Under the Oshima Volcano sits ruin of basaltic volcano named Okata basalts. Under the Okata basalts, it is believed to exist miocene volcanic complex (Yugashima Series) as the fragments of altered andesites and gabbro are contained in the tuff breccia. The Okata basalts is composed in pliocene epoch as strato-volcano. After it is folded, faulted and eroded deeply, the activity of the Oshima Volcano began covering the reef of ruined Okata basalts exposed on the surface of the pacific ocean in quaternary epoch. The volcanism succeeded violently intercalating peaceful time, when the pre-historic man lived at the sea shore, and at last, after the flow out of large quantities of lava, caulderon subsidence of the top of the volcano occurred. Before the subsidence, fault in the direction of N-S was formed along the east coast of the island, which is now represented as cliff of about 100 m in height. And also, accompanying the subsidence, fault was formed at the north part of the caldera in the direction of NE-SW to obscure the oval form of the caldera. After the subsidence central cone Miharayama began its development. The broad crater named “Inner Somma” seems to be formed in the pre-historic time and the historic activities occurred in it to form narrow vents or small cinder cones here and there in each case. The violent activities to flow lava over the wall of “Inner Somma” occurred only two times in the record and the recent activity is the third case.
The writer intended here to sketch in broad outline the eruption of Mt. Miharayama, on the island of Oshima, and to have a general view of the works of Japanese volcanologists with reference to the eruption. The eruption was one of·the greatest activities of the volcano in the historical time, characterized by frequent explosions of the Strombolian type. A summit eruption took place suddenly on July 16, 1950, ejecting ash and lava fragments and welling out molten lava, and continued without interruption until September 23. Besides, the volcanic activity resumed on February 4, 1951, and continued incessantly until about the end of March, but it declined and became intermittent at the beginning of April and was completely dead on July 28. During the eruption, a large quantity of new lava flowed out over the rim of the central cone after an interval of 172 years. Nearly one hundred able volcanologists In our country made various invaluable studies on the eruption from geophysical, geochemical, geological and petrological viewpoints. Moreover, the eruption went a long way toward the popularization of scientific knowledge on the volcano and its activity.
The processes of activity of Mihara volcano in 1950 are described. Topographic changes as revealed by frequent plane-table surveys and photographic analyses during the activities are also written. The amount of lava and ejecta during 1950 was estimated at 50, 000.000 tons.
The writer made frequently topographical surveys of inside of the inner somma especially level changes of the surface of fresh lava, which was spouted by the present eruption and accumulated there in a form of lava-pool. On the other hand, the eruptions occurred several times up to June, 1951, each of which lasted for several weeks or several days. As the result of the present surveys, it was found that raising and lowering of the lava surface rookplace repeatedly in the close relation with severe eruptions, especially, rapid and marked subsidence of the lava preceded by severe eruptions. In addition, the writer estimated the total volume of lava, overflew the inner somma and flow down over its flank this year, about 107m3. that is 2 ×107 tons. (Aug. 20, 1951)
On the basis of the relation between the amplitude of volcanic tremors, and the distance from their origin, it was made clear that these tremors of continuous train caused by the Strombolian eruption propagate two-dimentionally along the earth's surface from the active crater to each station. From the assumption that the attenuation of amplitude is entirely caused b the solid viscosity of the mountain formation consisting of various volcanic ejecta including lava beds, its coefficients were estimated at the range from 1.0×109 to 1.6×109 C.G.S. for the extremely upper part of the formation and from 2.2×109 to 2.7×109 C.G.S. for a little deeper part. From observations of flowing velocity and other necessary elements of the lava river descending the flank of the inner somma, the viscosity coefficient of the fresh lava was. calculated with the result of a range from 5.6×103 poises at 1125°C to 2.3×105 poises at 1038°C. It must be added that the temperature of fresh lava was measured at various depths of the lava river with the aid of thermo-junctions.
Repeating geomagnetic dip surveys in Ooshima Island, the writer found out marked changes in the earth's magnetic field at the early period of the 1950 eruption of Volcano Mihara, the changes being recognized as the demagnetization in the deep interior of the volcano due to temperature-rise. In order to study the geomagnetic change in detail, a continuous recording of the change in geomagnetic declination was begun since April this year, while a survey of three geomagnetic components was also made in May.
The gravity survey was carried out by the North American gravitymeter at about 50 stations at Mihara volcano in Ooshima Island over four times from September 1950 to April 1951 during the active periods of eruption. From these four surveys the gravity change with time was observed, the results of which being shown Pn. Fig 1, 2 and 3, and the results are summarized as fellows : 1. From the present investigation, gravity change with time was cleared in connection with the volcanic eruption, and the variation of subterranean mass distribution was found. 2. In the dormant season of this volcano, the value of gravity increases, particularly, it is evident in the somma domain. 3. After the beginning of the volcanic activity, the value of gravity decreases. 4. And, during the period in which the eruption is very violent, the decreasing rate of the gravity is very large. (0.20 mg16/ month).
Geochemical studies of volcanic activities of Volcano Ooshima, Izu during the period from March to May, 1951, are reported briefly. Max. temperature of fresh lava is 1120°C, flowing velocity on the slopes of 3035°is about 4 m/sec, and there is no remarkable rotation of ejecta in the air. Many kinds of bluish green copper compounds, yellowish brown iron compounds, white gypsum and etc are found. Bluish sublimates show the weak radioactivity and radon content of volcanic gas is about 13 mache. Volcanic gases contain remarkable chlorine compounds and frel hydrogen. Relation between magnetic substances and juvenile substances is discussed.
The general magnetic properties of the lava of Volcano Mihara are briefly summarized. A particular property. of the Mihara lava is an extremely large value of thermo-remanent magnetism. Their Q-value amounts to 100 or more.
The forms of crowns and the cross sections of stems of Pinus thunbergii on the coastal region of Izu-Oshima are analyzed and the distributions of their characters are researched from the standspoint of climatic landscapes. They coincide with nearly the distribution of weathering of houses, electric poles etc, on this region. Both of them show the climatic characters, esperically the distribution of force and directions of the prevailing winds.
The map has been compiled by Takeshi Honda based, on Dr. Seitaro Tsuboi's. The distribution of 19501951 Lava, a subsided central crater-pit and a newly-built cone were sketched with the naked eye on July 30, 1951. Abbreviaed symbols on the map are as follows : A : Rocks of Demolished Igneous Rocks. B : Somma Lava. C : Somma Ejecta. D : Scoriae from Flank Openings. E Central Cone Ejecta. F : Phreatic Explosion-Crater. G : An-ei Lava 1778. H : 19501951 Lava. + : The Ôshima Weather Station. Δ1 : Gojinka-chaya Δ2 : Kako-chaya. Here Chaya means a tea-house. (1) : Karihingasui (2) : Koshimizu (3) : Yaegawa (4) : Sakugawa (5) : Yamanokawa (6) : Traces of an old spring. (7) : Ajiko (8) : Mominosawa (9) : Akamizu (10) : Yokawa (11) : Ootaki (12) : Kawa (13) : Akamai (14) : Funo no Taki (15) : Shoiri Taki (16) : Motogawa (17) : Otaisama