Resumen
RESUMEN. El Llaima es uno de los mayores volcanes de los Andes del Sur (37°-46°S) con un area de ca. 500 km2 y un volumen de ca. 400 km3. Es un volcan compuesto, mixto y de escudo con una caldera cubierta y 40 conos adventicios de escoria. Se encuentra activo desde el Pleistoceno superior y en el se reconocen dos unidades principales, de las cuales la superior es enteramente postglacial. El volcan tiene una mayor proporcion de lavas respecto de piroclastos, con una composicion principalmente basaltica a andesitica (51-59% Si02) y, en menor proporcion, productos andesiticos siliceos a daciticos (60-65% Si02). Dentro de la base de la unidad postglacial se destaca una sucesion que revela una intensa actividad explosiva entre los ca. 13.200 y 7.200 a A.P. La evolucion de esta etapa comienza con el emplazamiento de la Ignimbrita Curacautin de composicion basaltica a andesitica (51-58% Si02) rellenando el fondo de los valles alrededor del volcan y extendiendose hacia la Depresion Central hasta mas de 100 km al oeste. Abarca un area de unos 2.200 km2 y se estima que tiene un volumen de ca. 24 km3, con espesores maximos expuestos de hasta 50 m. Esta ignimbrita habria estado ligada a la generacion de una caldera. La sucesion incluye depositos de caida de pomez, los cuales representan los productos mas siliceos del volcan (65% SiO2) y que fueron eyectados a los ca. 8.800 a A.P. mediante una erupcion pliniana cuya columna habria alcanzado unos 40 km de altura. La erupcion piroclastica culmina con depositos de oleadas piroclasticas de base de composicion andesitica y andesitica silicea (59-62% Si02), originados, probablemente, debido al colapso de la columna pliniana hacia el sureste, dando lugar a acumulaciones de 6-8 m de espesor promedio. La etapa de actividad altamente explosiva del Llaima culmino a los ca. 7.200 a A.P. con el emplazamiento de oleadas piroclasticas menores hacia el sector oriental. El tiempo transcurrido desde entonces al presente, tiene una implicancia relevante en la modalidad de vigilancia que el volcan requiere, siendo particularmente importante investigar el tamano actual o heredado de su camara magmatica. ABSTRACT. The Llaima is one of the largest Andean volcanoes, with an area of ca. 500 km2 and a volume of ca. 400 km3. It is a complex composite-shield volcano, with a buried caldera and 40 parasitic scoria cones. The volcano is active since the Late Pleistocene and two main units can be distinguished: an older one deeply eroded during the last glaciation and a younger one entirely Post-glacial. The volcano is composed mainly of lava flows with a small amount of pyroclasts, of basaltic to andesitic composition (51-59% SiO2). Silicic andesites to dacites are very scarce (60-65% SiO2). Within the base of the postglacial unit, a remarkable pyroclastic succession discloses an intense explosive activity between ca. 13,200 and 7,200 a S.P. This evolutive stage starts with the emplacement of a large pyroclastic flow (Curacautin Ignimbrite) of basaltic to andesitic composition (51-58% SiO2), filling the valley bottoms around the volcano and spreading into the Central Depression for more than 100 km to the west. It covers around 2,200 km2 and has an estimated volume of ca. 24 km3, with 50 m thick maximum exposures. This ignimbrite probably formed an 8 km diameter caldera as a remnant of the primitive collapsed stratovolcano. Above, within the succession, a conspicuous plinian pumice air-fall deposit dates ca. 8,800a S.P. The eruption column reached around 40 km high and the wind dispersed the plume toward the south-east. About 4 km3 of dacitic pumice (65% SiO2) were erupted and it represents the most silicic products recognized in the whole volcano stratigraphy. This explosive event ended with 6-8 m thick base surge deposits, with an andesitic to silicic andesite composition (59-62%SiOJ They were probably generated as the plinian column collapsed toward the south-east. The explosive stage of Llaima volcano ended about 7,200 a S.P. with the emplacement of small surge deposits toward the east. After this 6,000 years long explosive period, an effusive stage took place building the present volcanic edifice through strombolian, Hawaiian and minor subplinian-type eruptions. The pyroclastic deposit volumes and the eruptive behavior suggest that Llaima has a large magma chamber that de serves geophysical and geodetic research and continuous monitoring.