Straddling the Chile-Argentina border, Volcán Socompa is a massive, 6051-m-high dacitic stratovolcano that lies immediately north of the only railway line between the two countries.  Socompa is the youngest and southernmost of a 6000-m-high NE-SW-trending chain of volcanoes including Pular and Pajonales.  In contrast to the latter two volcanoes, no glacial moraines have been observed on the youthful and relatively uneroded Socompa volcano.  Collapse of the NW portion of Socompa volcano about 7200 years ago, during an eruption similar to that at Mount St. Helens in 1980, produced a 600 sq km debris-avalanche deposit that extends about 40 km from the summit and is one of the world's largest and best exposed.  Post-collapse eruptions have constructed dacitic lava domes that have filled much of the head of the collapse scarp.  No historical eruptions are known from Socompa.