Mechanisms of Steam-blast Eruptions Inferred from the Mineralogy of Volcanic Ash

Steam-blast eruptions are classified into three categories: (1) hydrothermal eruption caused solely by a phase change of hydrothermal water within a hydrothermal system; (2) phreatic eruption caused by a new thermal input derived from a magma body in a sub-volcanic aquifer; and, (3) ultravulcanian eruption (gas eruption), a type of vulcanian eruption, which is caused by gas degassed from magma accumulating under a lava plug. It is proposed that these can be classified from a petrological analysis of eruption products based mainly on the authors’ previous contributions. Volcanic ash from hydrothermal eruptions is characterized by abundant altered lithics. At some composite volcanoes, altered lithics exhibit a wide variety of alteration types including siliceous, advanced argillic, phyllic, and potassic alterations, which are considered to originate from alteration zones of composite volcanoes. The association of alteration zones are correlated with those around porphyry copper deposits. The products of phreatic eruptions are composed mainly of strongly acid altered rocks, but may also contain fresh volcanic rock fragments. The rocks are derived from selectively/partially altered rocks under the crater. Ultravulcanian eruptions mainly release fresh lithic fragments and may also emit sulfur compound minerals (mainly sulfate), but the products contain no alteration minerals indicating hydrothermal acid leaching.