Structure and Evolution
Located on the Chile-Argentina border, Socompa is the largest of a chain of volcanoes on a NE-SW trending portion of the active volcanic front of the Andes (Figure 33.1). Immediately south of Socompa is the only railway line crossing the Andes between Chile and Argentina. Associated with this important link are an army base at Monturaqui (~10 km from the western base of the volcano; M) and frontier and customs control posts on the frontier itself. Thus, although the volcano is in a remote part of the Atacama desert, road and rail communications are well maintained. On the volcano, there is abundant evidence of recent activity in the form of fresh lava flows and ash deposits of post-glacial age. The most conspicuous recent feature is a massive débris avalanche deposit (DF) which formed when the NW sector of the ancestral cone catastrophically collapsed ~ 7,500 yr ago (Francis et al., 1985; Francis & Wells, 1988; Ramirez, 1989; Kelfoun & Druitt, 2005). The deposit covers more than 600 km2, and is the world's best exposed example of a major debris avalanche deposit. Collapse of the north-west flank of the volcano left an amphitheater (A) occupying a 70° sector of the volcano, which is still an important feature, with steep cliffs >400 m high in places (Figure 33.2). An accumulation of toreva (slide) blocks up to several km across and several hundred meters high which slid more or less intact for several kilometres form an elongate massif in the mouth of the amphitheater (Figure 33.3;33.4).
Prior to collapse lava flows were distributed radially around the cone, and their truncated remants are exposed on the rim of the amphitheater scarp. Extrapolation of the existing slopes suggests that the original cone may have been up to 300 m higher than the existing edifice. Catastrophic failure of the north western flank of the volcano appears to have triggered a pyroclastic eruption, similar to that of Mt. St. Helens May 1980. Evidence for this explosive activity is manifested by a plinian ash deposit, now distributed predominantly to the northeast of the volcano, and an apron of pumiceous pyroclastic flows (the "Campo Amarillo"; CA), which covers the proximal parts of the avalanche deposit and is closely analagous to the "pumice plain' of Mt. St. Helens May 1980.
Dacite lavas extruded within the amphitheater have partially restored the profile of the original volcano. Five small explosion craters located on the vent regions of the extrusive dacites in the summit region represent the youngest eruptive activity of Socompa.
No current activity is known, though C.F. Ramirez (personal communication, 1988) reported that a faint smell of sulphur was detectable when he climbed to within 100 m of the summit.
The rocks of Socompa are dominantly dacites with subordinate andesites (Deruelle, 1978; Ramirez, 1989). The lavas are typically porphyritic and the andesites contain olivine, augite, hypersthene, plagioclase and opaques, while the latter two, hornblende, biotite characterize the dacites. The youngest flows and domes are dacitic (Francis et al.,1985).
Casertano,L., 1963. Catalogue of the active volcanoes of the world; Part XV, Chilean continent I.A.V.C.E.I. Naples, Italy, 55pp.
Deruelle, B., 1978. Calc-alkaline and shoshonitic lavas from five Andean volcanoes (between latitudes 21°45' and 24°30'S) and the distribution of the Plio-Quaternary volcanism of the south-central and southern Andes. Jour. Volcanol. Geotherm. Res. 3:281-298.
Francis, P.W., Gardeweg, M. Ramirez, C.F., & Rothery, D.A., 1985. Catastrophic débris avalanche deposit of Socompa volcano, northern Chile. Geology 13:600-603.
Francis, P.W., & Wells G.L., 1988. Landsat Thematic Mapper observations of débris avalanche deposits in the Central Andes. Bull Volcanol. 50:258-278
Kelfoun, K., and Druitt, T.H., 2005. Numerical modeling of the emplacement of Socompa rock avalanche, Chile, J. Geophys. Res., 110, B12202, doi:10.1029/2005JB003758.
Ramirez, C.F., 1989. The Geology of Socompa volcano and its débris avalanche deposit, northern Chile. Unpub. MPhil Thesis, Open University, Milton Keynes, UK. 232pp
Wadge, G., Francis, P.W., Ramirez, C.F., 1995. The Socompa collapse and avalanche event. Journal of Volcanology and Geothermal Research, 66: 309-336.