bordered by the high Monturaqui basin. A water table exists at depths of , but surface runoff is only ephemeral. Sector collapse Socompa suffered a major sector collapse during the Holocene, forming one of the largest terrestrial collapse deposits. The deposit left by the collapse was first discovered on aerial photography in 1978 but the correct interpretation as a landslide occurred in 1985; at first it was interpreted as a form of moraine, then as a large pyroclastic flow and the collapse scar as a caldera. The collapse removed about 70° (about ) of Socompa's circumference on its northwestern side, descended over a vertical distance of about and redeposited it over distances of over , at a modelled speed of . As it descended, the collapse landslide accumulated sufficient energy that it was able to override topographic obstacles and climb an elevation of about ; secondary landslides occurred on the principal deposit and there is evidence that the landslide was reflected back from its margins. The collapse occurred in several steps, with the first parts to fail ending up at the largest distances from the volcano; it is not established whether the collapse happened in a single event or as several separate failures. The total volume of material removed was about , which was dilated as it flowed and eventually ended up as a deposit with a volume of ; thorough mixing of the avalanche material occurred as the landslide progressed. The summit of the volcano was cut by the collapse and some lava domes embedded within the volcano were exposed in the rim of the collapse amphitheatre; before the collapse the volcano was about high. The collapse left a triangle-shaped collapse scar, which was partly filled by blocks left over by the collapse. The walls of the amphitheatre were about high, so high that secondary landslides occurred. The largest of these detached from a dome northwest of the summit and descended a horizontal distance of , forming a landslide structure notable in its own right and covering about . The central section of the collapse amphitheatre was not a simple collapse structure, but instead contained a secondary scarp. At the mouth of the collapse scar, the walls were lower, about . After the principal collapse, lava flows and pyroclastic flows – some of which emerge from the western rim of the collapse scar – filled up the scar left