LAUSR

Abstract Cotopaxi is an active, hazardous and ice-covered stratovolcano 60 km southeast of Quito, (Ecuador) whose last major eruption occurred in 1877. During 2001–2002, volcanic unrest characterized by volcanic seismicity and deformation ended without eruptive activity. On April 2015, a new increase of seismicity, SO2 emissions, thermal anomalies and edifice deformation, evolved into the onset of a new eruptive cycle, beginning August 14. We sampled and measured the ash fall deposits to the west of Cotopaxi between August 14 and 24, 2015. The ash collected was analyzed using grain size, X-Ray fluorescence, X-Ray diffraction and scanning electron microscope (SEM-EDS), revealing the eruptive products to be compound of dense fragments (mostly lithics), diverse types of scoria, pumice, free fractured crystals, glassy particles and aggregates. Most of hydrothermal alteration is observed during the initial stage of the eruption (14–15 August; including Cu oxides and Fe minerals in the lithics). The glassy particles were blocky morphology, and textural changes were recognized over 10 days of eruption, varying from null or low vesicularity to low-to-moderate vesicularity, occasionally exhibiting molten or subrounded textures. The bulk ash has a basaltic-andesitic composition (~ 55.67 wt% of SiO2), while clusters of selected particles (likely juvenile) analyzed through SEM + EDS reveal dacitic composition (65.67 and 65.8 wt% SiO2). Plagioclase, clinopyroxene and orthopyroxene are the main minerals present, with accessory anhydrite, melanterite and pyrite (these typically observed during the initial stage of eruption). These variations in addition to the geophysical background, led us to interpret this eruption as the result of the volcano's hydrothermal system disruption due to a shallow, low-volume magma input, which initially evolved into phreatic activity at surface level. Further activity up to 24 August was triggered by the indirect interaction between magma and the depleted hydrothermal system, generating a magmatic-hydrothermal eruption. The issue is important for evaluating unrest periods at active stratovolcanoes, and the impact of their initial, low-volume ash falls in neighboring communities.