Olca & Paruma
Structure and Evolution
Two potentially active centers are found on the 15 km long ridge which extends westwards from the pre-Holocene volcano Cerro Paruma (CP) and which defines the frontier between Chile and Bolivia in this area. Volcan Olca (O) is the westernmost center on the ridge. A previously unnamed volcano, Volcan Paruma (P), is situated about 7 km east of Volcan Olca and about 2 km west of Cerro Paruma (Figure 16.1, 16.2). Olca rises from a base level of about 4,200 m to its summit of 5,450 m. Lava flows from an early phase of activity, identifiable to the west of the present volcano, extend for up to 5 km and are overlain by some smaller, younger flows which are probably post-glacial. A summit crater is present, but is not pristine. The southern flanks of the edifice are extensively glaciated and moraines extend down to approximately 4,400 m. Volcan Paruma is the source of several conspicuous, fresh lava flows, the most notable of which are two unusually long, narrow, unbranching flows. One extends northwards; the other southeastwards, for distances of ~7 km, and both have average widths of about 300 m, giving length: width ratios of ~23:1. The lack of any unusual topographic control makes such a ratio unusual for Central Andean lava flows and may suggest a more mafic composition (lower viscosity) than the typical andesites erupted from other central volcanoes. By contrast, a pristine, stubby lava erupted from the east of the present summit, is limited in extent and is characterised by conspicuous flow ridges which argue for a viscous rheology, more typical of a glassy dacite (Figures 16.1 & 16.3). These flows clearly post-date the glacial moraines that are extensively developed on the south flanks of the volcano. Two rather degraded craters are visible immediately west of the source of youngest lava. While Olca and Paruma are the most distinctive vent areas on the ridge, other vents clearly exist. The relative degrees of preservation of Olca and the other centers eastwards to Volcan Paruma suggest that the ridge may be the manifestation of an overall eastward migration of activity. Based on the morphology of the summit lava flows from Volcan Paruma , the activity there appears to have been more recent than at Olca, although Olca is reported to have suffered a flank eruption between 1865 and 1867 (SEAN, March 1990). It is not clear if this eruption was from the newly named Volcan Paruma. Worner et al. (2000) dated two flows on the western side of Olca to Pleistocene age, and reported that eastern flows, and those on Paruma, appear much older, contradicting these other descriptions.
There is abundant evidence of fumarolic alteration along the ridge between the two eruptive centres. Streams of altered material are conspicuous on the northern and southwestern flanks. Up until the 1980s, the volcano used to be worked extensively for sulphur (Carrasco pers. comm., 1989). Casertano (1963) suggested that Olca was in a solfataric stage and reported that the altered regions contain abundant sulphur, and that vapours (fumaroles?) could be observed emanating from one of several "yellow pits" on the western flanks. Although, persistent fumarolic activity has been reported in recent years from the Bolivian side of the ridge near Volcan Paruma (Carrasco, pers. comm., 1989), observations at Olca did not reveal any evidence for fumarolic activity in August 1989 (Worner et al., 1989 unpub. data).
Renewed fumarolic activity in November 1989 coincident with seismic activity (M ~2) on the coast at Iquique and in March 1990 (SEAN, 1990; ONEMI, personal communication, 1990) has been reported.
Worner et al. (2000) described the lava as amphibole andesite.
Casertano,L., 1963. Catalogue of the active volcanoes of the world; Part XV, Chilean continent I.A.V.C.E.I. Naples, Italy, 55pp.
Scientific Event Alert Network Bulletin, March, 1990. v. 15, No. 3, p17.
Worner, G., Hammerschmidt, K., Henjes-Kunst, F., Lezuan, J., Wilke, H., 2000. Geochronology (40Ar/39Ar, K-Ar and He-exposure ages) of Cenozoic magmatic rocks from Northern Chile (18-22ÁS): implications for magmatism and tectonic evolution of the central Andes. Rev. geol. Chile, 27(2):205-240. ISSN 0716-0208.