters thick and flows 100 meters—the length of a football field—in one year. In comparison, Antarctica is hundreds of meters thick and can flow anywhere from a few meters a year to as much as thousands of meters a year—a football field in a month! The most rapidly moving ice is lubricated at its bed, which lowers the resistance to movement, and so it tends to slide more than deform.