southern part of the volcano, some of which are lined up in north-south direction. The northern sector of the volcano was apparently more heavily affected by glacial erosion; conversely, several craters in the southern sector appear to be young. The nunatak shows clear evidence of glacial action, including glacial striations and coverage by glacial drifts; the Viedma glacier may once have crossed the nunatak in its central part. Rock samples taken from the nunatak are Jurassic metamorphic rocks, including gneiss and schists, and no evidence of magmatic rocks was found in a 1958-1959 expedition. Different observations have yielded different sizes and numbers of supposed craters; González-Ferrán et al. 1995 reported several craters and calderas with sizes ranging , while Kobayashi et al. 2010 observed fewer craters and none of them larger than . These craters were later interpreted as being actually glacial cirques containing tarn lakes. Geology Off the southernmost west coast of South America, the Antarctic Plate subducts beneath the South American Plate at a rate of . This subduction process is responsible for volcanism in the Austral Volcanic Zone. The Austral Volcanic Zone is one of four volcanic zones in the Andes, the other three are the Northern Volcanic Zone, the Central Volcanic Zone and the Southern Volcanic Zone, all of which are separated from the Austral Volcanic Zone and each other by gaps where no volcanic activity occurs. Unlike the Austral Volcanic Zone, volcanism in these other zones is controlled by the subduction of the Nazca Plate beneath the South American Plate. The Austral Volcanic Zone was identified as such in 1976, but some volcanoes were identified and localized later. Owing to their similar composition the three northerly volcanoes Lautaro, Viedma and Aguilera are grouped as the "northern Austral Volcanic Zone". Volcanism in the Austral Volcanic Zone is not well known before the Holocene; it is likely that a lull of volcanism occurred before the Pliocene while the Chile Triple Junction was being subducted in the region. Magmas from the Austral Volcanic Zone are adakitic owing to the melting of the subducting slab and the interaction of these melts with the crust and mantle. The basement consists of metamorphic rocks of Paleozoic age. Outcrops of the basement are found around the volcano and could even occur on the edifice that crops out from the glaciers. The regional basement is formed by these Paleozoic and younger metamorphic and