Heat transport process associated with the 2021 eruption of Aso volcano revealed by thermal and gas monitoring

Abstract

Abstract The thermal activity of a magmatic–hydrothermal system commonly changes at various stages of volcanic activity. Few studies have provided an entire picture of the thermal activity of such a system over an eruptive cycle, which is essential for understanding the subsurface heat transport process that culminates in an eruption. This study quantitatively evaluated a sequence of thermal activity associated with two phreatic eruptions in 2021 at Aso volcano. We estimated plume-laden heat discharge rates and corresponding H2O flux during 2020–2022 by using two simple methods. We then validated the estimated H2O flux by comparison with volcanic gas monitoring results. Our results showed that the heat discharge rate varied substantially throughout the eruptive cycle. During the pre-eruptive quiescent period (June 2020–May 2021), anomalously large heat discharge (300–800 MW) were observed that were likely due to enhanced magma convection degassing. During the run-up period (June–October 2021), there was no evident change in heat discharge (300–500 MW), but this was accompanied by simultaneous pressurization and heating of an underlying hydrothermal system. These signals imply progress of partial sealing of the hydrothermal system. In the co-eruptive period, the subsequent heat supply from a magmatic region resulted in additional pressurization, which led to the first eruption (October 14, 2021). The heat discharge rates peaked (2000–4000 MW) the day before the second eruption (October 19, 2021), which was accompanied by sustained pressurization of the magma chamber that eventually resulted in a more explosive eruption. In the post-eruptive period, enhanced heat discharge (~ 1000 MW) continued for four months, and finally returned to the background level of the quiescent period (< 300 MW) in early March 2022. Thus, despite using simple models, we quantitatively tracked transient thermal activity and revealed the underlying heat transport processes throughout the Aso 2021 eruptive activity.