New observations on the recent eruptive activity of Sumaco Volcano (Ecuador), based on geochronology, stratigraphy and petrography

Abstract

Sumaco is a stratovolcano located in the northern SubAndean zone of Ecuador, 105 km east of Quito. The flanks of Sumaco are jungle-covered, and scarce outcrops are found along stream banks, trails and, in quarries. Based on our field studies, eruptions of Sumaco during the historical era were associated with small-scale Strombolian activity. Earlier more robust activity, which pertains to the last 4400 years, had vigorous eruptions whose ∼10 cm thickness ash fallout deposits are identified more than 20 km from the vent.Stratigraphic sections studied around the volcano's outer flanks identify five principal ash-fall layers corresponding to Sumaco's activity. The ash layers contain white micro-vesiculated pumice, displaying clinopyroxene, plagioclase, traces of amphibole, and biotite crystals. Also, gray lithics with pyroxene phenocrysts and scarce plagioclase crystals are present. The amount of volcanic glass shards is meager in almost all samples. The thickest ash layer, “SUM-JS-44″, was dated by 14C, obtaining an age of 3029-2888cal AD.Analysis of a sediment core obtained from the “Guagua Sumaco” near-source lagoon allowed us to identify nine ash layers; seven are from Sumaco. Dating an underlying layer of peat (OL3) beneath the six ash layers provides an age of 1483–1644 cal AD, showing that Sumaco volcano experienced at least six separate eruptive pulses during the last centuries, and thus it should be considered potentially active.Based on morphological observations, Sumaco's edifice consists of three phases: PaleoSumaco, Recent Sumaco, and Current Sumaco, differentiated mainly by two scars representing flank collapses that produced far-reaching debris avalanches, whose breccia deposits are observed around the cone's depositional apron.The aim of our research is to understand the evolution of Sumaco's edifice and verify the relative youthfulness of the volcano and its eruption styles. These inputs are then applied in determining three eruption scenarios.