Volcanic History And Eruption Scenario Of Iwate Volcano, NE Japan.

Iwate Volcano is one of the active volcanoes in NE Japan, and is located about 20km northwest of Morioka City having a population of 300 thousand. Iwate Volcano is a composite storato-volcano. On the based of topographical features, the volcano is divided into two volcanic bodies, i.e., Nishi-Iwate and Higashi-Iwate. Nishi-Iwate Volcano has a 2.5km by 1.5 km-wide caldera. Higashi-Iwate Volcano is a basaltic stratocone, which buried a few collapsed craters. The volcanic activities are divided into 4 stages for Nishi-Iwate Volcano and at 3 stages for Higashi-Iwate Volcano. The chronological relationship of these stages is interfinger. Eruptive rocks are mainly composed of tholeiitic basalt to andesite. Calc-alkaline andesite to dacite are minor component, and its eruption stage is restricted. Volcanic activity started at about 0.3 Ma, and constructed the Nishi-Iwate main stratocone. Summit of the Nishi-Iwate main stratocone collapsed and formed the Nishi-Iwate caldera without a catastrophic eruption at during 70 to 40ka. After that, central cones were formed. Magmatic activity of Higashi-Iwate Volcano were inclined to be activated after large-scale sector collapse. The magmatic activities continued about 10 to 20 thousands years, and reconstructed a volcanic cone in the collapsed-wall. The volcanic sequence, which was composed of the large-scaled collapse and the subsequent cone reconstruction, repeated 3 times through 100 ka. The most recent volcanic cone at the Higashi-Iwate, that is Yakushidake Volcano, grew after about 7 ka. The Yakushidake Volcano exhibited various types of activities, such as lava effusion, fall-out tephra and pyroclastic surge emission, small-scaled slope failure associated with eruption and so on. Historical magmatic eruptions at this volcano were recorded in 1686 and 1732. The 1686 eruption exploded pyroclastic surge and fallout tephra from the summit crater. This eruption induced lahars due to snow melting on the volcanic flank, and caused damage to houses. The 1732 eruption was a flank eruption at the northeastern part of the volcano. Several scoria cones were formed along fisher vent, and a basaltic andesite lava flowed down about 2.5 km. In 1930's and 60's, fumarolic activity intensified at around the summit crater. In the Nishi-Iwate caldera, the phreatic activity occurred at Ojigokudani fumarolic area about 7000 years ago and is continuing now. Small-scaled 1919 phreatic eruption occurred at Ojigokudani. From March 1998 to 2003, the ascending and intrusion of magma induced volcanic-seismisity, crustal deformation and fumarolic activities. After all, the eruptive activity did not arise.