Describe how glaciers move. Which property or properties of ice allow this movement?

The movement of glaciers is referred to as flow. Glacial flow happens two ways: plastic flow and basal slip. Plastic flow involves movement within the ice. Under high enough pressure, the normally brittle ice begins to distort and change shape-a property known as plasticity. The weight of overlying ice exerts this pressure on the ice below, causing it to flow. Plastic flow begins at about 50 meters below the glacier surface. Basal slip is the second cause of glacial movement. Due to gravity, the entire ice mass actually slips and slides downhill along the ground . The upper 50 meters of a glacier is not under enough pressure to have plastic flow. The surface of the glacier behaves differently than the ice below. This uppermost zone of a glacier is brittle, and it is referred to as the zone of fracture. This brittle topmost ice piggybacks a ride on the flowing ice below. The zone of fracture experiences tension when the glacier moves over irregular terrain. This tension results in gaping cracks called crevasses.