The Socompa collapse and avalanche event

Abstract

Failure of the northwestern flank of the Socompa volcano, north Chile, about 7000 years ago created a 40-km-long debris avalanche deposit. The deposit is remarkably well-exposed. We have combined spectral reflectance data, field observations and analysis of digital topographic surfaces to reconstruct the sequence of collapse events, and to constrain the volumes and sources of materials comprising the avalanche. Collapse involved three principle components: the debris avalanche deposit itself (volume 26 km 3, after 30% expansion); large slide blocks (11 km 3); and detached blocks (23 km 3) which remained in the upper part of the amphitheatre. The debris avalanche deposit comprised two main units and seven subunits. Landsat Thematic Mapper data enabled estimation of the proportions of spectrally distinct lithological components within these heterogeneous units. About 35% of the deposit area is composed of ignimbrites and gravels derived from a 100–200-m-thick slice of basement rock underlying the northwestern flank of the volcano. Dacite domes and lavas, originally situated in the southern and western parts of the amphitheatre, constitute much of the remainder of the deposit. Blocks composed largely of sequences of lavas and pyroclastic deposits, originally located in the northern and eastern parts of the amphitheatre, slid coherently 7 km northwestwards and accumulated near the mouth of the amphitheatre. Modelling of locations of centroids in the original cone and of displaced components shows that potential energy loss during collapse was 3.8 × 10 17 J (cf. 0.7 × 10 17 J for Mount St. Helens 1980). Significantly, components of the avalanche with least potential energy travelled the furthest laterally, perhaps demonstrating efficient transfer of momentum from rear to front of the avalanche. Retrogressive flank failures of the original cone contributed successive components to the avalanche; most of these combined to form a single moving mass, but some appear to have been emplaced separately.