十和田火山平安噴火(噴火エピソードA)の噴出物層序及び噴火推移の再検討

抄録

The Heian eruption (Eruptive episode A) is the latest activity of Towada Volcano, Northeast Japan that occurred in the 10^th century A.D. The activity comprised both magmatic and phreatomagmatic eruptions, that produced deposits of corresponding faces. It is proved that the Heian eruption started with a magmatic eruption and was followed by a phreatomagmatic eruption, then the magmatic-phreatomagmatic cycle repeated once, and finalized by effusion of a large pyroclastic flow. This eruption sequence is constructed on the basis of our new recognition of the second phreatomagmatic deposits in the proximal area (OYU-4), and correlation of the proximal base surge deposits (OYU-S) with the distal phreatomagmatic deposits (OYU-2). The Towada caldera including the vent of the Heian eruption (Nakanoumi caldera) is known to be abundant water in the Heian Period. The Heian eruption indicates that the magmatic eruption does occur in the existence of abundant water in the early stage of eruption and it migrates into the phreatomagmatic explosion as time passes. This eruption sequence provides evidence that the silicic magma does not necessarily cause phreatomagmatic eruption even in the presence of enough water.