Between 10 000 and 500 yr ago the Socompa volcano in northern Chile experienced a catastrophic collapse of a 70° sector of the original cone, causing a debris avalanche that descended nearly 3000 m vertically arid traveled more than 35 km from the volcano. The deposits cover some 490 km2 and have a minimum volume of 15 km3. Parts of the original cone slumped in a nearly coherent form and are now preserved as large blocks more than 400 m high. The primary avalanche traveled northwestward over sloping ground before coming to rest transiently, forming a prominent marginal ridge, and then slid away northeastward to form a secondary flow, overriding much of the primary avalanche deposit. Abundant, prismatic, jointed dacite blocks within the debris avalanche deposit and a thin, fine-grained pumiceous deposit beneath it suggest that the collapse was triggered by magmatic activity and may have been accompanied by a violent lateral blast. Collapse was followed by eruption of pumiceous pyroclastic flows and extrusion of voluminous dacite domes.