Structure and Evolution
Volcan Lascar (Figure 27.1), located about 32.5 km south-east of the village of Toconao, is the most active volcano in the central Andes and is characterised by persistent fumarolic activity, occasional vulcanian and steam eruptions. Since 1984, there have been four cycles of lava dome building within the summit crater, dome subsidence, and vulcanian to plinian eruptions. These culminated in the largest historic eruption of Lascar on April 19 and 20, 1993, which sent ash to the SE as far as Buenos Aires. The volcano has since produced further vulcanian eruptions in December 1993, July 1995, and July 2000, and a change in morphology with the opening of a fracture at the summit.
An elongate series of six overlapping craters, trending roughly north-east constitutes a summit ridge, giving the volcano a flat topped appearance from the ground. An active crater (AC) is located near the centre of this cluster. It is about 800 m in diameter, 300 m deep, and the rim of the crater is at an altitude of about 5,400 m. Large young lava flows occur on the volcano, particularly on the northern and western slopes; the most conspicuous of which extends about 6 km down the northern flanks to within a few kilometers of Talabre village (L; Figure 27.2). Casertano & Barozzi (1961) suggested that this flow may have erupted as recently as the 19th century, although this remains unsubstantiated.
In recent historic times Lascar has been active on several occasions, although details are sketchy. For example, a 1965 ash eruption which resulted in a plume being dispersed into the atmosphere was never formally documented (J. Guest pers. comm. 1987 ). A brief but powerful vulcanian eruption of Lascar occurred on 16 September 1986, when ash was dispersed as far as Salta in Argentina, some 285 km south east of the volcano (SEAN, August 1986, April 1987, May 1987; Glaze et al., 1989a). Other minor eruptive episodes were noted during March, 1988 (SEAN, April 1988), and June, 1988 (SEAN, August 1988).
At the present day, Lascar is the only volcano in the region to be erupting lava (Figure 27.2). Background activity varies from steam emissions to fumarolic "blue haze" emissions, but occasional minor vulcanian eruptions occur and slugs of ash and vapour are emitted. Such small eruptions have occurred in 1994, 1996, 2000, 2001, 2002, 2003, 2005, and 2006-2007 (Buenos Aires Ash Advisory Center).
Lascar is situated in a thinly populated region, but the small hamlet of Talabre, originally situated in the steep-walled Quebrada Talabre directly beneath the volcano and only 12 km from it, was at risk from mudflows and pyroclastic flows. Talabre was relocated to higher ground in 1987. Mudflows are unlikely at present due to climate, and the so the largest threat is from further explosive eruptions of tephra.
Lascar has erupted lavas ranging from andesite to dacite in composition (57.83% to 64.93% SiO2; Deruelle, 1978, 1982, 1985). These are typically porphyritic, containing plagioclase, augite, hypersthene, hornblende and biotite (Deruelle, 1978, 1985) with the pyroxenes being more common in the andesites. There is abundant evidence in hand specimen for magma mixing in the andesite-dacite juvenile clasts in some of the pyroclastic flows. Ash from the 1986 eruption was a hypersthene-andesite (Glaze et al., 1989a) suggesting that the magma composition at Lascar has been constant for much of its history. Several pre-1986 bombs on the crater rim are of dacitic composition. Harmon et al. (1984) report that the lavas of Lascar are characterised by d18 O of 7.3-8.4, 87Sr/86Sr of 0.70560 - 0.70720, and Pb-isotope ratios of 18.74, 15.62, 38.66 (206/204, 207/204, 208/204 respectively). Deruelle (1985) presents major and trace element analyses for 13 Lascar lavas.
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