On December 16, 1988, after 26 years of dormancy since the last eruption in 1962, Tokachi-dake began to erupt from the 62-II crater. The eruption started with phreatic explosions. Then, on December 19, the activity changed into phreatomagmatic explosions of Vulcanian type and continued intermittently until March 5, 1989. Although the composition of the essential ejecta, mafic andesite, is similar to those of 1926 and 1962 eruptions, the mode of the present eruption is considerably diffrent The present eruption consists of a series of 23 discrete cannon-like explosions, being frequently accompanied with small-scale pyrcclastic surges and flows. The total volume of ejecta amounts to approximately 6×10⁵ m³, of which about 20% is essential ejecta. A complete sequence of events was compiled and distribution maps of the ash-fall, ballistic blocks, and pyroclastic surges and flows were drawn for each of the larger eruptions. The pyrrolastic surges and flows of the present eruption were small scale, low temperature pyroclastic flows, rich in accessory clasts and unaccompanied by sector collapse. Therefore, the sudden melting of snow causing disastrous mudflows, as in the case of the 1926 eruption, fortunately did not occur.

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Twenty-three small-scale eruptions took place at Tokachi-dake from December 16, 1988 to March 5, 1989. The pyroclastic fall deposits, ballistic fragments, and pyroclastic surge and flow deposits were dispersed over the flank and leeward areas of the volcano. Because the pyroclasts of each eruption were well-preserved in snow during the winter, the stratigraphy and distribution of these deposits could be studied in detail. The volume of the pyroclastic fall deposits are nearly equal to those of the pyroclastic surge and flow deposits. The total volume of these pyroclasts is estimated to be 7.4×10⁵ m³. Judging from the sequential changes of the volume and composition of the pyroclasts, the characteristic features of the eruption can be summarized as follows: At first, a vent was opened by ejection of altered rock fragments in December, 1988. Then, essential fragments were ejected in January, 1989. Finally the activity level of magma declined and the altered rock fragments content increased again in February to March, 1989.

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The 1988-89 explosive eruption of Tokachi-dake volcano brought about ash-and block-fall, pyroclastic surge, and small-scale pyroclastic flow deposit. Petrographical, mineralogical, and petrochemical studies were made on the essential ejecta of olivine-bearing orthopyroxene-clinopyroxene mafic andesite and asscciated glassy rocks of peculiar origin. The mafic andesites (SiO₂=52-53 wt.%) have modal, mineralogical, petrochemical, and Sr isotope compositions similar to those of the 1962 mafic andesites of Tokachi-dake. However, the 1988-89 mafic andesites occur mainly as angular fragments which are less porous and possess more crystalline groundmasses than the 1962 mafic andesites. These observations suggest that the viscosity of the 1988-89 magma was higher than that of the 1962 magma. Mineral chemistries of the 1988-89 and 1962 mafic andesites display episodes of magma mixing. The glassy rocks occur as discrete fragments and are also found in ejected blocks of vent breccia. The glassy rocks are composed of deep brown glass, pale brown-colorless glass and dust, showing eutaxitic texture. The rocks also carry preserved clay and andesite shards. Areas which are gray frequently contain high-silica (SiO₂=94%) pseudomorph after plagioclase displaying optical isotropy. The area-perimeter relationship for the deep brown glass is fractal, indicating the glass movement turned to be in a turbulent flow condition. Bulk chemistries of the glassy rocks have a wide compositional range (SiO₂=61-7 9wt.%) and are similar to stoneware clay being extremely depleted in CaO and Na₂O and enriched in Al₂O₃. The Sr isotopic data of the glassy rocks are similar to those of the associated andesites from Tokachi-dake volcano. Variations in Fe₂O₃-SiO₂-Al₂O₃ compositions indicate that the glassy rocks are derived from an andesite suffered from alteration under strongly or weakly acidic condition. It is considered that the glassy rocks were derived from the melting or softening of an altered andesite around the vent. The above evidence suggests that low fluidity of the ascending mafic andesite and glassy product with a turbulent flow mode may have brought about the explosive nature of the 1988-89 eruption.

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During the 1988-1989 eruptive activity of Mt.Tokachi, repeated field observations and strengthened efforts for seismometrical observations were made by the National University Team. Nine eruptions during the later eruptive period were observed by three temporary seismic projccts; (1) a new seismic station (HNG) installed at the possible nearest site to the 62-II crater, (2) a small array and (3) nine temporary seismic stations using direct recording system at the volcano and surroundings. These observations revealed that both low-frequency earthquakes (LF) and volcanic tremors (TR) were characterized by Rayleigh type surface waves. The incident azimuths and the apparent velocities at the array are common for both LF and TR. They are inferred to be originated from the limited small region at the shallow depth beneath the 62-II crater. On the other hand, the wave characteristics and the sources of explosion earthquakes (EX) are different from those of LF and TR. EX are inferred to be located beneath the crater, but their depths are somewhat variable.

