Glaciers in the Western US
The vast majority (nearly 97%) of the glaciers in the United States are found in the Western States. Although the bulk of these are found in Alaska, small glaciers are found as far south as California and Nevada. These glaciers have a profound impact on the West’s scenery, geology, and water resources. Furthermore, ongoing research into how they have changed since the last major ice age is proving invaluable to our understanding of climate change.
What is a glacier?
A glacier is a large mass of ice (usually covered by snow) that is heavy enough to flow like a very thick fluid. Glaciers form in areas where more snow accumulates than is lost each year. As new snow accumulates, it buries and compresses old snow, transforming it from a fluffy mass of snowflakes into ice crystals with the appearance of wet sugar, known as firn. As this firn is buried yet deeper, it coalesces into a mass of hard, dense ice that is riddled with air bubbles. Much of this transformation takes place in the high part of a glacier where annual snow accumulation outpaces snow loss—a place called the accumulation zone. At a depth greater than about 50 meters (165 feet), the pressure is high enough for plastic flow to occur. Ice flow is driven by gravity, and it causes movement downhill and out from the center (Figure 6.1). Once the ice becomes thick enough, it flows outward to the ablation zone, where the ice is lost due to melting and calving (Figure 6.2). The boundary between these two zones, the equilibrium line, is where annual ice accumulation equals annual ice loss. Because the altitude of this line is dependent on local temperature and precipitation, glaciologists frequently use it to assess the impact of climate change on glaciers.
Figure 6.1: As dense glacial ice piles up, a glacier is formed. The ice begins to move under its own weight and pressure.
Most broadly, there are two types of glaciers: smaller alpine glaciers and larger continental glaciers (Figure 6.3). Found in mountainous areas, alpine glaciers have a shape and motion that is largely controlled by topography, and they naturally flow from higher to lower altitudes. Glaciers confined to valleys are called valley glaciers, while bowl-shaped depressions called cirques are located in mountainous areas.
Figure 6.2: Cross section of an alpine (valley) glacier showing snow being converted into glacial ice and the two major zones of a glacier’s surface. The red arrows show the direction and relative speed of different parts of the glacier. The longer the arrow, the faster the ice is moving.
Figure 6.3: The cirque and valley glaciers shown here are types of alpine glaciers. Both alpine and continental glaciers flow downhill. Although this is obvious with an alpine glacier, it is not as much so with a continental glacier. In the case of the latter, the glacier flows from its thicker middle to its lower edges.
Continental glaciers are much larger, and they are less controlled by the landscape, tending to flow outward from their center of accumulation. Ice sheets are large masses of ice that cover continents (such as those found in Greenland) or smaller masses that cover large parts of mountain ranges (ice fields, such as the Juneau Ice Field in Alaska). Because ice fields often appear to be crowning a mountain range, they are sometimes called ice caps as well. Mountains fringing the ice sheets cause the descending ice to break up into outlet glaciers (streams of ice resembling alpine glaciers) or broad tongues of ice called piedmont glaciers, such as the Malaspina Glacier of Alaska.
