Tasman Glacier, New Zealand: Twentieth-century thinning and predicted calving retreat

The nature and rate of the transition from a thinning, melting ablation zone to a retreating, calving terminus is examined at the debris-mantled Tasman Glacier. The debris mantle has existed since the earliest glaciological observations were made in 1863, indicating that debris cover is the normal glaciological state regardless of historic mass-balance change. The relationship between debris thickness and ablation rate has been derived from short-term heat flow calculations. Extrapolation over time and space indicate that the thermal effect of the debris mantle has resulted in a post-1890 reduction in glacier surface gradient which, through positive feedback involving ablation rate, ice velocity and particle emergence paths, has caused upglacier spread of supraglacial debris and upstream migration of the locus of maximum ablation. This has lead to the preservation of a long ice tongue at low gradient while preventing terminus retreat from the outwash head, and has made the glacier vulnerable to calving. Since the late 1970s, thermokarst melting has formed an ice-contact proglacial lake in which water depths now exceed ca. 130 m against the ice front. Since 1994, evidence of extending and accelerating flow may indicate the imminent onset of rapid calving. Predicted retreat scenarios suggest a rapid retreat of at least 10 km will probably cause major (possibly catastrophic) rock and debris avalanches into the enlarging proglacial lake as debuttressing of mountainsides progresses. (C) 1999 Elsevier Science B.V. All rights reserved.