This post was originally published on snow.news, my free Substack on snow science and the state of the snowpack.
A glacier is a large, perennial mass of ice and snow that moves slowly downhill under the influence of gravity. A glacier will form if the accumulation of snow and ice exceeds their loss through ablation (via melting, evaporation, sublimation, and other forces).
As the snow accumulates, it compresses over time, turning into firn before becoming glacial ice, as shown in the illustration below.
Glaciers around the globe are threatened by warming and are used as a barometer of climate change effects. The graphic below, from the U.S. Environmental Protection Agency, shows a precipitous decline in reference glaciers around the planet from 1956 to 2023.
The photographs below show how Alaska’s McCall Glacier changed from 1958 to 2023.
Past Ice Ages covered many of the West’s mountains in ice, and the current landscape still reflects these past glaciations, which both erode and deposit material. At the height of the Last Glacial Maximum, around 20,000 years ago, about one-quarter of the Earth’s land area was covered by glaciers.
The diagrams below show some common glacial landforms, followed by definitions of terms most relevant to the American West.
Mini-glossary of glacier-related terms
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Arête: sharp, narrow ridge formed between two cirques or glacial valleys
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Cirque: bowl-shaped depression carved by a glacier at the head of a valley
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Col: saddle-shaped pass between peaks formed by two glaciers on either side of a ridge
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Comb: jagged ridge caused by glacial erosion
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Crevasse: deep crack in a glacier’s surface caused by stresses from the ice’s movement
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Drumlin: elongated hill of glacial sediment shaped by the flow of ice
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Esker: long ridge of stratified sediment deposited by meltwater streams flowing beneath a glacier
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Firn: snow that has been compacted and crystallized but not yet converted to glacial ice
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Glacial erratic: rock or boulder transported by a glacier far from its origin
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Glacial step: step-like formation caused by differential erosion in glacial valley
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Hanging valley: a tributary valley at a higher elevation due to glacial erosion of the main valley; often includes a waterfall
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Horn: pyramid-shaped peak formed by the erosion of at least three cirques
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Kettle lake: depression formed by the melting of a block of ice in glacial outwash
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Mass balance: the difference between the amount of snow/ice gained through accumulation and the volume of snow/ice lost through ablation; glaciers grow due to a positive balance and shrink due to a negative balance
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Moraines: deposits of glacial sediment, including terminal (at the toe of a glacier), lateral (along the sides of a glacier) and medial (when lateral moraines join at the intersection of glaciers)
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Nunatak: a peak that protrudes above the surrounding glacier or ice sheet and is not covered by ice
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Outwash plain/delta/fan: formation created by the deposition of sediment as meltwater flows out of a glacier and deposits sand/gravel in a spreading formation
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Tarn: small mountain lake in a cirque
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U-shaped valley: a valley with a wide, relatively flat floor and steep sides, formed by the carving of a glacier (in contrast to a V-shaped valley caused by river erosion)
Where are glaciers in the West?
According to the Glaciers of the American West, an online resource by Portland State University researchers, there are 8,348 glaciers in eight Western states, not including Alaska, with the most by far in Washington. These glaciers and permanent icefields range from “rivers of ice on Mount Rainier that are over 8 km (5 mi.) long to tiny patches of ice in the Rocky Mountains not much larger than a city block,” according to the site.
The diagram below shows the general locations where the remaining glaciers are found in the American West—not all purple areas are perennially frozen!
