沼沢火山における火砕流噴火の多様性 : 沼沢湖および水沼火砕堆積物の層序

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書誌事項

タイトル別名
  • Two Contrast Styles of Pyroclastic Flows of Numazawa Volcano, NE Japan : Stratigraphy of the Numazawako and Mizunuma Pyroclastic Deposits
  • ヌマザワ カザン ニ オケル カサイリュウ フンカ ノ タヨウセイ ヌマザワコ

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抄録

Numazawa volcano is a small caldera volcano in NE Japan, and comprises dacitic lava domes, the Mizunuma Pyroclastic Deposit, and the Numazawako Pyroclastic Deposit, in ascending order. The Mizunuma eruption involving 1-2 km3 dacitic magma occurred at about 51 ka. This eruption started with phreatic explosions (unit I), followed by a Plinian eruption of inversely-graded pumice fallout (unit II), and terminated with generation of Pyroclastic flows (unit III). The Mizunuma Unit III pyroclastic flow deposit was made up of densely-welded, thick, pumiceous tuff breccia confined to the valley. The recent Numazawako eruption at 5.0 ka, involving 2.5 km3 dacitic and andesitic magmas, started with major pyroclastic-flow generation (unit I), and followed by a minor Plinian phase producing normally-graded pumice fall deposits (unit II). The Numazawako Unit I pyroclastic flow deposit consists of the massive pumice flow facies filling topographic lows and the stratified surge facies covering topographic highs : this flow devastated an area of ＞450 km2 around the volcano to the maximum distance of ＞20 km from the vent. The basal part of the Unit I deposit is the coarsest and contains abundant hydrothermally-alterated lithic fragments. As a result of this eruption, 2-km-wide Numazawako caldera was formed. The Units II and III of the Mizunuma Pyroclastic Deposit represent the normal sequence of pyroclastic-flow eruption : the transition from fall to flow activity was resulted by the extrusion of magma with progressively lower gas content as the eruption proceeded. In this case, effective emission of heated groundwater around the conduit before the magmatic eruption, produced the Unit I phreatic deposit and inhibited violent interactions between external water and ascending magma. On the other hand, the Numazawako eruption is unusual, as it started with the generation of destructive pyroclastic flow. Its mass-eruption rate is sufficient for rapid column collapse, in order to produce a high-speed flow at the beginning of the eruption. This eruption was probably initiated by phreatic explosions which disrupted large parts of the conduit sealing compressed steam and sudden decompression of magma which generated a highly-discharged column.

収録刊行物

  • 火山

    火山 40 (2), 67-81, 1995

    特定非営利活動法人 日本火山学会

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