Human remains of Middle Zyomon (Jomon) age (about 4500 years old) have been found between a tuff layer and a lava flow exposed at Tatunokuti on the western coast of O-sima, Izu, south-southwest of Tokyo. These rocks were formed during the period of pre-caldera or Somma lava activity. As the surface of the tuff layer had been subjected to weathering and stream erosion before the remains were laid down, the volcano appears to have been dormant or at least weakly active for some time before the pre-historic men lived at this place. The time interval between this stage and the close of the Somma lava activity is estimated as at least 1000 years, and probably about 2000 years . Therefore the formation of the summit caldera may be dated as 3000 or 2000 years old.

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Nantai Volcano was formed by the following successive events; 1. Main stage of activity characterized by the alternate eruption of lavas and pyroclastic rocks of olivine-pyroxene andesites. 2. Period of quiescence. 3. Later stage of activity. a) Eruption of non-porphyritic glassy andesite scoria (scoria fall followed by scoria flow). b) Eruption of hornblende-pyroxene dacite pumice (pumice fall followed by pumice flow). c) Outflow of viscous lava flow of hornblende-pyroxene dacite. The eruption of the scoria fall was probably caused by explosive foaming of magma within the vent. Scoria fragments were shot into the sky; their angular shape suggests continued explosive foaming of scoria even after they left the crater. The scoria fall was followed by the scoria flow. This change would be attributed to the decline of explosive foaming within the vent due to a decrease in vapor pressure of the magma. Scoria fragments were no longer shot up but overflowed the crater rim and flowed down the slope. The facts that the scoria flow deposit was strongly welded, and that lithic fragments derived from near the crater are included in scoria blocks suggest the low visdosity of the foaming magma.

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Volcano Sakura-zima resumed explosive eruption in its summit crater after a long repose on Oct. 13. 1955, and is still active up to the present (Oct. 1957). In order to throw a light on the relation between explosive eruptions and volcanic earthquakes, we applied to this volcano the same method of seismometrical observation as being carried out in Volcano Asama. As the result, we observed four types of volcanic earthquakes originating from Sakura-zima: Namely, 1) volcanic earthquakes of the A type, 2) volcanic earthquakes of the B type, 3) earthquakes accompanied by explosive eruptions and 4) volcanic pulsation of continuous train (or harmonic tremor). On the basis of the seismometrical observation at four or six stations, we determined localities of these earthquakes of the four types. As the result, it was made clear that the epicentral area of the B type earthquakes, the explosion earthquakes and the volcanic pulsation agree with the locality of the active crater on the summit, and the A type earth quakes are originated from deeper part of the volcano. From the illustration of seismic activity and explosive eruption in Fig. 5, it seems that frequency of the B type earthquakes harmonizes with the eruptive activity. After precise examination of the development of the mentioned phenomena, we find that marked swarms of the B type earthquakes preceed in many cases to respective swarms of explosive eruption by ten or fifteen days. However, the problem is not always simple. It is neccessary to continue the observation not only in active state of the volcano but also in its calm state.

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