Volcanic eruptions and tsunami events in Hokkaido are mainly attributed by the subducting Pacific plate. Marsh and lagoon deposits along the Pacific coast of eastern Hokkaido contain depositional records of the Holocene large-scale tsunamis and explosive volcanic eruptions. Coring and sampling using the Geoslicer technique at 182 sampling sites from 10 areas revealed thickness of widespread pyroclastic fall deposits and extent of the area covered by tsunami deposits. The pyroclastic fall deposits constitute mostly fine volcanic ash less than 3 centimeter thick and are well-preserved indicating calm depositional environments. Source volcanoes and ages of the pyroclastic fall deposits are identified by the phenocryst assemblage, shape and major element composition of glass shards, and stratigraphy. The pyroclastic fall deposits of Tarumai-a (Ta-a), Tarumai-c (Ta-c) and Hokkaido-Komagatake-c2 (Ko-c2) from southwestern Hokkaido, and Baitoushan-Tomakomai (B-Tm) from northern Korea/China are abundant throughout the study area. The Usu-b (Us-b) and Tarumai-b (Ta-b) pyroclastic fall deposits from southwestern Hokkaido can only be found in the southern part of the study area. Most of the pyroclastic fall deposits have more extensive distributions suggesting underestimated eruptive volumes by previous researches.

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This paper reviews the principles and the methods used to investigate the ascent process of water-saturated magmas based on crystal texture and composition, which are especially relevant to recent progress made in decompression experiments. The primary cause of syneruptive crystallization is an increase in the liquidus temperature and resultant undercooling due to a decrease in dissolved H₂O in the melt. Analyses of ejecta crystal texture provide time-resolvable information. Firstly, recent decompression experiments have improved knowledge regarding crystallization kinetics and have confirmed correlations made between ascent conditions, texture and crystal composition. The number density of groundmass microlite, crystal size distribution, and crystal form all reflect eifective undercooling (ΔT_eff). This increases with an increased pressure drop (ΔP), and also with an increased decompression rate (ΔP/Δt). If ΔP or ΔP/Δt match in an experiment series, we can compare the texture of the run products with respect to the other parameters. The experiments confirmed that the rates of crystal growth and nucleation show classical bell-shaped curves with ΔT_eff. Growth rate reaches a peak at a lesser ΔT_eff than for nucleation. Variation of ΔT_eff causes a shift in crystallization style. Secondly, decompression experiments help in estimation of the rate and style of magma ascent during a specific eruption through replication of crystals found in ejecta. To best reproduce natural ascent conditions one needs to know a) the number of syneruptive ascent stages, each of which can be assigned an approximate constant speed, b) temperature and pressure of magma at the start and end of each ascent stage, c) how texture evolved during the syneruptive ascent. Ascent conditions are estimated for each stage based on the experimental reproduction of the texture and composition of the natural ejecta. Accuracy in the estimation of ascent rate is thus influenced by errors in quantitative analysis of natural ejecta and the correlation of the decompression rate with textural parameters. Such experimental approaches improve our interpretation of eruption mechanisms by enabling us to combine observation made upon the eruptive product and time-resolved geophysical data. The application of this approach to the 2000 A.D. eruption of Usu Volcano in Japan is presented. With further improvement in the experimental techniques both laboratory and observational studies on ejecta will play a more important role in linking various volcanology research fields.

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