The geology of Nevados de Chillán volcano, Chile

Abstract

Nevados de Chillán volcano is a large composite stratovolcanic complex in the Southern Volcanic Zone of the Chilean Andes. It is one of the highest-risk volcanoes in Chile due to high levels of historic activity and rapid development of economic activity in the area. High precision ⁴⁰Ar/³⁹Ar and ¹⁴C geochronology, geochemistry and petrology have been employed in addition to photogeology and field mapping to elucidate the evolution of this volcano and assess its hazards. Nevados de Chillán has been active since at least 640 ka when a large group of subglacial andesite flows were erupted. Since 100 ka, sequences of andesite and dacite lavas have been erupted into both subaerial and subglacial environments. Ignimbrites were erupted at around 40 ka and may have been associated with caldera collapses. Two separate eruptive centres have evolved since 40 ka: the Cerro Blanco and Las Termas subcomplexes. The two centres are 6 km apart, yet have contemporaneously erupted geochemically distinct magmas. Subglacial lavas have been identified on the high flanks of the volcano and ⁴⁰Ar/³⁹Ar dating has confirmed their eruption during recent glaciations (isotope stages 4 and 2). Tephra fall deposits have been dated by ¹⁴C analysis of interstratified organic material and indicate that no proximal tephra fallout deposits older than 9 ka remain. Tephra dispersal indicates that Holocene activity has involved vulcanian to subplinian eruptions. At least, 3 pyroclastic flow eruptions have occurred during the Holocene and lahar deposits are common in the valleys around the volcano. Historically, the Santa Gertrudis vent erupted during 1861-1865 and the dacite lava cone complexes Nuevo and Arrau were constructed during 1906-1943 and 1973-1986, respectively. Historic records indicate that lahars and landslides are major hazards to economic developments on the lower flanks and valleys