NONEXPLOSIVE, CONSTRUCTIONAL EVOLUTION OF THE ICE-FILLED CALDERA AT VOLCAN-SOLLIPULLI, CHILE

A radar and gravity survey of the ice-filled caldera at Volcan Sollipu lli, Chile, indicates that the intra-caldera ice has a thickness of up to 650 m in its central part and that the caldera harbours a minimum of 6 km(3) of ice. Reconnaissance geological observations show that th e volcano has erupted compositions ranging from olivine basalt to daci te and have identified five distinct volcanic units in the caldera wal ls. Pre- or syn-caldera collapse deposits (the Sharkfin pyroclastic un it) comprise a sequence which evolved from subglacial to subaerial fac ies. Post-caldera collapse products, which crop out along 17 of the 20 km length of the caldera wall, were erupted almost exclusively along the caldera margins in the presence of a large body of intra-caldera i ce. The Alpehue crater, formed by an explosive eruption between 2960 a nd 2780 a. BP, in the southwest part of the caldera is shown to post d ate formation of the caldera. Sollipulli lacks voluminous silicic pyro clastic rocks associated with caldera formation and the collapse struc ture does not appear to be a consequence of a large-magnitude explosiv e eruption. Instead, lateral magma movement at depth resulting in empt ying of the magma chamber may have generated the caldera. The radar an d gravity data show that the central part of the caldera floor is flat but, within a few hundred metres of the caldera walls, the floor has a stepped topography with relatively low-density rock bodies beneath t he ice in this region. This, coupled with the fact that most of the po st-caldera eruptions have taken place along the caldera walls, implies that the caldera has been substantially modified by subglacial margin al eruptions. Sollipulli caldera has evolved from a collapse to a cons tructional feature with intra-caldera ice playing a major role. The po st-caldera eruptions have resulted in an increase in height of the wal ls and concomitant deepening of the caldera with time.