津久井 雅志

安政三年八月二十六日(1856年9月24日)に北海道駒ヶ岳で大規模な噴火が起こった.軽石・火山灰が駒ヶ岳東麓〜道東に降下するとともに南麓から東麓に火砕流が流下した.降下火砕物により2名,火砕流により南東7kmにある留ノ湯(とめのゆ)の湯治客ら20数名が死亡・行方不明となった(噴出物はKo-c_1,たとえば山田,1958,地団研専報; 勝井・他, 1989,地質調査所火山地質図).爆発的な噴火は6時間以内に終了したが,小規模な噴煙活動は少なくとも1ヶ月続き,溶岩円頂丘の形成があったらしい(田中館, 1918,地質学雑誌, 25).また山頂の楕円形火口内に安政火口を形成した(加藤. 1909,震災予防調査会報告62).当時の記録は幕府箱館奉行所関連史料,地元の救援記録,厚岸国泰寺日鑑,民間の史料,道内調査隊記録などに詳細に残されている.

島弧火山岩のBe同位体比(¹⁰Be/⁹Be)は、沈み込み帯における元素の移動を理解するのに有用なトレーサーとなる。本研究の目的は、伊豆島弧のスラブ深度の異なる火山岩のBe同位体比を系統的に分析し、U系列放射非平衡および微量元素比との比較から、非平衡が深部で起きたものかの検証を行うことである。

本研究は宇宙線生成核種の¹⁰Beを沈み込み帯マグマのトレーサーとした研究である。沈み込み帯火山岩からは海嶺玄武岩等と比較して高濃度の¹⁰Beが確認されている。地球形成時の初期¹⁰Beは既に壊変して消滅していることから、島弧火山岩の¹⁰Beは沈み込んだ堆積物起源であると考えられる。本研究では伊豆島弧火山岩試料(大島、三宅島、新島)の¹⁰Be濃度およびBe同位体比を示し、他の島弧との比較やBe同位体比の島弧横断弧方向での変化を調べることが目的である。<BR> 結果は伊豆島弧試料の¹⁰Be濃度は他の島弧よりも低い値が確認された。また、島弧横断方向でのBe同位体比の結果は、大島試料で高い同位体比を示したが、三宅島-新島間では明確な差が見られなかった。Be同位体比の変化は含水鉱物の脱水過程および流体の関与を反映していると考えられるが、その詳細については現在検討中である。

Hachijo-Nishiyama (meaning of the West Mountain) is an active composite volcano situated, next to another Pleistocene composite volcano Higashiyama (meaning of the East Mountain), on the Hachijojima Island, 300 km south of Tokyo. These two volcanoes stand on the volcanic front along the Izu-Mariana arc. Since the beginning stage of Nishiyama, ca. 10,000 yBP, the volcano repeatedly erupted in a shallow submarine environment. The explosively emitted pyroclastic materials covered almost whole the island. By 3,000 yBP, Nishiyama had grown as a subaerial volcano and lavas and fallout scoria were erupted without considerable effects of water. In this study, we presented petrographical and petrochemical descriptions of the lavas of Nishiyama in the latest 3,000 years. Based on these data, we discussed the differentiation processes of Nishiyarna volcano and the genetic relationship between the magma of Nishiyama and Higashiyama. Nishiyama volcano is totally composed of basalt to basaltic andesite with ubiquitous plagioclase, with or without mafic phenocryst minerals such as olivine, clinopyroxene, orthopyroxene, and titanomagnetite. In terms of SiO_2-Al_2O_3 relationship and amount of phenocrysts, we divide the differentiation trends of Nishiyama into three groups. P-group, A-group and T-group.

The activity of Miyakejima Volcano, in the Izu Arc, commenced on June 26, 2000AD, resulting in the formation of the 1.6km-across ""2000AD Caldera,"" or ""New Hatchodaira Caldera."" This summit subsidence caldera duplicated the position and size of the Hatchodaira Caldera of 2500y.B.P., probably utilizing the former caldera structure. In this paper we describe these two caldera-forming activities and discuss the mechanisms of caldera formation. The volume of magma discharged during the Hatchodaira eruption was 3.7×108m3 (DRE) comprising a new batch of basalt magma mixed with preexisting andesite magma. The 2000AD event, on the other hand, was accompanied only by 1.1×107m3 of ejecta in contrast to 6×108m3 of the missing volume. The major reason for the 2000AD subsidence is thought to be that the magma in the reservoir underneath Miyakejima Volcano intruded to form dykes in the adjacent area of Niijima and Kozushima, 30 km northwest of Miyakejima. The explosive events in 2000 were caused passively by decompression in the magma-evacuated reservoir, rather than active magma itself. This sequence of the 2000AD event had never been experienced by Miyakejima Volcano during the geologically described last 10,000 years.

