Structure and Evolution
Located 5 km west of the Chile-Bolivia frontier, Isluga has a broad, subdued conical shape (Figure 14.1; 14.2). It lies on the western end of an E-W trending group of volcanoes which includes Volcan Tata Sabaya (q.v) in Bolivia. The edifice itself appears to form the youngest, central part of an arcuate chain, concave towards the northwest, and consists of a complex pile of levˇed lava flows, which are particularly prominent on the southern flank of the volcano. There is evidence of both pre- and Post-glacial (Holocene) activity. The broad, flat, snow covered region in the summit may represent an in-filled crater, and to the west of this is a pristine circular summit crater ~400 m in diameter, with a rim of andesitic scoria (Figure 14.3; 14.4) . The crater walls are made of layered tephra deposits which have been strongly fumarolically altered and crusted over with sulphur (Wšrner et al., unpub. data 1989). The morphology of the numerous post-glacial lava flows suggests much recent activity. Casertano (1963) reported activity from the summit crater in the late 1800's and early 1900's. Reports from 1879 indicate activity in 1869 and again in 1878, during which 4 villages were destroyed by lava flows.
The crater was reported to be in a fumarolic state, with much steam activity in August, 1987 (Figure 14.3). There is also a small noisy fumarole about 150m below the crater on the SW flank (Wšrner et al., pers. comm. 1989).
The most recent lavas show a range of 60 - 63% SiO2, while older lavas from the ancestral volcano show a range of 56 - 59% SiO2 (Wšrner, et al., unpub. data 1989).
Casertano,L., 1963. Catalogue of the active volcanoes of the world; Part XV, Chilean continent I.A.V.C.E.I. Naples, Italy, 55pp.t
(1879). Volcanic Phenomena and Earthquakes during 1878. Nature, 20:378-379. doi:10.1038/020378a0.