Tlaloc is a late Pleistocene stratovolcano located NE of M¨¦xico City. It is the northernmost volcano of the N-S Sierra Nevada Volcanic Range, which consists from north to south of Tlaloc, Telap¨®n, Iztacc¨ª
huatl, and Popocat¨¦petl volcanoes. Tlaloc has always been considered the oldest (and extinct) volcano of the Sierra Nevada Volcanic Range. Recent field data revealed that Tlaloc was very active during late Pleistocene through a series of explosive eruptions. One of these eruptions produced the Multilayered White Pumice (MWP) a rhyolitic pyroclastic sequence. The eruption began with a 24-km high Plinian column MWP-F1 that was dispersed to the NE by prevailing winds. It was interrupted by fountaining of the column with the generation of a pyroclastic density current that emplaced MWP-S1 layer. Then, followed five unstable sub-Plinian columns (MWP-F2 to F6) that reached altitudes between 16 and 19 km. Fall deposits as a whole are 1 m thick at 12 km from the vent, cover a minimum area of 577 km
2 for a total volume of 4.68 km
3 (DRE 1.58 km
3). The eruption ejected a total mass of 3.45 ¡Á 10
12 kg at different mass discharges. The last sub-Plinian column (MWP-F6) collapsed and produced dense pyroclastic density currents that deposited pumiceous pyroclastic flows (MWP-PF) following main ravines to the north and east of the vent. These density currents filled gullies with 23 m-thick deposits at a distance of 12 km from the vent totaling a minimum DRE volume of 0.2 km
3. Pyroclastic flow deposits charred tree trunks that yielded an age of 31,490 + 1995/? 1595 yr B.P. that closely date the age of the eruption. Rain during this phase of the eruption generated syn-eruptive lahars (MWP-DF). Post-eruptive lahars (MWP-ED) finally swept the volcano flanks.
The MWP deposits consist of abundant white pumice (up to 96 vol. % ), rare gray pumice, cognate lithics, accidental altered lithics, xenocrysts. White and gray pumice clasts contain phenocrysts of quartz, plagioclase, sanidine, biotite, rare Fe-Ti oxides, monazite, zircon and apatite. Xenocrysts are represented by plagioclase, microcline, orthoclase and quartz likely coming from a deeper plutonic body. Both pumices have a rhyolitic composition (74.98 ¡À 1 wt. % SiO2 in water free basis) representing one of the most acidic products of Tlaloc and the entire Sierra Nevada Volcanic Range. The rhyolitic liquid was likely extracted from a mush-like plutonic body reaching the minimum predicted vesicularity values for Plinian fragmentation for which the triggering mechanism of the MWP eruption is attributed to the high viscosity of the magma that provoked overpressure of the magmatic system. The parameters studied suggest that eruption behavior evolved from Plinian to sub-Plinian eruptive columns over time due to conduit dynamics rather than variations in magma properties of the MWP products.
