The Tsurumi volcanic region is one of the most seismically active regions in the tectonic zone from Beppu Bay to the Shimabara Peninsula. It is characterized by frequent earthquakes repeatedly. In the seismic activity, two different seismic appearances are recognized. One of them is named as the seismic cluster “A” of which S-P times observed at the station on the summit of Tsurumi Volcano, are less than 0.2 seconds. These events are located in the tor of volcano and have very small magnitude. And then. the ever,ts have occurred as short bursts of spatially-clustered activity that typically lasted one or two hours and the number of events in one seismic active duration amounts to a few hundreds. Another seismic activity is named as the seismic cluster “B” of which S-P time are from 0.7 to 1.0 seconds. These events have also occurred as a seismic swarm. But one seismic active duration of these events is longer than the duration of the seismic cluster “A”. Most of these events are ｌocated at the southern base of Tsurumi Volcano as a belt from east to west fault plane is of these events shows a combination of both normal and strike-slip mechanism with the right lateral type, of which the fault plane is directed from east to west and the north area part is dipped. T axis maintains a horizontal north-south orientation. The strike of this fault plane is coincident with the Yufuin fault. Therefore the seismic activity of the seismic cluster “B” is due to the activity of this fault.

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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 eruption 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 especially TV reports for documentation of eruption are discussed.

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Many genetic models have been accepted for the origin of intermediate and felsic magmas which formed considerable volume of volcanic rocks. Among these models, fractional crystallization is generally thought to be the most fundamental theory. This model, however, fails to explain some important geophysical and geological evidences such as, the relationship between eruptions and ground deformation, changes in composition during steady state volcanism, and frequency of chemical composition of erupted magma. To explain these phenomena and problems comprehensively, this paper proposes the model of crustal anatexis as the origin of intermediate and felsic magmas. A broad spectrum of magma can be generated through crustal anatexis owing to the heating of the crust by mantle-derived basaltic magmas. It is not necessary for this model to assume a large magma chamber in which fractional crystallization occurs. Contamination inevitably accompanies crustal anatexis. Magma mixing will effectively occur within the conduit during the ascent of the series of magmas toward shallow magma chambers. Behabior of P₂O₅ contents may support crustal anatexis. Natural volcanic rocks show similar P₂O₅ trend as that obtained from melting experiments of basalts. In general, frequency of eruption of felsic magma increases as the cmst thickens. It is therefore concluded that the lower crust is thickened by the intrusion or accretion of basaltic magma of mantle origin. Based on the frequency of chemical composition of volcanic rocks and the behavior of P₂O₅ contents, origin of intermediate and felsic magmas in bimodal volcanism which are typical in extensional regions can be reasonably explained by crustal anatexis. Calcalkaline andesitic volcanism, which is predominant along subduction zone, can also be explained by the same mechanism, assuming that the source materials are amphibole-bearing rocks, e.g. amphibolites, which have formerly undergone metamorphism under the wet conditions at depth. Petrographical evidences for magma mixing in calc-alkaline volcanic rocks are well preserved probably due to relatively lower temperature of calc-alkaline magma as compared to other volcanic rocks series.

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