Glaciers are among the most conspicuous and dynamic geologic features on Mount Rainier in Washington state. They erode the volcanic cone and are important sources of streamflow for several rivers, including some that provide water for hydroelectric power and irrigation. Together with perennial snow patches, glaciers cover about 36 square miles of the mountain's surface, about nine percent of the total park area, and have a volume of about 1 cubic mile. To the casual observer, glaciers may seem to be rigid and unchanging, but in fact, they deform and flow continuously. Glaciers flow under the influence of gravity by the combined action of sliding over the rock on which they lie and by deformation, the gradual displacement between and within individual ice crystals.
Maximum speeds occur near the surface and along the centerline of the glacier.
Flow rates are generally greater in summer than in winter, probably due to the presence of large quantities of meltwater at the glacier base. The position of the snout, or terminus, of a glacier may change as the relative quantities of snowfall and glacier melt change. If summer melt exceeds winter snowfall, the terminus retreats, whereas if snowfall exceeds summer melt, the terminus advances. These changes in terminus position do not occur instantaneously, but typically take several years or more to become apparent. Glaciers are therefore sensitive indicators of climate changes.