Viedma is located in the southern Patagonian Andes, southwest of Mount FitzRoy. The lake of the same name lies to the southwest. The area is poorly accessible, and the geological history poorly known. Viedma lies within the Austral Volcanic Zone. This zone consists of five known volcanoes, from north to south: Lautaro, Aguilera, Reclus, Monte Burney and Fueguino. These volcanoes form a long chain, the most southerly of which is a volcanic complex of lava domes and lava flows on Cook Island. The existence of the putative volcanic edifice of Viedma was advanced in the 1950's, and it was originally thought to be mostly buried beneath glacial ice. In 1956 Louis Lliboutry proposed that volcanic activity may occur in fissure vents buried beneath glaciers; between eruptions they would be concealed below the ice. Lliboutry considered dark bands on the ice to be tephra deposits, a view supported by a 1958-1959 expedition that found pumice on the Viedma Glacier. Other reports of volcanic phenomena in the region exist, and the existence of a large caldera beneath the Southern Patagonian Ice Field has been suggested as an explanation for volcanism there.
The Viedma Nunatak was originally interpreted to be the site of the putative volcano, after observations made in 1944-1945 and 1950 by survey flights. The collected data included controversial sightings of features interpreted asfumaroles, which also led to Nunatak becoming known as the "Volcan Viedma"; but the lack of any clear evidence for its volcanic nature led to the idea being discredited. The Nunatak is about high above sea level. Most of the mountain is buried beneath the Viedma Glacier of the Southern Patagonian Ice Field and only parts of it crop out. The outcropping Nunatak is elongated in north-south direction. A number of structures originally interpreted as craters and concave depressions are found locally, especially on the southern part of the volcano, some of which are lined up in north-south direction. The northern sector of the Nunatak 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 Jurassicmetamorphic 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 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 being 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. Owing to their similar composition, the northerly volcanoes Lautaro 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 sedimentary rocks as well.
Eruption history
Tephra that were originally linked to Viedma has been found in the Laguna Potrok Aike, including three tephra layers 26,991 - 29,416, 40,656 - 48,219 and 41,555 - 57,669 years before present, but these may have originated instead from Lautaro. Holocene tephra found in Lago Cardiel and dated 3,345 - 3,010 years before present was also originally ascribed to Viedma, but this attribution cannot be regarded as confirmed.
Historical
Many observations referring to volcanic activity on the Southern Patagonian Ice Field exist, including reports of ashfalls, layers of tephra on glaciers, and columns of smoke rising from the ice. A subglacial eruption was thought to have occurred in 1988, which deposited ash and pumice on the Viedma Glacier. These materials later gave rise to a lahar. The eruption had melted part of the ice and formed a network of valleys; it was assumed that it had taken place at some point between September and November of that year.
Hazards
There are no major population centres close to any volcano in the Austral Volcanic Zone, and the volcanoes are largely unmonitored. Among the know eruptions are large Holocene explosive eruptions, while historical eruptions took place in 1908 at Reclus and 1910 at Monte Burney. Future eruptions of volcanoes in the Austral Volcanic Zone may lead to ash fall at large distances from the relevant volcano, with the potential to create interruptions in air traffic and direct ash damage.
