Abstract
Volcanoes are often monitored by geophysical and geochemical instruments that aim to track and anticipate their eruptive activity. However, assessment of the state of the volcano at any given time, and its evolution towards eruption or change in eruptive activity is notoriously difficult. Once explosive activity has begun, the study of volcanic ash can provide crucial insights on whether the activity is mainly driven by the hydrothermal system, shallow gas accumulation, or/and a new stalled intrusion close to the surface. We present the results of a study of ash componentry from part of the current volcanic crisis that started in December 2015 at Nevados de Chillán Volcanic complex (Chile) which we integrate with seismic and visual data. We identified three main stages: (i) an early one that lasted for about a year and includes two months of increased seismicity and significant amount of juvenile ash fragments, and thus suggesting some explosions were fed by a shallow magmaintrusion. (ii) A second one which lasted for about six months with cycles of quiescence and explosions, and a predominance of lithic particles in the ash, suggesting that the explosions were probably driven by the dynamics in the upper part of the system, including shallow gas accumulation or/and the hydrothermal system, rather than by fresh magma intrusion. (iii) Finally, after about two years of unrest and intermittent explosions, seismicity increased again and the ash became dominated by juvenile particles, and led to the extrusion of a dome. The timing and sequence of events that we report is broadly similar to other volcanoes that have produced dome eruptions such as Soufriere Hills (Montserrat), Unzen (Japan) and Sinabung (Indonesia). Our study highlights the usefulness of integration of volcanic ash studies with other monitoring data and importance of integration of many case studies to gain a more comprehensive understanding of the processes and evolution of dome-forming eruptions.