[S-VC33] Mechanism of volcanic eruptions

[SVC33-05] Mechanism of Non-Eruptive Deflation (NED) events based on the analysis of volcanic ash at Sakurajima volcano, SW Japan

Nanami Homma¹, *Taketo Shimano¹, Atsushi Yasuda², Masato Iguchi³ (1.Graduate School of Science and Engineering, Kagoshima University, 2.Earthquake ResEarthquake Research Institute, University of Tokyo, 3.Kagoshima City)

Keywords:Sakurajima, Volcanic ash, NED, Vulcanian eruption

Sakurajima volcano has been intermittently erupting since 1955, mainly in vulcanian style. These eruptions may sometimes be cyclic in a short term, but vary irregularly in time scales from weeks to years with strombolian and continuous ash emitting activity. Geophysical observations showed inflation and increasing strain in the shallow part of conduit prior to the explosion, followed by deflation. Based on these observations, it has been proposed that a typical vulcanian eruption occurs when degassed crystallized magma is plugged at shallow level in conduit, causing the gas to increase pressure and exceed the strength of the plug to break eventually with explosion. Based on this model, an empirical correlation between ground deformation and eruptive mass is used to estimate ash volume by using strain observation as well as seismic activity. On the other hand, however, the mechanisms and fundamental processes of eruption transition or time sequence are still in debate and need to be verified based on the analysis of eruptive materials.

Recently, Iguchi et al. (2022FES) reported another phase of activity called Non-Eruptive Deflation (NED) at Sakurajima volcano. This phenomenon is characterized by few amount of eruptive material but by gas emission and ground deflation similar to vulcanian explosions. In this study, we report the characteristics of the materials during NED and discuss the mechanism of NED and the difference from other eruptive activities.
We first classified ash samples into 10 categories based on the color and shape of particles, which are considered to reflect crystallization and degassing processes, respectively, and obtained component ratios of different eruption styles as well as NED. The crystallization process was also considered based on texture investigation using BSE images and matrix glass chemical analysis by EPMA.
Almost half of the componentry of NED samples are occupied by black dense lava particles which are very similar to those of explosive eruptions, but are richer in vesicular particles than explosive eruptions. In particular, pumice and scoria in NED samples have less-evolved matrix glass composition than in explosive eruptions. These results suggest that NEDs are caused by the ascent of unevolved and volatile rich magmas albeit small eruptive amount. A possible mechanism for the lack of explosion even for gas-rich magma rise would be the change in sealing properties of the cap-rock, or plug, being heterogeneous and poorly sealed to hold pressure in contrast to the case of explosion.