Abstract

Pio XI glacier, with an area of 1263 km 2 and a length of 65.7 km, is the largest of the Southern Patagonia Icefield (SPD. During the period 1945-1995 it experienced a net advance of ca. 10 km., a unique behavior considering thai virtually all neighboring glaciers are retreating. Two maps at 1:50,000 scale, produced from 1975 and 1995 aerial photography, are used to compute volume changes for this glacier. This is done by digitizing the maps and creating digital terrain models at 200-m resolution. Volume changes for a reduced sample area, in combination with the hypsometric curve of the glacier, are analyzed in terms of the recent advance experienced by the glacier. The sample area (4.5% of the glacier area) for the volume change analysis shows an average thickening on the glacier for the period 1975-1995 of 44.1 m. which represents 2.2 m a -1. The corresponding volume change was 2.52 km -3, with a larger thickening in the lower part of the ablation area. The hypsometric curve of the glacier shows a low sensitivity of the glacier to ELA variations. Nevertheless, the increase in temperature has shifted the ELA upwards, reaching close to a threshold, from where the glacier would begin to retreat in the future if the ELA elevation trend continues.