Glaciers in the Northwest Central US
Glaciers have had a profound impact on the Northwest Central’s scenery, geology, and water resources. Today, small cirque glaciers and larger valley glaciers are found largely in the mountains of Wyoming and Montana, while a few small glaciers and perennial snowfields can be seen in Idaho. Ongoing research into how these glaciers 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.
Most broadly, there are two types of glaciers: smaller alpine glaciers and larger continental glaciers. 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. 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). 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.
Figure 6.1: As dense glacial ice piles up, a glacier is formed. The ice begins to move under its own weight and pressure.
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.
While only the two broadest categories of glaciers are discussed here, glaciers exist in a variety of forms. Even these broadest of distinctions are not quite so clear-cut (e.g., continental glaciers often have tongues that feed into valleys, which may become alpine glaciers).
In summary, glaciers grow when it is cool enough for an ice sheet to accumulate snow more quickly than it melts. As they grow, ice sheets become so massive that they flow outwards, covering an increasing area until melting at the margins catches up to the pace of accumulation. Glaciers that reached the Northwest Central States flowed from centers of accumulation far to the north (in what is now Canada), and glacial growth southward through the Midwest was more a result of this lateral flow than of direct precipitation from falling snow. By 18,000 years ago, the ice was in retreat due to a slight warming of the climate—it was not actually flowing backward, but melting faster than it was accumulating and advancing.