What makes it a glacier?

Science Center Objects

The retreat of alpine glaciers in Glacier National Park (GNP), Montana, reflects changes in recent climate as glaciers respond to altered temperature and precipitation.  USGS scientists in GNP are studying glacier retreat to understand glacier-climate interactions and to provide a scientific foundation for land managers.

A glacier is a perennial accumulation of ice, snow, water, rock and sediment that moves under the influence of gravity. A glacier forms when winter snowfall exceeds summer melting. Many years of accumulation result in compaction of the bottom layers of snow, turning them to ice. The weight of the snow mass causes the ice to become flexible and move downhill. Glaciers must have sufficient ice mass to flow under gravity. This usually requires glaciers to be 100 ft. thick or more. Retreating glaciers are those that melt more than they accumulate new snow. Glaciers are dynamic – changing in response to temperature, precipitation, and other geologic processes. Ice is continuously cycling through a glacier, going from snow to ice, and that moves down and out as water. The ice does not persist forever in the glacier, although the glacier remains if there is enough snow accumulation to offset melting. The USGS Climate Change in Mountain Ecosystems Program minimum size criterion for a glacier is 0 .1 km2 (100,000 m2), or about 25 acres. Below this size, the accumulated ice generally does not move and is stagnant.