Source and behavior of pyroclastic density currents generated by Vulcanian-style explosions of Popocatépetl volcano (Mexico) on 22 January 2001

Abstract

Since 1994 Popocatépetl volcano has extruded 85 lava domes, each of which has been destroyed by Vulcanian-style explosions. The paroxysmal Vulcanian explosion of 22 January 2001 has been unique by generating multiple pyroclastic density currents (PDCs). Scientists and Civil Protection authorities have used those PDCs to remap the high-risk hazard zone of the volcano, although little study has been carried out to understand the generation and evolution of those PDCs. Here, we examine 38 stratigraphic sections and analyze the granulometry, componentry, vesicularity, and petrology (glass, melt inclusions in plagioclase) of collected samples to interpret the origins of the Vulcanian explosions and the PDC behavior. The 22 January event generated two series of dense PDCs that detached from the base of the eruption column. The first set of PDCs traveled northwest across a glacier on the volcano, partially scouring and melting the ice that hours later generated a lahar. Two pulses of these PDCs were confined within a large ravine, surpassed the slope break, and traveled up to 3 km away from the vent. The deposits are matrix-supported throughout their extent. The second set of PDCs traveled to the northeast, mainly as unconfined currents that split into two lobes prior to the slope break. The farthest traveled current followed a western ravine and reached the forest 3.5 km from the summit. The eastern lobe buried a local hiking hut and reached 2.1 km. The deposits from these northeastern currents are massive and block-supported. All deposits (volume of 5.71 × 105 m3) have finger-like morphology with levees and frontal snouts. All PDCs flowed as a dense basal granular current with turbulent upper clouds at minimum speeds of 7–10 m/s.The components of the deposits are, in order of abundance, black scoria (53–62 wt% SiO2), banded scoria, pumice (63 wt% SiO2), dome lithics (60–61 wt% SiO2), accidental lithics, and free crystals. Juvenile pumice and scoria contain phenocrysts of plagioclase (An47±10), clinopyroxene (En48±3), orthopyroxene (En64±1), ilmenite, and titanomagnetite set in a rhyolitic glass (pumice clast; 74.39 wt% SiO2) or microlite-rich matrix (scoria clast). The dome lithics also contain these phenocrysts as well as rare olivine phenocrysts (Fe2+/(Fe2 + Mg) = 0.1). Dissolved H2O-CO2 contents in melt inclusions hosted by pumice-plagioclase phenocrysts suggest the felsic magma was stored at depths of 2–4 km. Thus, the 2001 event was triggered by the injection of mafic magmas into a small volume of felsic magma, probably forming dikes left from the previous eruptive episode. We propose that the Vulcanian-explosions tap a column of magma (~2 km depth) equivalent to the volume that contained the felsic dikes. This mechanism has been acting during the destruction of all summit domes of the ongoing eruption of the volcano.