The Western Branch of the East African Rift Valleys has two active volcanoes Nyiragongo and Nyamulagira, which erupted in 1977 and 1980 for the last five years. A base line network consisting of 18 benchmarks was set up in 1977 in and around the volcanoes to reveal the horizontal movements across the rift valley near the volcanoes and also those in the volcanic area. The measurements were carried out by Geodimeter 6-BL in 1977 and 1979. The maximum compressional strain observed at the southeastern flank of Nyiragongo was 5×10^-5 with a NNW-SSE direction, which agrees well with nearly linear, N15°W trending surface fissures accompanying the 1977 Nyiragongo eruption. The maximum compressional strain at the Nyamulagira crater was 1.5×10^-5 with a ENE-WSW direction, which is consistent with the NE-SW direction of the new fissure which resulted in the 1980 Nyamulagira flank eruption. These two short-term strain patterns can be explained by a hypothesis that there exist two domains with different, long-term stress systems in this volcanic area. They have the stress axes of the maximum horizontal compression in the NE-SW and NNW-SSE directions. The former stress is probably in control of a preferred alignment of monogenetic volcanoes at the flank of Nyamulagira. The latter may most probably be responsible for the volcanic activities along the linear zone across Nyiragongo and Nyamulagira. The measurements of horizontal distance across the rift valley could not give any fruitfull results because 8 benchmarks were destroyed by some persons during the two years. Tilt measurements at three sites in the volcanic area were also performed with the automatic level equipped with a micrometer. Each site set up in September, 1979 has four orthogonal leveling lines of about 100m length. It is found that the tiltings at the western and eastern sites of the rifting axis were 3μ radians down to a NNE direction and 10μ radians down to a nearly W direction, respectively, over the six months including the 1980 Nyamulagira flank eruption.

View full abstract

Mt. Ontake, a dormant volcano in central Japan since the dawn of the Japanese history erupted on 28th October, 1979. We set up an IC thermometer in a thermal spring well about 80m deep at Nigorigo-Onsen, 5km north-west from the summit of Mt. Ontake, 9 days after its eruption and continued the temperature measurements for a year. We found that the temperature at the well decreased exponentially with time and dropped about 1.5 deg. by the end of October, 1980. The temperature decrease must have been somehow related to the eruption of Mt. Ontake which may have affected the temperature distribution near the well source and path of the ground water. Besides the secular temperature decrease, the short period temperature variation of the well is characterized by intermittent pulse-like variations about 1 hour duration and about one-week period variations. The latter variation is well correlated with atmospheric pressure variation. These variations are reasonably well explained by a kind of geyser model with a pressure sensitiv buffer cavity.

View full abstract

Pyroclastic material is considered to be a kind of “fossil” suggesting the pressure-tempe-rature condition when it was formed. From this point of view, as a case study, the tephra associated with the 1783 eruptive activity of Asama Volcano is investigated. The total thickness of the tephra at our outcrop is about 2 meters. The formation is composed of several pumice layers alternating with thin ash beds. The mean bulk density of each pumice layer is measured by using about 100 pieces from the corresponding layer. The resultant values obtained from the lowest pumice layer to the highest one change as 0.79, 0.70, 0.69, 0.76, 0.82, 0.73, 0.70, 0.73 and 0.75 gr/cm³. The error is within 0.03 gr/cm³. The values, therefore, range roughly from 0.70 to 0.85 gr/cm³. One of our most interesting findings is that there is a peak in the change of the value. Thus, according to the order from the lowest pumice layer to the highest one, the value decreases from 0.79 to 0.69 gr/cm³ and next, increases to 0.82 gr/cm³ which is the peak value, and the value again decreases to 0.7 gr/cm³ and slightly increases. The peak can be eventually explained by attributing not to the worse vesiculation throughout the magma but to the coexistence both vesiculated pumice and worse vesiculated one, of which bulk density ranges roughly from 0.85 gr/cm³. Since the eruptive activity was great, many old documents relative to it still remain. Much information obtained from them is compiled. The course of the eruptive activity can be inferred from the compiled information. The formation of pisolite is generally considered to be closely related to rain. Therefore, taking account of minglement of pisolite, as a key material, into the pumice fall deposit, we can correlate the result from the measurement of mean bulk density with the course of the eruptive activity inferred form the old documents, in which the information relative to rain is found out. Conclusively, we can determine the date when each pumice layer was formed and also, we might be able to say that the whole tephra was formed wihin about a week. Moreover, it might be able to say that the peak in the mean bulk density as mentioned above was attained just before the occurrence of the two pyroclastic flows termed as “Agatsuma” and “Kambara” and the occurrence of the lava flow termed as “Onioshi-dashi”.

View full abstract

The compositions of the primary tholeiitic, high-alumina and alkali-olivine basalt magmas (THB, HAB and AOB magmas, respectively) which are derived directly from the upper mantle beneath the volcanic arcs, are obtained by calculating the average compositions of liquid in equilibrium with the mantle peridotite, which can produce basalts in NE Japan arc through olivine maximum fractionation. Anhydrous high-pressure melting experiments on these three basalts indicate that the AOB and HAB magmas coexist with olivine, orthopyroxene and clinopyroxene at 1360℃ and 17.5 kbar and at 1340℃ and 15 kbar, respectively. The THB magma, on the other hand, coexists with olivine and orthopyroxene at 1320℃ and 11 kbar. The volcanic arc magmas are believed to contain significant amounts of water which affect the P-T conditions of the phase equilibria at high temperatures and pressures. However, the detailed petrographic studies on the rock suites in volcanic arcs revealed that the island arc primary basalt magmas contain water not more than 3 wt.% at generation in the upper mantle. Combining this with the experimental results, the THB, HAB and AOB magmas are suggested to segregate from the mantle at temperatures of about 1300℃ and at pressures of 11 kbar (THB), 16 kbar (HAB) and 20 kbar (AOB), respectively. As the temperatures of segregation of the magmas given above are too high for a stable mantle geotherm, the mantle diapir is the most probable mechanism for magma generation under the volcanic arcs. Due to the heat of formation of liquid in the diapir, the temperature of the diapiric mantle must be higher at deeper levels. The required temperature of the upper mantle is 1400℃ at a certain depth between the descending slab and depth of approximately 70km.

View full abstract