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Mode of eruption and deposition of the hachinohe phreatoplinian ash from the towada volcano, japan

Department of Geography, Tokyo Metropolitan University
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  • Explosive Eruption
  • Magma-Water Interaction
  • Accretionary Lapilli
  • Towada Volcano


The Hachinohe ash (SiO_2=69%) is a 16 km^3 phreatoplinian fallout deposit erupted from the Towada volcano 13,000 years ago. It is composed of alternating beds of fine ash and pumice lapilli. Fine ash beds are made up of closely packed accretionary lapilli with open matrix. A number of accretionary lapilli are also found in pumice beds. The dynamic contact between erupting magma and lake water was responsible for the phreatomagmatic character of this deposit. In the course of the eruption, discharge rate of magma occasionally rose so that mass ratio of water to magma became lower than the ratio of an explosive maximum, when the style converted into a relatively magmatic eruption. Pumice beds indicate timing of these occurrences. The premature fallout of fine ash from eruption clouds in the form of accretionary lapilli or particle aggregates will be an inevitable process operating in phreatomagmatic eruption clouds. In the case of the Hachinohe eruption, it is supposed from spherical shapes of accretionary lapilli that they were frozen up during the descent before reaching the ground.

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