[S-VC32] 火山噴火のダイナミクスと素過程

[SVC32-P07] Characteristics of Ash Particle from Maar Complex, Lamongan Volcanic Field, East Java, Indonesia

*Andriansyah Gurusinga¹、Tsukasa Ohba¹ (1.Akita University)

キーワード：volcanic ash, LVF, maar eruption

The morphology and shape of volcanic ash particles are controlled by a combination of the physical properties of magmas and the conduit process of magma ascent. At maar eruptions, where typically basaltic magma has erupted, they are dominated by explosive phreatomagmatic activities to form lithic rich to fine-grained deposits. Volcanic ash from these eruption activities produced a significant portion of fine-grained particles by strong magma fragmentation. The variation and origin of ash particles in maar eruptions, which may significantly relate to eruption dynamics, remain unclear by detailed observation. We chose fine-grained layers from three different regions in the Lamongan Volcanic Field (LVF) maar complex. We quantified the texture of ash particles using classical shape parameters and the eruptive parameters using geochemical analysis to evaluate the eruption conditions. Componentry analysis of ash particles bears component types and distribution patterns related to the eruption process and vent history. The samples were divided into seven types from two main categories such as primary ash (black non-vesicular [bnv], black vesicular [bv], clear-brown [cb], and pale-brown [pb]) and accessory ash (orange-brown [ob], free crystal [fc], and altered [alt]) particles related to the eruptions. The primary ash particles from the maar eruption represent some magmatic properties. The bnv and bv particles have higher SiO₂ (48-51 wt.%), higher viscosity (323-707 poises), and lower temperature (1096-1125°C), whereas the cb and pb particles present lower SiO₂ (45-47 wt.%), lower viscosity (82-161 poises), and higher temperatures (1151-1169°C). The bnv and bv ash particles have equidimensional shapes and sub-angular outlines in common, but cb and pb ash particles have sub-equant shapes and sub-rounded outlines. Ash particles from LVF also reflect longer residence in shallower magma chamber resulting in high crystallinity texture. Our results suggest that heterogeneities in textures and morphologies of the ash particles (bv, bnv, cb, and pb) in maar eruption related to the difference of magma properties in conduit sequence during the eruptions.