Abnormally fast flow of a glacier over a period of a few months to years, during which the glacier margin may advance substantially. A surge-type glacier exhibits quiescent phases, typically lasting some decades, during which velocities are lower than in a 'normal', non-surge-type glacier. The ice discharge is thus too small to maintain the longitudinal profile of the glacier, which thickens in its upper reaches and thins in its lower reaches. Surges recur at quasi-periodic, glacier-specific intervals, and transfer large quantities of ice from the thickened upper part to the thinned lower part. Velocities during the surge are often greater by an order of magnitude than those during the quiescent phase. A surge-type glacier will almost always be out of balance. That is, a surge-type glacier cannot be in steady state. Surge-type glaciers may end on land or in water, and the proportion of glaciers that are of surge type varies from region to region. The mechanism of surging is poorly understood. Surges seem, however, to be related to changes in the subglacial hydrological regime and not primarily to climatic fluctuations. Although surging is best documented on smaller glaciers, many larger outlet glaciers of ice caps have been observed to surge, and there may be a connection with the unsteady behaviour exhibited by some ice streams.