The geology of Nevados de Chillan volcano, Chile

Nevados de Chilian volcano is a large composite stratovolcanic complex in t he Southern Volcanic Zone of the Chilean Andes. It is one of the highest-ri sk volcanoes in Chile due to high levels of historic activity and rapid dev elopment of economic activity in the area. High precision Ar-40/Ar-39 and C -14 geochronology, geochemistry and petrology have been employed in additio n to photogeology and field mapping to elucidate the evolution of this volc ano and assess its hazards. Nevados de Chilian has been active since at lea st 640 ka when a large group of subglacial andesite flows were erupted. Sin ce 100 ka, sequences of andesite and dacite lavas have been erupted into bo th subaerial and subglacial environments. lgnimbrites were erupted at aroun d 40 ka and may have been associated with caldera collapses. Two separate e ruptive centres have evolved since 40 ka: the Cerro Blanco and Las Termas s ubcomplexes. The two centres are 6 km apart. yet have contemporaneously eru pted geochemically distinct magmas. Subglacial lavas have been identified o n the high flanks of the volcano and Ar-40/Ar-39 dating has confirmed their eruption during recent glaciations (isotope stages 4 and 2). Tephra fall d eposits have been dated by C-14 analysis of interstratified organic materia l and indicate that no proximal tephra fallout deposits older than 9 ka rem ain. Tephra dispersal indicates that Holocene activity has involved vulcani an to subplinian eruptions. At least, 3 pyroclastic flow eruptions have occ urred during the Holocene and lahar deposits are common in the valleys arou nd the volcano. Historically, the Santa Gertrudis vent erupted during 1861- 1865 and the dacite lava cone complexes Nuevo and Arrau were constructed du ring 1906-1943 and 1973-1986, respectively. Historic records indicate that lahars and landslides are major hazards to economic developments on the low er flanks and valleys.