Evolution of the 2015 Cotopaxi Eruption Revealed by Combined Geochemical and Seismic Observations

International audienceThrough integration of multiple data streams to monitor volcanic unrest scientists are able tomake more robust eruption forecast and to obtain a more holistic interpretation of volcanic systems.We examined gas emission and gas geochemistry and seismic and petrologic data recorded duringthe 2015 unrest of Cotopaxi (Ecuador) in order to decipher the origin and temporal evolution of thiseruption. Identification of families of similar seismic events and the use of seismic amplitude ratios revealstemporal changes in volcanic processes. SO2 (300 to 24,000 t/d), BrO/SO2 (5–10 × 105), SO2/HCl(5.8 ± 4.8 and 6.6 ± 3.0), and CO2/SO2 (0.6 to 2.1) measured throughout the eruption indicate a shallowmagmatic source. Bulk ash and glass chemistry indicate a homogenous andesitic(SiO2 wt % = 56.94 ± 0.25) magma having undergone extensive S-exsolution and degassing duringascent. These data lead us to interpret this eruption as a magma intrusion and ascend to shallow levels.The intrusion progressively interacted with the hydrothermal system, boiled off water, and producedhydromagmatic explosions. A small volume of this intrusion continued to fragment and producedepisodic ash emissions until it was sufficiently degassed and rheologically stiff. Based on the 470 kt ofmeasured SO2 we estimate that ~65.3 × 106 m3 of magma were required to supply the emitted gases.This volume exceeds the volume of erupted juvenile material by a factor of 50. This result emphasizes theimportance of careful monitoring of Cotopaxi to identify the intrusion of a new batch of magma, whichcould rejuvenate the nonerupted material