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Why do they move?
Glaciers periodically retreat or advance, depending on the amount of snow accumulation or ablation that occurs. This retreat or advance refers only to the position of the terminus, or snout, of the glacier. Even as it retreats, the glacier still deforms and moves down slope, like a conveyor belt. For most glaciers, retreating and advancing are very slow occurrences, noticeable only over a long time. However, when glaciers retreat rapidly, movement may be visible over a few months or years. For instance, massive glacier retreat has been recorded in Glacier Bay, Alaska. Other glaciers have been photographed at intervals showing dramatic recession.
Alternatively, glaciers may surge, racing forward several meters per day for weeks or even months. In 1986, the Hubbard Glacier in Alaska began to surge at the rate of 10 meters (32 feet) per day across the mouth of Russell Fjord. In only two months, the glacier had dammed water in the fjord and created a lake. This illustrates how quickly a surging glacier can change its surroundings.
NSIDC's Glacier Glossary - Search and browse terms related to glaciers in NSIDC's comprehensive cryospheric glossary.
NSIDC Glacier Photograph Collection - NSIDC archives a Glacier Photograph Collection of historical photos, which includes both aerial and terrestrial photos for the 1880s to 1975. The photos are primarily of Alaskan glaciers, but coverage also includes the Pacific Northwest and Europe.