An attempt is made in this paper to estimate the volume of magma in primary stage of volcanic activity as a function of content of K₂O in a differentiating residual magma. In the test it is found that present method of volumetry (equation (1)) seems applicable in case of Usu Volcano in Hokkaido, Japan.

抄録全体を表示

Spindle-shaped volcanic tremors being similar to sinusoidal waves are frequently observed at the Sakurazima Volcano. They are called tremors of C-type by the Kagoshima District Meteorological Observatory (KDMO). It has hitherto been presumed that they have some connection with the intrusion of lavas into the base of crater. We have, however, no reliable evidences as yet. The authors reexamined the tremors marked as C-type by KDMO, using particle motions and periods of waves. The results are as follows. The mean periods of tremors range from 0.4 to 2.0 sec, but most of them from 0.5 to 1.3 sec. The peaks of frequency distributions of periods are at 0.7, 1.0 and 1.3 sec and they seem to vary from time to time. The investigations of particle motions show, however, that the tremors of different type of waves are mixed into the same group as C-type and that the predominant part of one tremor is composed of P or S type of waves and the other is surface waves. Nevertheless, the possibility of variance of predominant period with time is not denied because tremors of the same type of waves have different periods one another. The tremor of C-type is composed of superposition of waves having slightly different periods though it is very similar to sinusoidal waves. The mean period still has some meanings because the periods of waves composing the tremor are confined to a narrow range.

抄録全体を表示

The welded tuffs of pumice-flow origin are made of deformed and consolidated glassy fragments. On the other hand, viscous glass particles at high-temperature and at high-pressure deform and comsolidate, and then they grow into dense compact body. The sintering of glass proceeds by the mechanism of viscous flow. At the early stage, we get nearly the same density from both the equations of particle-deformation model and pore-closing model for the same pt/η value. Slightly welded specimens (Shikotsu welded tuff) were experimented under various load-pressures and temperatures. These experiments indicated that the degree of sintering is controled by pt/η value. The density-increase of glassy fragments is sepresented by the following equation : dD/dt=3/4・p/η・(1-D).

抄録全体を表示

Sikotu Caldera near Sapporo has been studied from the viewpoints of the distribution of its welded tuff, the subsurface structure, and the origin of its formation. The age of the caldera formation was already determined by means of ¹⁴C as 32, 000 years B. P.. In this paper, magnetic properties of the welded tuff from the caldera was discussed from the standpoints of volcanology and paleomagnetism. The specimens were collected mainly at an outcrop 20 meters high at a distance of 18 km from the caldera. First, paleomagnetic intensities of geomagnetic field at that period were determined by Thellier's method, as about 1.4 times the present one. Second, the temperatures of deposition of the ejecta at various depths of the same outcrop were deduced by comparison between the natural remanent magnetization and the partial thermal remanent magnetization. The middle part of the outcrop might arrive at the spot with the temperature higher than Curie point and then cooled acquiring the N. R. M. while the upper and lower parts were near 500℃. Such distribution of the temperatures of deposition is not due to the effects of cooling at the upper and lower boundaries but may be due to the changes of the original temperatures of the ejecta within the volcano, or due to the differences in cooling of the ejecta during the travels, at each stage of the eruption.

抄録全体を表示

The Cheju Island, southern Korea is covered by alkaline volcanic rocks of Pliocene and Pleistocene which are divided into three groups, i.e. trachytes, trachyandesites, and trachybasalts in order of eruption epoch. The trachytes are characterized by aegirineaugite, kaersutite-ike amphibole, Fa-rich olivine and plagioclase with anorthoclase mantle. The trachyandesites show the reversed magnetic polarity which belongs perhaps to the “MATUYAMA reversed epoch”. The sketch of the interior of the “Sankaido” which is one of the lava tunnels were also given.

抄録全体を表示