RECENT FLUCTUATIONS OF GLACIAR-PIO-XI, PAGAGONIA - DISCUSSION OF A GLACIAL SURGE HYPOTHESIS

The Southern Patagonia Icefield remains one of the least studied glaci al areas in the world, although it is one of the largest mid-latitude ice bodies. The largest glacier, Glaciar Pio XI, located on the wester n maritime side, has been advancing during the last 50 years. In 1993 the tidewater front of the glacier was overriding trees at least 300 y ears old. In a worldwide context of rapid deglaciation and retreat, wh ich is particularly affecting Patagonia, there is no simple explanatio n for the exceptional advance of Glaciar Pio XI. Careful examination o f the fluctuations of the glacier front shows that there have been per iods of enhanced movement, separated by retreat periods. Due to the ma gnitude and characteristics of the advance periods, they are postulate d to correspond to glacial surges. Four explanations are presented to account for the surging behavior: increased sliding due to enhanced ba sal water pressure, as is typical in surging glaciers; increased slidi ng resulting from periods of increased geothermal activity associated with Volcan Lautaro; increased precipitation during periods before the surges; and reduction of the rate of calving as the depth of water de creased due to proglacial sedimentation. Dendrochronological, geomorph ological, and glaciological data, collected during 1992, 1993, and 199 5, have been interpreted in combination with satellite imagery, air ph otographs, and historical maps. It is concluded that the data availabl e are insufficient to permit selection of any one of the proposed mode ls.