The Southern Patagonia Icefield remains one of the least studied glacial areas in the world, although it is one of the largest mid-latitude ice bodies. The largest glacier, Glaciar PíO XI, located on the western maritime side, has been advancing during the last 50 years. In 1993 the tidewater front of the glacier was overriding trees at least 300 years old. In a worldwide context of rapid deglaciation and retreat, which is particularly affecting Patagonia, there is no simple explanation for the exceptional advance of Glaciar Pío XI. Careful examination of the fluctuations of the glacier front shows that there have been periods of enhanced movement, separated by retreat periods. Due to the magnitude and characteristics of the advance periods, they are postulated to correspond to glacial surges. Four explanations are presented to account for the surging behavior: increased geothermal activity associated with Volcán Lautaro; increased precipitation during periods before the surges; and reduction of the rate of calving as the depth of water decreased due to proglacial sedimentation. Dendrochronological, geomorphological, and glaciological data, collected during 1992, 1993 and 1995, have been interpreted in combination with satellite imagery, air photographs, and historical maps. Its is concluded that the data available are insufficient to permit selection of any one of the proposed models.

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