Abstract

HERNANDEZ, Javier; CARRASCO-NUNEZ, Gerardo  and  AGUIRRE-DIAZ, Gerardo. Eruptive dynamics of the Ezequiel Montes Pumice in the Amazcala caldera, central Mexico. Rev. mex. cienc. geol [online]. 2009, vol.26, n.2, pp.482-500. ISSN 2007-2902.

The Pómez Ezequiel Montes (PEM) is a late Miocene sequence of plinian fallout deposits associated with the formation of the Amazcala caldera, 30 km NE of Querétaro city in central Mexico. The Pómez Ezequiel Montes is a stratified sequence of fallout deposits interlayered with sporadic thin pyroclastic surge layers, and some horizons of reworked material. The whole sequence is very thick (32 m in the SE type locality) and widely distributed along two main dispersal axes, oriented to the SW and the SE, with respect to the caldera centre. Deposits are dominated by gray pumice of rhyolite composition, with minor quantities of lithic clasts; such composition is fairly homogeneous along the whole sequence. Both the size of the pumice fragments and the thickness of the identified units diminish to the SW and SE and the isopach maps confirm that the Pómez Ezequiel Montes was emitted from the Amazcala caldera. The physical parameters obtained from the study of 14 sections and correlation of unit A (selected by its widespread distribution) reveal that this unit in the SW lobe represents an event associated to a 23-km height plinian eruptive column with an exit velocity of about 100 m/s. This eruptive column produced a widespread pumice fallout with a minimal volume of 2.6 km³ and a mass of 1.45×10¹² kg. The mass discharge rate estimated for this eruptive column is of ~6×10⁷ kg/s.

Keywords : explosive volcanism; Plinian eruption; Amazcala caldera; Mexico.

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