Abstract

During the late Pleistocene, a large Plinian eruption from Nevado de Toluca volcano produced a complex sequence of pyroclastic deposits known as the Upper Toluca Pumice. This eruption began with a phreatomagmatic phase that emplaced a hot pyroclastic flow (F0) on the east and northern flanks of the volcano. Eruption decompressed the magmatic system, almost immediately allowing the formation of a 25-km-high Plinian column that was dispersed by winds predominantly 70° to the northeast (PC0). Next, three other Plinian columns were dispersed in a northeast to east direction, reaching heights of 39, 42, and 28 km, resulting in fall layers (PC1, PC2, and PC3), respectively. These Plinian phases were interrupted several times by phreatomagmatic and collapse events that emplaced pyroclastic flows (F1, F2, and F3) and surges (S1 and S2), mainly on the eastern and northern flanks of the volcano. The eruption ended with the extrusion of a crystal-rich dacitic dome at the vent.

The juvenile components of the Upper Toluca Pumice sequence are white, gray, and banded pumice, and gray lithic clasts of dacitic composition (63%–66% SiO2) and minor accidental lithic fragments. The fall deposits (PC1 and PC2) covered a minimum area of 2000 km2 and constitute a total estimated volume of 14 km3 (∼6 km3 DRE [dense-rock equivalent]). The mass eruption rate ranged from 3 × 107 to 5 × 108 kg/s, and total mass was 1.26 × 1013 kg.

Charcoal found within Upper Toluca Pumice yielded an age of 10,500 14C yr B.P. (12,800–12,100 14C calibrated yr B.P.), somewhat younger than the earlier reported age of ca. 11,600 14C yr B.P. This new age for the pumice falls within the Younger Dryas cooling event. The eruption emplaced 1.5 m of pebble-sized pumice in the City of Toluca region and ∼50 cm of medium to fine sand in the Mexico City region. Distal lahar deposits derived from the Upper Toluca Pumice event incorporated mammoth bones and other mammals in the basin of Mexico. A future event of this magnitude would disrupt the lives of 30 million people now living in these cities and their surroundings.

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