The Neogene Hokutan Group irl the Tanngo Penlnsula is mainl) composed of various kinds of hydrovolcanic products. The Amino Formation and Tango Forrnatlon in the Hokutan Grourp consist of intermediate to felsic subaqueous lave fio,vs. subaqueous pyrcclastic fiow deposits, phreatomagnatic fallout deposits. base surge deposits. and shallow intrLlsive bodles accompan_ied with I)ep erites. Shape analysls of grains within thin sections are preser,ted for various kinds of hydrovolcanic rocks in the Amino Formation and Tango Formation. Morphometrlc parameters used in this paper are as follows: (1) relative percentage of planer (P). convex (V). and concave (C) peri-meters of a grain (“roundness” according to SZADICZKY-KARDOSS (1933)); (2) shape parameter defined by Pe , where Pe is the perimeter of a grain. S is the area of a graln. as a quantitative indicator for the shape irregularity of grains: (3) fractal dimension estimated by -(log N) (log r). where N is the number of segments for the perimeter of a grain, r is the length of a segment. as a quantitative indicator for the surface irregulatiry of grains. Results of the shape analysis distinguish the following three processes of fragmentation: (1) brlttle fracturing of solid magma; (2) vesiculatlon caused by expansion of magmatic gas: (3) fragmentation of liquid magma caused by a fuel-coolant interaction (FCI). The self-triggering FCls do not occur between ascending magma and pore water of host material, such as wet sedlments and wet volcaniclastics filling the vent. Therefore, it is necessary that the surfaces of magma are enough disintegrated by other triggering agents. Trlggering agents for the formation of peperites by FCls were stream explosions caused by mixing wet host materials and magma due to fiuid instability or vesiculatlon caused by expansion of magma-tic gas. Triggerlng agents of FCls are suppressed by higher viscosity of magma or a larger amount of hot magmatlc gas.

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In the night of August 21, 1986, a large amount of CO₂ gas was emitted from Lake Nyos, Located in the western Cameroon. The outburst started about 2030 hours and lasted about 4 hours. 1700 people and more than 3000 cattle died as relatively heavy masses of gas swept through valleys. The highest level of the lethal gas was more than 100 m above the lake and the main stream of the gas reached as far as 16 km downstream. The water of the lake was highly distubed and the vegitation was stripped off by the splashing water more than 20 m (80m in maximum) above the lake level. Most scientists agree that the main constituent of the lethal gas was CO₂ and the presence of significant amount of other toxic gases such asH₂S and SO₂ seems unlikely as a cause of “burns” or skin lesions.

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Geochemical and limnological survey was carried out at lake Nyos, Cameroon, where the tragic gas disaster took place on 21 August 1986, killing more than 1700 people and an uncomtable number of animals. The water temperature and concentrations of dissolved chemical species increase with depth. The chemistry was overwhe1mingly dominated by CO_2(sq) and HCO₃^- with a substantial concentration of ferrous iron below 40m. Very low concentrations of Cl- and SO₄^2- argue against the volcanic gas injection hypothesis as a cause of the disaster. The gases discharged from the lake consisted mainly of CO₂ but sulfurous gases such as SO₂ and H₂S were hardly present. Stable isotopic ratios of carbon and helium strongly indicated a mantle origin of the gases. The present level of the sissolved CO₂ is about 20% of the saturation. Assuming the pre-event saturation of the lake with respect to CO₂, the maximum amount of CO₂ discharged is calculated to be O.63km³, which is consistent with the estimates based on the field observation. Density of the present lake water calculated from the profiles of temperature and chemical compositions increases with depth, indicating the stable stratification of the lake, The CO₂ gas burst was caused by upward displacement of deep water creating oversaturation of CO₂ with formation of CO₂ bubbles which move upwards dragging a part of ambient water. Since this is a self-sustaining process, most of the dissolved CO₂ would have been released from the lake by the above process once triggered by the increased input rate of warmer, CO_2- saturated groundwater into the lake bottom during the rainy season. Since exsolution of CO₂ from water and adiabatic expansion of CO₂ bubbles would have lowered the water temperature, the circulation of water may have been restricted in relatively small cells, thus preventing the initial chemical stratification from a drastic change. The lake Nyos tragedy and the Monoun event in 1984 preesnt a new type of natural disaster. It is essential to accumulate the geochemical and limnological baseline-data as presented here in order to understand the causes of the disaster in more detail and to predict possible future gas bursts from crater lakes of the world under similar geological situations.

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A large volume of lethal gas was released from lake Nyos in the North-West provine of Cemeroon on 21 August 1986. Some 50 days after the event, a hydro-physical and -chemical survey of Lake Nyos was carried out by four japanese scientists including the author. Vertical density distribution of lake water showed remarkably stable stratification through whole lake water column. The result of chemical analysis showed high concentration of dissolved CO₂ and Fe which contribute stable stratifications. Assuming initial saturation of dissolved CO₂ in lake water, released gas volume was estimated to be 0.86(km³) as volume difference between the saturated gas content and the residuals in the present lake. Quasi-adiabatic coolng due to expansion of CO₂ gas during ascent through the water column, led to temperature drop of water, which was estimated for every 1Om leyer thickness assuming acomplete thermal exchange between expanding gas and lake water. The calculated temperature profile assuming initially uniform distribution of temperature at 23.38℃ agreed well with the measured profile by CTD. The height of gas cloud over the lake and flow velocity of draining gas from the rim of the lake were evaluated using a simple geometrical flux model. In the case of sinusoidal change of ejection with equal volume mixing and head loss coefficient of C_ｄ=0.3, the maximum gas height reached to more than 100 m and the corresponding drain velocity was 19 m/sec.

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Lake Nyos is a maar-type volcano formed by phreatomagmatic eruption along the fissure. The rock is weakly alkaline olivine basalt very rich in crustal xenoliths and mantle-derived spinel-lehrzolite nodules. Stratiied surge deposits made of scoria and ash drape the pre-Cambrian(?) granitic basement along the lake shore. Lake Nyos is a part of Cameroon Volcanic Line which contains many fields of monogenetic volcanoes. Among them is another maar-type crater lake Monoun, which erupted similar lethal gas(CO₂) in 1984 and killed 37 people. The rocks of these young monogenetic volcanoes range from weakly to strongly alkaline basalts and basanitoids. The older(Tertiary?), plateau-forming basalts which constitute immediate basement are also alkaline basalts and basanitoids.

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