What makes it a glacier?

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.

Resources:

Key, C. H., D. B. Fagre, and R. K. Menicke. 2002. Glacier retreat in Glacier National Park, Montana. Pages J365-J381 In R. S. Jr. Williams and J. G. Ferrigno, (eds.) Satellite Image Atlas of Glaciers of the World, Glaciers of North America - Glaciers of the Western United States. U.S. Geological Survey Professional Paper 1386-J. United States Government Printing Office, Washington D. C., USA.

Related Links:
All About Glaciers, National Snow and Ice Data Center
Glaciers and Glacial Features, Glacier National Park website