The temporal significance of transitions from melting to calving termini at glaciers in the central Southern Alps of New Zealand

Termini of valley glaciers in the central Southern Alps are undergoing a three-phase sequence of retreat that began in the 1950s. An initial phase of melting and slow downwasting under thickening supraglacial debris mantles is associated with stationary or slowly retreating termini. This is followed by a transitional phase of disruption of insulating debris mantles allowing the rapid growth of shallow thermokarst lakes, the shape and location of which vary according to glacier gradient and marginal topography. A third phase of rapid calving retreat of ice cliffs into deepening proglacial lakes develops from the transitional phase. At no stage are terminal responses simply related to climate. Present transitions to calving retreat are related to the morphology of glacier termini, particularly to large outwash heads. Landform evidence indicates that similar calving phases do not appear to have occurred since Late Pleistocene deglaciation, so that present changes to glacier termini are of great significance in the context of the whole Holocene. At millennial timescales, the melting-calving transition is effectively instantaneous and represents a sharp threshold heralding rapid deglaciation. The commencement of calving is imminent at the stationary terminus of Tasman Glacier which will soon retreat rapidly as a result.