The 12.1 ka Middle Toluca Pumice: A dacitic Plinian-subplinian eruption of Nevado de Toluca in Central Mexico

摘要

The Nevado de Toluca volcano erupted explosively approximately 12.1 ka ago, producing a Plinian-subplinian eruption that deposited the Middle Toluca Pumice (MTP). The MTP consists of white and gray juvenile pumice, gray dense juvenile lapilli, and red altered lithic lapilli. The pumice is dacitic (63.54-65.06 wt.% SiO_2) with phenocrysts of plagiocla-se >orthopyroxene> hornblende ± ilmenite and titanomagnetite, and biotite xenocrysts set in a groundmass of rhyolitic glass (70-71 wt.% SiO_2). The MTP has a dispersal axis to the ESE covering an area of 92 km~2, with a minimum volume of 1.8 km~3 (DRE). Stratigraphic relations, grain size, componentry, and vesicularity analyses suggest that the eruption occurred in five major phases: (1) an opening magmatic phase that generated a 20-km-high Plinian column dispersed to the SE; (2) a hydromagmatic explosion followed with the establishment of a subplinian eruptive column (18-19 km high) dispersed tephra to the SE and gradually waned; (3) hydromagmatic explosions emplaced dilute pyroclastic density currents followed by the formation of an eruptive column of unknown height; (4) immediately after, a new magmatic explosion established another eruptive column; and (5) the collapse of the latter column generated two pumiceous pyroclastic density currents that were fully dilute proximally, but transformed into two granular-fluid pyroclastic currents that traveled 19 km from the source.