Reconstruction of the Sibinal Pumice, an andesitic Plinian eruption at Tacaná Volcanic Complex, Mexico–Guatemala

The Tacaná Volcanic Complex, located on the Mexico–Guatemala border, has had numerous small historical explosions, the latest being a phreatic explosion in 1986. The stratigraphic record, however, suggests that much more voluminous eruptions have occurred in the past. Here, we document one such eruption, the Sibinal Pumice deposit, which occurred ca. 23,540 years B.P. The deposit consists of two pumice-rich units: 1) a lower stratified member (SM) that consists of at least seven, normally graded fall layers, interbedded with pyroclastic wet surge layers; and 2) an upper massive member (MM), made up of a single fall deposit. Both members can reach up to 2.5 m in thickness and are separated by a single massive, indurated, yellowish, pumice-rich reworked layer. SM was dispersed to the northeast (N70°E), with its 6-cm isopach covering an area of 275 km2, whereas MM was dispersed to the north (N22°E), with its 70-cm isopach covering an area of ca. 330 km2. Yellow, vesicular pumice fragments, light-gray lithics, and hydrothermally altered (red and black) dense lithics are abundant in both members, although altered lithics are more abundant at the base of each, reaching 15 vol.%. Pumice compositions range from basaltic to andesitic (48–61 wt.% SiO2, anhydrous basis), but are highly altered. They contain plagioclase (andesine–labradorite), augite, hypersthene, Fe–Ti oxides, and rare amphibole. No compositional or mineralogical differences occur between the units indicating a common magma source during the same eruption. The Sibinal Pumice eruption started with a weak, pulsating column that reached at most 19 km in height, ejecting 2.9 km3 of tephra (1.1 km3 DRE) at an average mass discharge rate of 4.7 × 107 kg/s, with repeated hydromagmatic explosions that generated wet surges down the slopes of the volcano. The eruption ceased for a while during which rainfall generated a widespread lahar that eroded the top of SM. The eruption began anew with a sustained and stable Plinian column that ejected 4.6 km3 (1.9 km3 DRE) of tephra at a mass discharge rate of 8.1 × 107 kg/s. The lack of any compositional differences in the magma suggests that the change in eruptive style was driven by influxes of external water provoking hydromagmatic explosions that cleared the conduit, then, the system changed to dry system and the eruption style changed from pulsating to stable Plinian. As the conduit became wider, the mass discharge rate also increased.