After about two winters, the snow turns into firn, which is an intermediate state between snow and ice. At this point, its density is about half that of water. Over time the larger ice crystals become so compressed that the air pockets between them are very tiny. In very old glacier ice, crystals can reach several inches in length. For most glaciers, this process takes over a hundred years.
Surface generally moves faster than the base.
Middle moves faster than the sides. When a glacier encounters an increase in the slope of its bed, crevasses form Form where the surface is in tension and close where it is in compression. Crevasses extend only down to the zone of plastic deformation where they close up. When a glacier encounters a very steep slope in its bed, flow may become chaotic as in an icefall. The irregular ice blocks are called seracs and may be extremely unstable.