Volcanic-hydrothermal system inferred from steam-blast eruption ash

Abstract

Steam-blast eruptions are classified into three: phreatic, hydrothermal, and ultra-vulcanian. Each eruption type is correlated to the mineralogical characteristic of the volcanic ash. Relatively unaltered lithic fragments and sulfate minerals (anhydrite, gypsum) are abundant in volcanic ash from ultra-vulcanian eruptions. Volcanic ash from hydrothermal eruptions abundantly contains alteration minerals from the alteration zones developed under the volcano, such as the advanced argillic alteration zone. The heating phenomena of phreatic eruptions can be identified by analyzing, e.g., sulfur isotopic compositions of adherent water-soluble components and coarse ash separately.