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A series of 23 explosive eruptions had occurred at Mt. Tokachi (Tokachi-dake), central Hokkaido, Japan during December 16, 1988-March 5, 1989. This eruptive activity was preceded by 6 years of long-term but sporadic precursors and three months of irreversible culminating precursors. All eruptions were small scale, but characterized by explosive nature and often accompanied by small pyroclastic surges and/or pyroclastic flows. Geophysical aspects on those pre-eruption stages and explosive characteristics were discussed. One third of the 23 explosive eruptions were preceded by common immediate precursors; an increase of small low-frequency events. However, important eruptions, which produced pyroclastic flow or magmatic blocks, were not always preceded by such precursors. Knowledges of those abilty and inability was informed quickly to the officials and that resulted in an urgent introduction of a mud flow detection system, as a precautional measure to mitigate the possible occurrence of the 1926 type mud flow disaster, Only one eruption, the January 20 eruption, ejected essential materials including large blocks, Various observational evidences including extensive low-frequency tremor indicate the activated situation before this magmatic eruption. One possible explanation of this low-frequency tremor is the enforced magma injection to the vent system.

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Mt.Tokachi was reactivated on December 16, 1988 at the 62-2 crater. Chemical compositions of the volcanic gases from the 62-1 crater have been determined periodically since 1984, and more frequently after the reactivation. There was little change in chemical compositions of volcanic gases collected on August 23, 1988, four months before the eruption. H₂ content in residual gas (R-gas) and HF/HCl ratio showed a remarkable increase on January 25, 1989. These facts indicate tha the volcanic activity was at the highest stage in January, 1989 in this eruptive period. Also, the relative propotion of HF+HCl, SO₂ +H₂S and CO₂ exhibited close correlation with the variation in the volcanic activity. The apparent eqilibrium temperatures for the reaction; SO₂+3H₂=H₂S+2H₂O, suggest that the volcanic gases from the 62-1 crater were contaminated with sulfur at a shallow depth around the summit area.

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Eighteen new K-Ar ages for Neogene dike swarms at Oki-Dozen and Tango Peninsula, western Honshu, were determined. The K-Ar ages of dikes at Oki-Dozen area are within a narrow range of 5.7 and6.3 Ma, which are much younger than previously reported results. Slight shift, from N10-15E to N15-70 W, of regional compressional stress axis might have occurred around 6.2 Ma. The present age data for dike swarms from Tango Peninsula area suggest the existence of at least 3 separate periods of their activities, 15-11 Ma, 6 Ma and 3 Ma. Peak of the Miocene igneous activity was during 15-13 Ma, just after the opening period or Japan Sea. Although the present data indicate that the regional compressional stress axis of this area has been NS direction since 15 Ma, which seems not in accord with the conclusion given by TSUNAKAWA (1986), the difference might be explained by the absence of dike swams at 12-9 Ma and less than 1 Ma in the investigated sites in the present study.

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22,000 years ago, about 100 km³ of magma erupted from the northrn end of Kagoshima Bay, southern Kyushu. The eruption produced 5 units of pyroclastic deposits; (1) 98 km³ of airfall pumice (Osumi pumicde fall, OS), (2) 13 km³ of oxidized, fine-grained Tsumaya pyroclastic flow (TSU), (3) Kamewarizaka breccia (KM) of the new vent-opening and enlargement stage, (4) 250 km³ of Ito pyroclastic flow (ITO) at the climactic stage, (5) ＞50 km³ 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 SiO₂ 74-76.5%(H₂O 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.

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Morphology, abundance and vertical distribution of vesicles were studied in a thick (40-60 m) andesitic lava flow, that lies in the underground of Kumamoto City. The vesicles are frozen bubbles that were fomed in the molten lavas at the time of its eruption. The lava can be divided into three zones: (1) an upper vesicular zone (2) a middle non-vesicular and compact zone and (3) a lower, thinner vesicular zone. The vesicles in the upper zone are elongated vertically, probably due to bouyancy-driven ascent of the bubbles, and those in the lower zone are flattened and elongated horizontally, that may be ascribed to a viscous shear flow at the bottom of the lava flow. Size distribution of the vesicles typically display nearly the log-normal distribution. Abundance, the mean size and the number density of the vesicles are greater in the upper zone than in the lower zone. Such vesicle distribution pattern is consistent to the hypothesis that the lava originally contained abundant bubbles when it was poured on the ground and then the bubbles started to ascent in the lava. Vesicles in the lower zone were the bubbles trapped by the advancing cooling front from the bottom surface of the lava. Bubbles that have escaped from the cold trap below have been accumulated in the upper zone and have been frozen in the lava upon cooling from the top surface. Mass balance calculation, however, indicates that much of the bubbles that were originally present in the lava, have been escaped through the lava surface. A dynamic cooling model was, therefore, proposed, that is to say, in the presence of surface flow in the lave during its cooling, impermeable lava crusts may not be maimtained so that gas bubbles may leak out of the lava into the air.

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