Monogenetic volcanoes with initial phreatomagmatic phases in the Ceboruco graben, western Mexico: The cases of Potrerillo I, Potrerillo II, and San Juanito

Abstract

Potrerillo I, Potrerillo II, and San Juanito volcanoes are part of the 28 Late Quaternary monogenetic eruptive vents concentrated along a ~2-km-wide and ~30-km-long stripe in the vicinity of Ceboruco stratovolcano (western Mexican Volcanic Belt). The studied volcanoes are closely spaced and located within a ~12 km2 sub-drainage basin, northwest of Ceboruco's edifice. Eruptive dynamics were inferred from sedimentary, stratigraphic, and pyroclast characteristics. Unlike Potrerillo I, which was formed by an entirely dry explosive magmatic eruption, evidence for the prevalence of initial short-lived phreatomagmatic phases was found for the Potrerillo II and the San Juanito eruptions that ended effusively.In this study we examine the nature and proportion of different types of pyroclasts within the exposed deposits of these three volcanoes, as well as their sedimentary features to better understand the initiation and progression of their eruptions. Our results indicate that both Potrerillo II and San Juanito tephra ring deposits show dominantly atypical, crudely bedded, coarse-grained deposits where the evidence for energetic, diluted pyroclastic density currents is scarce or subordinated. Instead, low energy, shallow seated, phreatomagmatic explosions generated medium-to-coarse grain-sized pyroclasts that were transported within dense tephra jets that were ballistically ejected, which on deposition formed non-turbulent currents with non-extensive lateral movement. The nature of such eruptive style was controlled by the substrate hydrogeology conditions that prevailed at the time of the eruption. So, magmas of different composition (dacitic for Potrerillo II, and basaltic andesite for San Juanito) followed similar dynamics when interacting with water. We propose that fractured aquifers with shallow water tables (or even surface water) existed at the time of both eruptions and interacted explosively with the magmas.In contrast, the construction of Potrerillo I scoria cone, which is related exclusively to explosive magmatic activity, does not show evidence of phreatomagmatic activity. This eruption may have occurred when the water table was deep, or surface water was unavailable. The variations in the depth of phreatomagmatic explosions, the availability of the water involved in the phreatomagmatic phases, and the absence or presence of these phases in the studied volcanoes were probably strongly influenced by the water-bearing conditions of the aquifers at the time of the eruption, which in turn, depended on the amount and distribution of annual rainfall.This study improves our understanding of phreatomagmatic eruptions of diverse magmatic compositions in similar settings. The recognition of associated hazards will allow to further define eruptive scenarios of monogenetic eruptions and help update the volcanic hazard map of the Ceboruco volcano area in the future.