[S-VC54_1PO1] Volcanic and igneous activities, and these long-term forecasting

[SVC54-P13] Boring Core Observation of the Izu Oshima Sembazaki Strain Meter Well.

*Yoshihisa KAWANABE¹, Shin'ya ONIZAWA², Kazuya KOKUBO³ (1.Geol. Surv. Japan, AIST, 2.Meteorological Research Institute, 3.Japan Meteorological Agency)

Keywords:Izu Oshima, boring core, basalt

We report the boring core observations of strain gauge well that Japan Meteorological Agency has been installed in the Sembasaki, Izu Oshima. The location of the well is at 34 42' 20.5168 "N, and 139 21' 40.7016"E, and well altitude is 51.2m, and drilling depth is 100m. For deeper than about 70m deep, the core was recovered. Depth from 70m to 86m is composed of volcanic breccia with thin layers of volcanic ash. Volcanic breccia is solidified and including fragments of various basalt, scoria and altered rocks. Some basalt fragments seems to be the essential with a quench rim. From the surrounding geology, this breccia can be compared to the Senzu Formation that is the product of explosive eruption at shallow sea in the first stage of Izu Oshima volcano. The core, depth of 86m (below sea level 34.8m) or deeper, is made of fresh aphyric basaltic lava flow. At least 2 flow units can be identified. Both lava flows contain very small amount of plagioclase and clinopyroxene phenocrysts. There is no evidence that is water-cooled to the lava flow. We performed the whole rock chemical composition analysis for basalt fragment of breccia and lava flows. All specimen have SiO₂ = 49.8 ~ 52.9wt% and significantly lower K₂O content, about 0.2wt%, than the basalts of Izu Oshima volcano, except for one breccia fragment. The lower K₂O content than that of the rock of Izu Oshima volcano is consistent with the characteristics of the old basement volcanoes such as Fudeshima volcano.In the sea floor of the west of the Izu Oshima, there are Semba spur accompanied by magnetic anomaly. Oshima et.al.(1987) pointed out that the Semba spur might be the older volcanic body and they named it Semba volcano. The height of the sea cliff is gradually increased from Sembazaki toward the north, and the highest around Tsuwai. The distribution of valleys around Tsuwai also shows the discordant rise in the foot of Izu Oshima volcano, and this discordant may lead to the Semba spur. The basalts from the Semba core indicate that the old basement volcano, Semba volcano, is also present in the Izu-Oshima southwest side.