Structure and Evolution
Volcan Lastarria is located on the Chile/Argentina frontier at the northern end of a chain of Quaternary eruptive centres. It is a NNW trending edifice with an oval basal plan, about 12 km long and 8km wide (Figure 36.1). Naranjo (1986) made a detailed study of the volcano and recognized three structural components: the ancestral volcano, South Spur (S); the modern volcanic edifice, Lastarria sensu stricto (L); and a geographically associated lava field, Negriales del Lastarria or 'Big Joe', on the lowermost southwestern flanks. There is no break visible between the ancestral and modern volcano.
The summit of the volcano (5, 706 m) is where the South Spur joins the main edifice of Lastarria volcano. The modern composite cone consists of lava flows, scoria and pyroclastic flow deposits (PF), and one dome. Slopes of the edifice are slightly concave, and steepen progressively upwards (5° - 30°). Young, blocky, stubby lava flows occur on the lower portions of the northern flanks, while the upper portions are covered with tephra. The summit complex consists of a line of four semi-nested craters; the youngest (SC) occurs in the north, and is a funnel shaped vent about 45 m deep and 150 m in diameter. Recent pyroclastic flows form an extensive apron on the northern flanks of the volcano, reaching as far as the Salar de Aguas Calientes. Failure of the southeast flank of the volcano produced a small but impressive debris avalanche (DF) which completely overan an older cone, (Naranjo & Francis, 1987; Francis & Wells, 1988). "Big Joe" is formed of several thick, long and extensive lava flows, which erupted from a single vent in the southeast, now plugged by a dome. This was the source of the largest single eruption (~14 km3) in the region of Lastarria, and is much greater than any single flow from Lastarria sensu stricto.
Powerful and persistent fumarolic activity has been known from the northwestern flanks since the earliest records were made (Figures 36.2; 36.3). Casertano (1963) reported solfataric and vapour activity at the end of the 19th century and in Dec. 1960. Even the most recent eruptions appear to have predated the Spanish colonists, as no records of large or small eruptions have been found. In 1900 L. Darapsky (vide Martin, 1917) said " Lastarria volcano... is the only one in the district which shows signs of volcanic activity, exhaling sulphur and water steams". Naranjo (1985) describes sulphur flows resulting from melting of fumarolic sulphur deposits on the northwestern flanks by thermal activity on Lastarria.
Persistent fumarolic activity is present on Lastarria. InSAR ground deformation studies have also detected recent ground inflation (2.5 cm/yr) from Lastarria to Cordon del Azufre and Cerro Bayo, which Froger et al. (2007) interpret as magma chamber growth due to the injection of new melt.
The area around Lastarria is unpopulated in a 150 km radius.
Naranjo (1986) reported that the lavas of Lastarria are dominantly high-K andesites and dacites (57 - 68% SiO2) containing plagioclase, pyroxene (augite and hypersthene) with hornblende and biotite in the more evolved rocks. Phenocryst contents maybe as high as 40% in the lavas and pyroclasts. There are significant differences between Lastarria and Big Joe. Big Joe is dominantly dacitic, whereas dacite occurs only in the pyroclastic deposits of Lastarria. At similar SiO2 compositions, the former are enriched in MgO and TiO2, while Big Joe is more enriched in CaO relative to MgO. Lastarria rocks are also enriched in Ni, Cr, Ba and Zr but depleted in Y realtive to Big Joe.
Casertano,L., 1963. Catalogue of the active volcanoes of the world; Part XV, Chilean continent I.A.V.C.E.I. Naples, Italy, 55pp.
Francis, P.W., & Wells G.L., 1988 Landsat Thematic Mapper observations of débris avalanche deposits in the Central Andes. Bull Volcanol. 50:258- 278.
Froger, J.-L., Remy, D., Bonvalot, S., Legrand, D., 2007. Two scales of inflation at Lastarria-Cordon del Azufre volcanic complex, central Andes, revealed from ASAR-ENVISAT interferometric data, Earth and Planetary Science Letters, 255(1-2):148-163.
Martin, C., 1917. Los volcanes activos de Chile. Rev. Chil. de Historia Natural, p242-250.
Naranjo, J.A., 1985. Sulphur flows at Lastarria volcano in the North Chilean Andes. Nature 313:778-780
Naranjo, J.A., (1986) Geology and evolution of the Lastarria volcanic complex, north Chilean Andes. Unpub. Msc thesis, Open University, England157pp.
Naranjo, J.A., & Francis, P.W., 1987. High velocity débris avalanche deposit at Lastarria volcano in the North Chilean Andes. Bull. Volcanol. 49:509-514.
Naranjo, J.A., 1992. Chemistry and petrological evolution of the Lastarria volcanic complex in the North Chilean Andes. Geological Magazine, 129(6):723-740.