Fieldwork and radiocarbon dating provide evidence of two major explosive eruptions and an intervening minor eruption. The Nevado, a central stratovolcano built up in late Pleistocene time, was eviscerated by a violent Vulcanian-type eruption about 25,000 yr B.P., which deposited thick, cold, lithic lahars around its flanks. After a period of widespread soil formation, dated at 24,500 yr B.P., a relatively minor Vesuvian-type eruption produced small amounts of pumice that fell only on the eastern and northeastern slopes of the volcano. A major Plinian-type eruption took place at 11,600 yr B.P., directed mainly east-northeast, which spread thick dacitic pumice over an area of at least 1,700 km2. This eruption probably produced the “tripartite” pumice, which forms a marker for the stratigraphy of early man in the adjoining Valley of Mexico. The vent orifice was subsequently plugged by a dome of vesicular dacite.

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