[SVC42-P12] Factors affecting the sequence of the Plinian eruption inferred from rock texture and physical properties - an example of the 1783 eruption, Asama volcano, Japan
Keywords:Asama Volcano, eruptive style, magma ascent process, textural analyses
Pumice clasts from fallout and pyroclastic flow deposits were chosen for measurement of density, microscopy, and chemical analyses. The apparent density ranges 0.6-0.9 g/cm3 in fall deposits, 0.7-0.95 g/cm3 in pumiceous pyroclastic flow deposits, and 0.7-1.2 g/cm3 in scoriaceous pyroclastic flow deposits. The ratio of the connected vesicles ranges 87-96% in fall deposits, 83-98% in pumiceous pyroclastic flow deposits, and 91-95% in scoriaceous pyroclastic flow deposits. The ratio of connected vesicles ranges widely in pumiceous clasts, pyroclastic flow deposits have lower ratio than fall deposit. Scoriaceous clasts have higher ratio than pumiceous clasts. Phenocryst types are plagioclase, orthopyroxene, clinopyroxene, and Fe-Ti oxide, and olivine is rare. SiO2 content of groundmass glass shows bimodal distribution around 67wt% and 72wt%. Pyroclastic flow deposits contain abundant low-SiO2 clasts than pumice-fall deposits. Magma temperature and water content are estimated from chemical composition of plagioclase and matrix glass (Putirka, 2008; Waters and Lange, 2015), then SiO2= 67 wt%, T=1060-1100℃, H2O=0.8-1.0 wt% and SiO2=72 wt%, T=1000-1015℃, H2O=1.2-1.4 wt%. Vesicles in pyroclasts in the upper units are more connected and elongated. Clast types in pyroclastic flows are similar to the lower part of fallout units, but scoriaceous ones have thick bubble wall glass. We analyzed crystal and vesicle textures by using the shareware program ImageJ and CSD correction (Higgins, 2006). Crystallinity range 7-14% and poor in microlite. The average vesicularity of each layer ranges around 75% in pumice-fall deposit, excepting middle level, and 65-67% in middle level fall deposit and pyroclastic flow deposit. Bubble number density (BND) ranges 2.0-4.0×1013 m-3, then the flow deposits show higher in BND than the fall deposits. Decompression rate for each layer can be estimated from BND, SiO2 content, water content, and temperature (Toramaru, 2006). It was calculated at 3.2-4.7×107 (Pa/s) in fall deposits, 5.0×107 (Pa/s) in pumice flow deposit, and 1.0×108 (Pa/s) in scoriaceous flow deposit.
The change of the decompression rate has a correlation with changes of the eruptive style from column formation to pyroclastic flow generation. Fall deposits accompanied by red ash in the middle layer, which indicates simultaneous generation of pyroclastic flows, show high decompression rate. Based on these results, it is suggested that the increase of magma decompression rate is densely related to generation of pyroclastic flows during the climactic phase of the 1783 eruption.