Volcán Ollagüe, also known as Oyahué, is a massive andesitic stratovolcano with a summit dacitic lava dome. A large Pleistocene debris-avalanche deposit extending westward separates the Salar de San Martín from the Salar de Ollagüe. Three youthful-looking silicic lava flows mark late post-collapse eruptions, but show evidence of glaciation and are thought to pre-date the last glacial advance at about 11,000 years ago (Freeley et al., 1993). A youthful-looking scoria cone on the lower WSW flank, La Poruñita, was initially considered to be of Holocene age (González-Ferrán, 1995), however Wörner et al. (2000) later obtained Potassium-Argon dates of 420,000 to 680,000 years. Active sulfur mines on the upper western and southern flanks are reached by a road that climbs to about 5500 m elevation. No historical eruptions have been recorded; activity has been restricted to periods of intense fumarolic activity, and a persistent steam plume emanates from a fumarole on the south side of the summit dome.