Structure and Evolution
Situated on the Chile/Bolivia frontier, Volcan Irruputuncu is a relatively small active composite cone which is the youngest and most southerly of a NE-SW striking chain of volcanoes (Figure 15.1). Structural relations suggest that the modern volcano formed in the southwest facing amphitheatre of an older, collapsed, pre-Holocene ancestral cone (15.2;15.3). Remnants of the collapse scarps are evident on the western and southern flanks of the edifice and the débris avalanche, which appears to be Post-glacial, forms a flat apron of material extending to the SW of the volcano (DF). The western and southern margins of the avalanche deposit have clearly defined flow fronts, and there are indications of linear flow features on the surface of the deposit, similar to those on the better known Aucanquilcha and San Pedro deposits (Francis & Wells, 1988). 'Healing' of the collapse amphitheater is manifested by several thick stubby lava flows (L) and a nested summit complex (SC) with two craters, the most southerly of which is fumarolically active. Wörner et al. (pers. comm. 1990 ) report that the summit area is covered with well sorted hydrothermally altered material possibly of airfall origin and that the NW crater rim contains large pristine prismatically jointed bombs up to 2.5 m in diameter. The pristine morphology of these features indicate that they are young. A dark apron of material conspicuous on the western flanks of the volcano, but merging with the shadow of the cone on the TM image, is a very recent thin debris flow. Young compositionally-mixed pumice deposits also indicate a phase of explosive activity during Irruputuncu's evolution.
There are no records of historic eruptions. The most recent deposits from the volcano are the two young lava flows (L) on the northwestern flanks of the volcano. These have the appearance of "rootless flows" as they have apparently detached themselves from their original eruption points on the summit. Remnants can be seen near the summit area (G. Wörner, pers. comm. 1990; Figure 15.3). Observations made over the last 10 years have detected persistent fumarolic activity from the summit of Irruputuncu (Casertano (1963); R. Carrasco pers. comm. 1989; Wörner et al., 1990 unpublished data). Casertano (1963) and reported that the southern of the two craters is the active one.
The most recent, and the only definite historic eruption, occurred in November 1995, when a phreatic eruption sent up a dark plume (Grunder, A.). Fumarolic activity in the southern active crater continues to the present day (G. Worner, personal communication, 1990; Fig. 15.3). Reports of more intense fumarolic activity coincidental with an earthquake (M-2) at Iquique on 6 November 1989 remain unsubstantiated (ONEMI, personal communication, 1989), as does the press report from Bolivia of an eruption from the volcano in December 1989. Normal levels of fumarolic activity were reported in March 1990 (SEAN, 1990).
The petrography of the modern Irruputuncu is homogeneous plagioclase-phyric hornblende andesite. Mafic inclusions indicating magma mxing are present. (Wörner et al., unpub. data, 1990).
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, Vol. 15, No. 3., March 1990.