Geological Survey Bulletin 1292 The Geologic Story of Mount Rainier

We frequently hear the question: "Why are there glaciers on Mount Rainier?" A glacier forms wherever snowfall repeatedly exceeds melting over a period of years. Above 6,500-7,000 feet on Mount Rainier, more than 50 feet of snow falls each winter, and not all of it melts before the next winter. The survival of this snow from one year to the next depends partly on the cooler temperatures at the higher altitudes, and perhaps also on the somewhat deeper snowfalls there.

A line that marks the limit on a mountain above which snow persists from one winter to the next is called the annual snowline, and this line on a glacier is called the firnline (fig. 11). Above the firnline, snow that falls each year packs down and changes into glacier ice as air is slowly forced out of it. This part of the glacier is its accumulation area, where more snowfalls each year than is lost by melting. Below the firnline is the ablation area, where melting predominates. The firnline on Mount Rainier's glaciers has been well above 6,500 feet in recent years. But some glaciers extend to altitudes below 5,000 feet — that is, far down into the ablation area. They do this by slowly flowing downhill. Solid ice flows by sliding on the hard bedrock under the glacier and by slipping along the innumerable surfaces within the ice crystals that make up the glacier.

TWO ICE STREAMS meet to form the half-mile-wide Cowlitz Glacier. One heads on the flank of the volcano and the other (Ingraham Glacier) at the summit. The firnline is a short distance above the junction of the glaciers. The high bare embankment at the extreme right is a lateral moraine that was formed about 1000 years ago when the glacier was thicker and about 1-1/2 miles longer. (Fig. 11)

The rate of flow and the rate of ablation govern the distance a glacier extends down into the ablation zone. If these rates remain fairly constant, the glacier will be in balance and its size will be about the same from year to year. But if changing weather patterns affect the rates of ablation or accumulation, or both, the glacier will either become smaller or grow larger. The change you are most likely to notice is in the position of the glacier's terminus, which may either recede or advance, but precise measurements of the upper reaches of a glacier also show volume changes there, some of which may not affect the glacier's terminus for many years, if ever.

Crevasses are a glacier's most awesome feature and are a constant hazard for climbers. They form where adjacent parts of a glacier are moving at different speeds. Some of Mount Rainier's glaciers may be flowing at a speed of several hundred feet per year along their centers but at a much slower rate along their margins. This unequal rate of flow produces stresses in the ice that cause it to break Groups of crevasses often form where the glacier flows over a steep place in its bed. The ice moves faster here, and pulls apart, and a crevasse is formed. Although a large crevasse may seem to be bottomless to the observer, most crevasses are less than l00 feet deep because ice pressure tends to close the open spaces in the ice below that depth.¹

Ice covers 37 square miles in the park today. Individual glaciers that make up this ice blanket are placed into three groups, depending on their place of origin. Those of one group originate at the volcano's summit and flow far down the valleys that radiate from the cone. Most of the snow that nourishes these glaciers probably falls on them at altitudes well below the summit. The largest examples of the group are the Emmons, Nisqually, and Tahoma Glaciers.

Glaciers of the second group originate on the flanks of the volcano, mostly at altitudes of about 6,000 feet, and they owe their existence to locations well South Tahoma, Carbon, and Inter Glaciers. Glaciers of the third group are on north-facing slopes in the mountains around Mount Rainier. They are mostly at altitudes of about 6,000 feet, and they owe their existence to locations well protected from solar heat, Glaciers representing this group are the Unicorn and Pinnacle Glaciers in the Tatoosh Range, a small unnamed glacier near the west end of Burroughs Mountain, and the somewhat larger Sarvent Glaciers east of Mount Rainier.

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¹For more information about glaciers read "Glaciers" by Robert P. Sharp, published in 1960 by the University of Oregon at Eugene.

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