Nevados Ojos del Salado
Structure and Evolution
Nevados Ojos del Salado is the highest active volcano in the world and is situated in the centre of an ENE striking chain of volcanic centres which extends for more than 80 km (Figure 43.1;43.2). Despite its remote location on the Chile-Argentine border east of Copiapo, its lower slopes are relatively more accessible than other centers in the region, since an international road crosses the frontier between Chile and Argentina via the Paso de San Francisco, 20 km north of the volcano. There have been reconnaissance studies of the region by Gonzalez-Ferran et al. (1977) and Baker et al. (1987) who suggest that the complex probably has a history dating back to the late Pleistocene. Astronomers searching for an high altitude observatory have also reconnoitered the volcano as a potential site (Brunier, 1989).
The complex has an oval basal plan, elongated in the E-W direction, and covers an area of about 70 km2. Thick pumice deposits cover the lower slopes of the complex, and were identified by Baker et al., (1987) as being the earliest phase of activity from this complex who also postulated that they may be related to the formation of an ancestral "somma" caldera described by Gonzales-Ferran et al., (1985) (Figure 43.3). There are clearly extensive young pyroclastic deposits but their source and stratigraphic relationships require field study. A welded airfall deposit may cover much of the north flank of the volcano.
Between 1000 and 1500 years ago, a large rhyodactic eruption occurred, and the most recent record of activity was a reported gas plume in 1993. Climbers report that the most recent activity is concentrated along a rift like feature which strikes N85°E along the summit of the complex (Gonzalez-Ferran et al., 1985), which consists of at least a dozen small cones, domes, explosion craters, and thick, young, lava flows.
There are persistent fumaroles from the summit area.
Baker et al. (1987) have published an account of the geochemistry of the Ojos del Salado region. The dominant rock type at Ojos del Salado sensu stricto is high-K dacite, with some rhyodacite. The mineralogy is plagioclase, biotite, hornblende, opaques, with minor amounts of quartz, hypersthene and augite. These evolved magmas are part of an evolutionary sequence from more mafic magmas in the earliest phases of activity in this region.
Brunier, S., 1989. Extreme Astronomy. Sky & Telescope, October, 1989.
Baker, P.E., O. Gonzalez-Ferran, & Rex, D.C., 1987. Geology and geochemistry of the Ojos del Salado volcanic region, Chile. Jour. Geol. Soc. Lond. 144:85-96
Casertano,L., 1963. Catalogue of the active volcanoes of the world; Part XV, Chilean continent I.A.V.C.E.I. Naples, Italy, 55pp.
Gonzalez-Ferran, O., Baker, P.E., & Rex, D.C., 1985. Tectonic-volcanic discontinuity at latitude 27°S, Andean range, associated with Nazca plate subduction. Tectonophysics 112:423-441
Mpodozis, C., Kay, S.M., Gardeweg, M., Coira, B., 1996. Geolog’a de la regi—n de Ojos del Salado (Andes centrales, 27¡S): implicancias de la migraci—n hacia el este del frente volc‡nico Cenozoico Superior. XIII Congreso Geol—gico Argentino, Buenos Aires, vol. 3, pp. 539Ð548.
Smithsonian Institution Global Volcanism Program, 1993. Nevados Ojos del Salado Monthly Reports.