Rock Glacier

A rock glacier is a tonguelike body of coarse rock fragments, found in high mountains above the timberline, that moves slowly down a valley. The rock material usually has fallen from the valley walls and may contain large boulders: it resembles the material left at the terminus of a true glacier. Interstitial ice usually occurs in the centre of rock glaciers. Where the ice approaches the terminus, it melts and releases the rock material, which then forms a steep talus slope. A rock glacier may be 30 metres (100 feet) deep and nearly 1 .5 km (about 1 mile) long.

A rock glacier may have wavelike ridges on its surface that curve convexly downstream; these indicate flowage. Maximum movements observed exceed 150 cm (4.5 feet) per year. The method of movement is thought to be either flowage of the interstitial ice or creeping by frost action.

by reducing the depth of the water at the glacier's terminus and thereby inhibiting iceberg calving, will allow the glacier to advance into deep water farther down the fjord. This advance phase is slow—typically 9 to 40 metres (30 to 130 feet) per year—and in an Alaskan fjord it may take a period of 1,000 years or more to cover a typical fjord length of 30 to 130 km (20 to 80 miles).

Such a glacier, in an extended position and terminating in shallow water on a moraine shoal, is in an unstable situation. If, for some reason, the terminus retreats slightly, the deeper water upstream of the shoal will cause an increase in iceberg calving; this will result in further retreat into deeper water, which will further increase the calving until the calving speed becomes so high that the normal processes of glacier flow cannot compensate. A rapid, irreversible retreat will result until the glacier reaches shallow water back at the head of the fjord. In contrast to the slow advance phase, the retreat phase may take only a few decades. The fastest glacier retreats observed during historical time (for instance, the opening of Glacier Bay, Alaska), as well as those inferred during the demise of the great Quaternary ice sheets, were caused by this mechanism. Information on the advance and retreat of tidewater glaciers should not be used to infer climatic change, however.

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