Miyakejima volcano is located in the Pacific, 200 km south of Tokyo, as a member of lzu-Mariana Arc active volcanoes. The main cone of the volcano has nested two calderas; outer Kuwanokitaira caldera, 4 km in diameter, and inner Hatchodaira caldera, 1.8 km by 1.6 km across. The central cone, Oyama, grew in the Hatchodaira caldera. The volcano-stratigraphy of Miyakejima during the last 7000 years was revealed and described in detail, mainly by tephrochronological method and historic records. 18 isopach maps and distribution of lavas were presented together with volume estimation of 42 erupted units. Based on the eruption-recurrence period, production rate and eruption style, the developing history was divided into four stages; 1) inactive stage of 7000-4000 yBP, 2) the Caldera-forming stage of 4000-2500 yBP, 3) the Oyama stage, from 2500 yBP to the 15th century, and 4) the Shinmio stage, 1469 A.D. to the present. The inactive stage of 7000-4000 yBP is characterized by small scale eruption with a long interval of quiescence. In the Caldera-forming stage, voluminous eruption of two accretionary lapilli and a scoria resulted in the formation of the Hatchodaira caldera. In the Oyama stage, caldera had been filled by products from central and lateral eruptions. It is noteworthy that phreato-magmatic eruptions from central vent prevail over dry magmatic eruptions. In the Shinmio stage, most eruptions took place from lateral fissures. The typical volume of materials ejected in a single eruption ranges from the order of l0^-3 km^3 to 10^-1 km^3 (DRE), most cases are about 10^-2 km^3, and a few exceed 10^-1 km^3 during the last 7000 years.

Hachijo island, located 300 km south of Tokyo, consists of two Quaternary volcanoes; Higashiyama in the southeast, and Nishiyama in the northwest. The eruptive history of the Higashiyama volcano was investigated. The volcanic edifice of Higashiyama is composed of Early stratovolcanoes I,II, Middle stratovolcano, and Late volcano. A series of dacitic pumice fall and flow eruptions took place ca. 22,000 yBP, at the end of the Middle stratovolcano, Sueyoshi stage. Evacuation of a large amount of magma in this stage caused formation of Mihara caldera of 1.4 km×0.9 km. The history of the Late volcano can be further subdivided into the Nakanogo and Mitsune stages. In the former stage, basaltic scoria-fallout eruptions frequently took place inside the Mihara caldera and east to south flank of the volcano. While in the latter stage, eruptive vents were restricted on the flank of the volcano. The major erupted magma type changed to differentiated andesite and dacite. The last eruption of Higashiyama which covered the whole volcano was estimated to be 5,000-4,000 yBP. Beside tephras from Higashiyama, at least seven hydromagmatic-eruption deposits from northwest of Higashiyama volcano are detectable. They are most likely the products resulted from the submarine to the earliest subaerial eruptions of the Nishiyama volcano, in 10,000-8,000 yBP.

Evolution of whole-rock chemistry of volcanic rocks in Fuji volcano is presented. Basaltic andesite is predominant during 66,000 to 80,000 y. B. P. ; they are poor in FeO^*, MgO, MnO, K_2O, TiO_2, P_2O_5, and abundant in Al_2O_3 and Na_2O, with slightly high K_2O/TiO_2, Rb/Y, and Zr/Y ratios. Basalts erupted during 22,000 to 66,000 y. B. P. show low FeO^*/MgO ratio (2.0 >), and are lacking in those with high K_2O, TiO_2, P_2O_5, Rb, Ba, and Zr, and low Al_2O_3 ; they are characterized by low K_2O/TiO_2, Rb/Y, and Zr/Y ratios. Eruption products during 10,000 to 22,000 y. B. P., especially in its later stage, are basalt with relatively high FeO^*/MgO, Rb/Y, and Zr/Y ratios, and K_2O, TiO_2, P_2O_5, Rb, Ba, and Zr contents, and low Al_2O_3 ; their chemistry resembles to those after 10,000 y. B. P.. Ejecta during 8,000 to 10,000 y. B. P. are basalt and high in FeO^*/MgO, Rb/Y, and Zr/Y ratios, and K_2O, TiO_2, P_2O_5, Rb, Ba, and Zr contents, and Rb/Y and Zr/Y ratios, and low in Al_2O_3. Volcanic rocks during 3,000 to 8,000 y. B. P. are basalt showing low K_20/TiO_2, Rb/Y, Ba/Y, and Zr/Y ratios, and K_2O, TiO_2, P_2O_5, Rb, Ba, Y, and Zr contents, and high FeO^*/MgO and Al_2O_3 ; they are rather similar to those erupted during 22,000 to 66,000 y. P. B. in chemistry. Erupted materials during 2,000 to 3,000 y. B. P. are basalt with low K_2O, TiO_2, P_2O_5, Rb, Ba, and Zr contents, and Rb/Y and Zr/Y ratios, and high Al_2O_3 and FeO^*/MgO ; their chemistry are akin to those during 3,000 to 8,000 y. B. P., but slightly enriched in incompatible elements. Eruption products after 2,000 y. B. P. are abundant in K_2O, TiO_2, P_2O_5, Rb, Ba, and Zr, and poor in Al_2O_3, with high FeO^*/MgO, Rb/Y, and Zr/Y ratios ; they are almost similar to basalts erupted during 8,000 to 10,000 y. B. P.. The Fuji volcano is classified into three magma-series on the view point of whole-rock Rb/Y and Zr/Y ratios : Early Ko-Fuji (66,000 to 80,000 y. B. P.), Late Ko-Fuji (10,000 to 66,000 y. B. P.), and Shin-Fuji (present to 10,000 y. B. P.). Basaltic magmas of each series may have probably been derived successively from chemically heterogeneous upper mantle. The high rate of change of source rock chemistry may reflect dynamic process in the upper mantle beneath the arc volcano.

