Most glaciers follow a regular and nonspectacular pattern of advance and retreat in response to a varying climate. A very different behaviour pattern has been reported for glaciers in certain, but not all, areas. Such glaciers may, after a period of normal flow, or quiescence, lasting 10 to 100 or more years, suddenly begin to flow very rapidly, to up to five metres per hour. This rapid flow, lasting only a year or two, causes a sudden depletion of the upper part of the glacier, accompanied by a swelling and advance of the lower part, although these usually do not reach positions beyond the limits of previous surges. Advances of several kilometres in as many months have been recorded. Even more interesting is the fact that these glaciers periodically repeat cycles of quiescence and activity, irrespective of climate. These unusual glaciers are called surging glaciers.

Although surging glaciers are not rare in some areas (e.g., Alaska Range and St. Elias Mountains), they are totally absent in other areas of similar topography, bedrock, climate, and so forth (e.g., western Chugach Mountains and Coast Mountains). Furthermore, glaciers of all shapes and sizes, from tiny cirque glaciers to major portions of a large ice cap, have been known to surge. The flow instability that results in glacier surges is generally caused by an abrupt decoupling of the glacier from its bed. This decoupling is the result of a breakdown in the normal subglacier water flow system, but the exact mechanisms that cause some glaciers to surge are not fully understood.

Tidewater glaciers

Many glaciers terminate in the ocean with the calving of icebergs. Known as tidewater glaciers, these glaciers are the seaward extensions of ice streams originating in ice fields, ice caps, or ice sheets. Some tidewater glaciers are similar to surging glaciers in that they flow at high speeds—as much as 35 metres (115 feet) per day—but they do so continuously. Tidewater glaciers share another characteristic with surging glaciers in that they may advance and retreat periodically, independent of climatic variation.

The physical mechanisms that control the rate of iceberg calving are not yet well understood. Empirical studies of grounded (not floating) tidewater glaciers in Alaska, Svalbard, and elsewhere suggest that the speed of iceberg calving is roughly proportional to water depth at the terminus. This relation can produce an instability and periodic advance-retreat cycles. For example, a glacier terminating in shallow water at the head of a fjord will have a low calving speed that may be exceeded by the ice flow speed, causing advance of the terminus. At the same time, glacial erosion will cause the deposition of sediment as a moraine shoal at the terminus. With time, the glacier will advance, eroding the shoal on the upstream face and depositing sediment on the downstream face. The shoal, 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 kilometres (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.