Quiescent-phase evolution of a surge-type glacier : Black Rapids Glacier, Alaska, U.S.A.

Black Rapids Glacier, a surge-type glacier in the Alaska Range. most recently surged in 1936-37 and is currently in its quiescent phase. Mass balance, ice velocity and thickness change have been measured at three to ten sites from 1972 to 1994. The annual speed has undergone cyclical fluctuations of as much as 45% about the mean speed. Ice thickness and surface slope did not change enough to cause the speed fluctuations through changes in ice deformation, which indicates that they are being driven by changes in basal motion. The behavior of Black Rapids Glacier during this quiescent phase is significantly different from that Variegated Glacier, another well-studied surge-type glacier in Alaska. The present medial-moraine configuration of Black Rapids Glacier indicates that a surge could occur at any time. However, ice velocity data indicate that the next surge may not be imminent. We believe that there is little chance that the next surge will cross and dam the Delta River.