An eruption in 1988 confirmed the presence of a postulated subglacial vent in the Patagonian Icefield NW of Viedma Lake (Kilian, 1991). A previously suggested vent location (Shipton, 1960) turned out to be a glacial nunatak of metamorphic and sedimentary rocks. The Volcán Viedma eruptive center is a subglacial dacitic volcano beneath the Patagonian Icecap west of the spectacular granitic spires of the Cerro Torre, Cerro Fitz Roy area. Only part of the older edifice rises above the surface of the icecap. Four large craters or calderas between 1.5 and 4 km in diameter are located along a N-S line. The 1988 eruption deposited ash and pumice on the Patagonian Icecap and produced a mudflow that reached Viedma Lake.