New evidence of Holocene pyroclastic density currents at Galeras volcano, Colombia

Abstract

The Galeras Volcanic Complex (GVC) is a composite volcano located between the Central and Western cordilleras in southern Colombia. It is considered one of the most active volcanoes in Colombia, with typically Vulcanian eruptions. The complex has been divided into different stages based on stratigraphic and geochronological differences. The most recent one is called the Galeras stage and is considered to have started ca. 4500 years. This study presents the results of new stratigraphic and geochronological analysis of the deposits that outcrop along the El Barranco river valley, NW of the GVC, which allowed us to identify newer and older deposits within the most recent stage (i.e., the Galeras stage). The deposits were analyzed via distribution, stratigraphy, geochronology, componentry and granulometry, as well as vesicularity and microtexture of the pumice fragments. We identified three previously unreported deposits, which results in a total of eight pyroclastic density current (PDC) deposits in the valley, which discordantly overlie andesitic lava flows of the previous stage (i.e., Genoy stage; 150–40 ka) of the GVC. They were named from Unit U1 to Unit U8, with units U1, U2 and U8 representing events not previously recorded in the eruptive history of Galeras volcano. Units U1 (8303 ± 97 and 8284.5 ± 90.5 cal BP), U2 (7667 ± 78 cal BP), U3 (5082 ± 198, 5096.5 ± 226.5, 5801.5 ± 481.5 and 5966 ± 235 cal BP) and U6 (2215.5 ± 101.5 and 2033 ± 91 cal BP) are interpreted as formed by PDCs generated by pumice-rich flows linked to the collapse of Subplinian eruptive columns, while units U5 (3201.5 ± 129 cal BP), U7 (1066.5 ± 109.5 and 944.5 ± 115.4 cal BP) and U8 (390.5 ± 80.5 cal BP) are interpreted as formed by PDCs generated by block and ash flows linked to the destruction of intracrater domes and the collapse of Vulcanian eruptive columns. U4 (4625 ± 181 cal BP) is associated with a “blast” type event, caused by the overpressure exerted by the gases of a viscous magma. The above mentioned record allows us to propose that the eruptive history of Galeras volcano began at the beginning of the Holocene (∼8300 years BP), and not 4500 BP, as previously established, which has hazard implications. Unit U8 in the town of La Florida as well as the nearby recorded deposits, indicates a probable PDC hazard for this population.