Description: Sequences and products of the Izu-Oshima 1986-1987 eruptions which started on November 15, 1986, were investigated tephrochronologically. The results are summarized as follows : 1) Summit eruptions (Crater A) During 15-20, Nov. 1986, Strombolian eruptions continued to make a lava lake from where lava flows spilt over and went down the slope of the central cone to the caldera floor (LA I～IV). Volcanic ash and scoria (TA-1～4) were dispersed to the eastern and western parts of the island. On 21 Nov., a little after the beginning of the fissure eruption (Craters B), Strombolian eruptions were reactivated and ejected large volcanic bombs and scoria (TA-5) from Crater A. On Dec. 18, 1986, small explosion occurred from the Crater A for three or four hours, ejecting a scoria fall (TA-6) and bomb. The level of the lava lake lowered about 5 meters. On Nov. 16, 1987, a phreatic explosion occurred to break the crust of the lava lake, and the lava drained back to the deep on Nov. 18. 2) Fissure eruptions in the caldera floor (Craters B) At 16 : 15, on Nov. 21, 1986, fissure eruptions (Craters B) started on the caldera floor and extended to the slope of the central cone. The eruptions became explosive one, generating lava fountains with the height of more than 1500 meters, with a high discharge rate of 8×106 ton/hour, producing pyroclastic cones and rootless (clastogenic) lava flows (LB I and III). Subplinian scoria falls were dispersed to west (TB-1) and east (TB-2). About 5 hours after the beginning, the activity waned to produce only volcanic ash (TB-3 and -6) and finer scoria falls (TB-4 and -5) and ceased on Nov. 23. A rheomorphic lava flow (LB II) occurred from the edge of the deformed cone on Nov. 23. 3) Fissure eruptions on the somma slope (Craters C) At 17 : 45, on Nov. 21, 1986, fissure eruptions occurred on the somma slope, and produced two lava flows (LC I and II), scoria cones, and vesicular scoria falls (TC-1 and -3) from the 11 craters. 4) The 1986 eruptions ejected 0.053 km3, 7.9×107 tons of lava and pyroclasts from A, B and C craters (Table 4).

Description: Significant progresses in recent studies on tephra are reviewed. In relation to volcanological aspects of tephra, recent works on mechanism of vesiculations and disruption of magma, formation of eruption column, transportation of tephra, recognition of types and magnitude on eruptions, and roles of interaction between external water and magma are discussed. The progresses are mainly based on the intensive researches on the recent eruptions observed by volcanologists such eruptions as the 1980 St. Helens, the 1977 Usu and so on. Attempts on quantitative understanding of eruption phenomena such as total eruption volume, initial population of grain-size, and dispersal/fragmentation values of tephra are discussed. Recent studies on phreatomagmatic eruptions on the basis of the experimental researches of magma/water interactions and field observations are remarkable progresses in volcanological studies. In relation to tephrochronological aspects, recent works on identification techniques of tephra layers, discovery of wide-spread tephras, dating methods of tephras, and applications of tephrochronology are summarized. Identification techniques of tephra layers based on determinations of refractive indices and major, minor and trace element compositions of glass and/or minerals were highly advanced in the last 10 to 20 years. Those are effective tools for identification of wide-spread tephras. Wide-spread tephras such as AT ash from Aira Caldera, K-Ah ash from Kikai Caldera, B-Tm ash from Baegdusan Volcano and others were recognized in Japanese Islands and also in deep-sea sediments around Japan. They were dated by radiometric and stratigraphic dating methods. Radiometric dating methods applicable to the late Quaternary tephras are ^〈14〉C, Fisson Track, Ionium, K-Ar, Thermoluminescence, and Electron Spin Resonance methods. Because the dates of tephras were mostly obtained by ^〈14〉C dating, the radiometric ages of tephras over 40000 years are lacking so far. The accurate radiometric ages of tephras for this range are strongly required for further developments of tephra studies. It is stressed that tephra studies will play great roles on establishment of eruption histories of poligenetic volcanoes, activity aspects of one cycle eruptions, life time of magma and zoning of magma chamber.