Glaciers move basically because of the force of gravity. Glaciers begin forming in places where more snow piles up each year than melts. Soon after falling, the snow begins to compress or become denser and tightly packed. It slowly changes from light, fluffy crystals to hard, round ice pellets. New snowfalls and buries this granular snow. The hard snow becomes even more compressed. It becomes dense, grainy ice called firn. The process of snow compacting into glacial firn is called firnification.
As years go by, layers of firn build on top of each other. When the ice grows thick enough—about 50 meters (160 feet)—the firn grains fuse into a huge mass of solid ice. The glacier begins to move under its own weight. The glacier is so heavy and exerts so much pressure that the firn and snowmelt without any increase in temperature. The meltwater makes the bottom of the heavy glacier slicker and more able to spread across the landscape. The movement could be a few centimetres to a few metres a day or even less or more.