The Sierra Nevada volcanic complex, located in one of the most inaccessible parts of the Central Andes, covers an area of about 225 km2 astride the Chile-Argentina border. The complex is of partial Holocene age and includes at least 12 volcanic vents with associated lava flows (de Silva and Francis, 1991). Craters up to 400 m in diameter and large andesitic lava flows that extend up to 7 km with well-developed flow ridges are present. The oldest part of the complex, at its eastern end in Argentina, includes two stratovolcanoes, one with a 1-km-wide summit crater.