La erupcion reciente en el Complejo Volcanico Lonquimay (diciembre 1988), andes del sur

Abstract

RESUMEN. El Complejo Volcanico Lonquimay, de edad pleistocena superior-holocena, forma parte del frente del arco volcanico de los Andes del Sur y presenta una composicion predominantemente andesitica. La nueva erupcion, ubicada en el pie noreste de su cono principal y sobre la fisura activa en 1887-89, se inicio el 25 de Diciembre de 1988, en un nuevo crater que evoluciono rapidamente a una pequena fisura con cuatro crateres, para luego de tres dias, concentrarse en el mas occidental, en donde se edifico el cono de piroclastos Navidad. La erupcion, de tipo estromboliana, con un inicio mas explosivo (VEI 2-3) y con columnas de emision que alcanzaron hasta 9.000 m, declino durante los meses siguientes, presentando columnas de 1.000-2.000 m en Abril (VEI 1). Desde el cono Navidad, con crater abierto al noreste, fluyo lava inicialmente de bloques, la cual evoluciono a transicional entre bloques y 'aa', y con tasas de emision declinantes (de 5 x10 6 a 10 6 m3/dia). Hasta el 5 de Abril se estima un volumen de lava emitido de ca. 142 x 10 6 m3. Simultaneamente, fueron eyectados piroclastos correspondientes a bombas de hasta 5 t a 500 m del crater, lapilli escoriaceo y ceniza de color gris oscuro a negro, esta ultima dispersada principalmente hacia el este. Las lavas y piroclastos corresponden a andesitas afaniticas, con microcristales de plagioclasa, clinopiroxeno, olivino y opacos, cuya composicion quimica (SiO2 = 57,8-58,9%) Y petrografica es similar a las anteriores del complejo. Aunque de magnitud reducida, la presente erupcion ha afectado a los habitantes de la zona y a la industria agropecuaria. ABSTRACT. The Lonquimay volcanic complex, of Late Pleistocene to Holocene age and predominantly andesitic composition, is part of the Southern Andes volcanic front. The present eruption, initiated on December 25, 1988, is located on the northeast foothill of the main edifice of the complex, along a fissure active in 1887-89. It started with one crater which shortly evolved to a small fissure with lour craters. After three days the explosive activity concentrated in the westernmost crater, where the Navidad (Christmas) pyroclastic cone was built. The eruption, of strombolian type, had a more explosive beginning (VEI 2-3), with emission columns that reached up to 9,000 m. During the following months it decreased, showing by early April columns between 1,000 and 2,000 m (VEI 1). From the Navidad cone, which is breached to the northeast, a blocky lava was erupted which later evolved to transitional between blocky and 'aa'. By April the emission rates of the flow has decreased from 5 x 10 6 to 10 6 m3/day, with an estimated volume of 142 x 10 6 m3 of erupted lava. Simultaneously pyroclastic material was ejected, including bombs up to 5 tons, 500 m from the crater, and dark gray to black scoriaceous lapilli and ash, the latter mainly dispersed eastward. The lavas and pyroclasts correspond to alanitic andesites with plagioclase, clinopyroxene, olivine and opaque minerals microcrysts of chemical (SiO2 = 57,8-58,9%) and petrographic composition similar to previous andesites of the complex. Although it is a rather small eruption it has harmed the inhabitants of the area, and largely the farming industry.