22,000 years ago, about 100 km^3 of magma erupted from the northrn end of Kagoshima Bay, southern Kyushu. The eruption produced 5 units of pyroclastic deposits; (1) 98 km^3 of airfall pumice (Osumi pumicde fall, OS), (2) 13 km^3 of oxidized, fine-grained Tsumaya pyroclastic flow (TSU), (3) Kamewarizaka breccia (KM) of the new vent-opening and enlargement stage, (4) 250 km^3 of Ito pyroclastic flow (ITO) at the climactic stage, (5) >50 km^3 of co-ignimbrite ash fall (AT ash). Phenocryst mineral assemblage throughout the whole sequence is ubiquitously plag+qtz+opx(Mg#45-60) +mt+il. One exceptional sample (ITO 11c) carries Fe-rich oliv (Fo 26-28) and cpx beside other phases. Fifty-five new XRF analyses of 10 major and 15 trace elements show that the majority of the erupted magma consisted of a remarkably homogeneous, high-silica rhyolite with SiO_2 74-76.5%(H_2O free and 100% normalized). The maximum fluctuation found both in major and trace elements is ±40%. These variations can be explained by the crystal-liquid separation near the roof of the magma reservoir. Mt-il temperatures and opx-mt-qtz pressures show narrow ranges, i e., 770±20℃ and 3-5 kb, respectively. Although the sample ITO 11c shows similar temperature, its calculated pressure is close to 0 kb. The bulk and mineral chemistry and the temperature-pressure estimation suggest that the magma reservoir was not distinctly zoned but was very homogeneous throughout prior to eruption.

Izu-Oshima volcano erupted at about 17:20 hours on November 15, 1986, after 12 years quiescence. The first eruptive activity, phase 1, was restricted to fire fountaining and overflow of lave from the south part of Mihara crater (A crater). The phase 2 eruption began at 16:15 hours on November 21, on the caldera floor. In this phase, fissure eruptions took place on the caldera floor (B craters) at 16:15 hours and another on the northwest slope of the somma (C craters) at 17:46 hours. The A crater activated again at 16:44 hours. Coarse scoria fell on the eastern coast of the island from subplinian column on B craters. Lavas from B craters spread on the caldera floor and another lava heading to Motomachi town streamed down from one of C craters. The eruption of C, B and A craters lasted until about 21:00 hours on 21, 01:00 hours on 22 and 04:30 hours on 22 respectively. The whole sequence of phase 2 emption was broadcasted by television and informed also by other mass communications. In this report, documentation of phase 2 is presented based on these media. Merits and limits of mass media espccially TV reports for dccumentation of eruption are discussed.

Stratigraphy and thickness distribution of the pyroclastic fall deposits formed during the eruption of Miyakejima volcano on October 3-4, 1983, were studied immediately after the deposition. Of the total mass of 20 million tons erupted, 8.5 million tons were ejected as basaltic scoria to form a complex set of air-fall deposits east of the fissure vents. One million tons of the latter were ejected from the upper fissures as fire-fountain products. The rest was the product of phreatomagmatic explosions which occurred in the lower fissures where ground water chilled the magma to form dense scoria blocks which devastated villages. Explosion craters and a tuff ring were formed along the N-S trending lower fissures. Account of the general distribution of the deposits, nature of the constituents, mutual stratigraphic correlation and correlation with observed sequence are given.