M4 Quick Slide and Lifts

Topic

Rapid sliding occurs when the base of a glacier reaches its melting point and liquid water appears at the interface between the ice and the underlying bedrock. Under these conditions, the glacier can slide over its bed at speeds ten to one hundred times greater than those achieved solely through the slow flow described earlier.

Ice streams represent the most extreme manifestation of this rapid sliding mode: bands of ice tens of kilometers wide that flow at speeds of one to ten kilometers per year, draining entire basins of polar ice sheets into the ocean. The acceleration of these ice streams is linked to the presence of soft, water-saturated subglacial till beneath them—a type of sediment that offers very little resistance to sliding.

Glacier surges are cyclical fluctuations in glacier velocity; a surging glacier alternates between an active phase—lasting decades, characterized by speeds ten to one hundred times the glacier's normal rate and a vigorously advancing front—and a quiescent phase, lasting from decades to centuries, during which the glacier returns to its normal flow speeds. In temperature-dependent surging glaciers, ice and heat accumulate in an internal reservoir until the base of that reservoir becomes warm enough to trigger rapid sliding; once this sliding begins, the accumulated ice is rapidly discharged, causing the entire system to cool and slow down again, thereby completing the cycle between the active and quiescent phases. In glaciers where surging is driven by water, the subglacial drainage system itself controls the surge: when drainage beneath the glacier is distributed across numerous small, disconnected cavities, water accumulates at high pressure—promoting rapid sliding—whereas when drainage is organized into a few large, efficient channels, those channels reduce water pressure and inhibit sliding.

The collapse of a floating ice shelf is yet another manifestation of rapid sliding: upon losing the buttressing effect provided by the floating ice shelf, the tributary glaciers feeding it accelerate drastically. Following the collapse of the Larsen B ice shelf in 2002, its tributary glaciers accelerated by factors of two to eight within just a few weeks—a sudden shift that no slow-flow model can replicate, requiring instead a full-flow model capable of accounting for tensile stresses throughout the entire glacier, thereby rounding out the range of rapid-sliding phenomena with which this description began.

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