[S-VC28] Active Volcanism

[SVC28-P05] Amorphous silica coating on flank deposits of the 1783 CE (Tenmei) eruption at Asama volcano

*Takayuki Nakatani¹, Sarina Sugaya², Maya Yasui³, Satoshi Okumura², Michihiko Nakamura² (1.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.Department of Earth Science, Graduate School of Science, Tohoku University, 3.Department of Earth and Environmental Sciences, College of Humanities and Sciences, Nihon University,)

Keywords:Rock coating, Amorphous silica, Acid rain, Vog, Asama volcano

Recently, rock coatings have attracted attention in the broader field of geosciences because of their potential to record the changes in their immediate environment. Despite the potential for silica-rich rock coatings to evaluate past volcanic activity and its impacts on the surface environment, detailed reports on the silica-rich rock coatings at the proximal area are still rare, and its occurrence and formation processes are not fully understood. In this study, we report the amorphous silica-rich coating on the juvenile volcanic lapilli and blocks of the 1783 CE (Tenmei) pyroclastic flow deposits 1.9–3.6 km northeast of the Maekake crater of Asama volcano in Central Japan. These coatings are up to 660 µm in thickness and composed of fine-grained volcanic ash particles with silica-rich cement. Raman spectroscopic measurements showed that the interstitial silica-rich phase is hydrous amorphous silica and is considered to be a reaction product of ash particles with acidic volcanic rain and fog like volcanic smog (vog) as it has been recorded in Hawaii. The formation of amorphous silica without jarosite and the preferential alteration of plagioclase compared with pyroxene indicate that the reaction occurred with a sulfuric acid solution of pH < ~3. The dissolution of sintered volcanic ash and the resultant precipitation of amorphous silica would have been facilitated by the porous structure and large surface area of the ash layer. The thickness of the coating and degree of alteration in the plagioclase phenocrysts adjacent to the coating tend to decrease with an increase in distance from the volcanic crater, which is consistent with the absence of fumarolic areas in the flank. Since 1783 CE, the Asama-Maekake volcano has repeated Vulcanian and phreatic eruptions, especially in the first half of the 20^th century. The amorphous silica coating recognized in this study is assumed to have formed mainly during this period, which can thus be an indicator of the acidity and distribution of past acidic volcanic rain and fog.