Volcano Oshima, Idzu

(1) both the position now occupied by the Idzu peninsula and its environs have been scene of volcanic agitation since the Tertiary period. Here the vulcanism of the two different petrographic lineages - andesitic and rhyolitic - have displayed their activity, and built up the Idzu peninsula. Lavas of these two lineages continued to be extruded down to the Quaternary period. The andesitic or basaltic lavas built up the volcanoes Fuji, Ashitaka, Hakone, Atami, Amagi, etc. on the main land and many insular and submarine ones in the pacific ocean, along the line that runs in the direction N.N.W. - S.S.E., conforming to the conspicuous "Fuji volcanic chain." The lavas descending from the Tertiary rhyolitic rocks formed the rhyolitic islands off the Idzu peninsula to the west of this volcanic chain. Contrary to the general opinion the writer agrees with TSUJIMURA that these rhyolitic islands are as young as the volcanoes of the Fuji volcanic chain. What is commonly called "Idzu Shichito" comprises the Quaternary insular volcanoes of both of the above-mentioned Lineages, and is to be subdivided into the Oshima group and the Niijima group, consisting respectively of basaltic and rhyolitic islands. The members of the former group are Oshima, Toshima, Udoneshima, Miyakejima, Mikurashima, and Hachijojima. The rocks of these volcanic islands show a very strong resemblance to one another and appear to be closely comagmatic. By correlating these rocks and those in the related region in the main land, where the general magmatic change is from acidic to basic, it appears that the birth of the basaltic insular volcanoes was at a late date, probably late Diluvium, by which time the magma had become strongly basic. (2) The insular volcano Oshima consists mainly of double homates - a somma and a central cone -, and is built up of alternating lava-flows, both superfluent and effluent, and ejecta beds. The somma has eight parasitic knobs and two phreatic explosion-craters on its flanks, and is truncated at the top with a ring-wall that surrounds a huge oval caldera, about 3 km. in diameter. The wall is not completely closed but has two gaps, a great one on the northeastern and a small one on the south-western sides. The central homate, Miharayama, standing in the caldera, has an active summit crater, 700m. across. Its volcanic products cover the ground not only within the inside of the encircling wall but have also spread down to the sea shores through the gaps in the wall. Besides the above, there are along the western half of the northern coast, several small demolished igneous bodies now covered by the volcanic materials of the somma. (3) The rocks constituting the volcano are of several types as already tabulated on p. 67, but they are readily grouped under three main headings, - basaltic bandaite, miharaite, and basalt. Basaltic bandaite predominates amongst the lavas of the somma. in it, the phenocrysts of calcic bytownite are scattered through the granular intersertal groundmass consisting of labradorite, augite, magnetite, and a small quantity of glass. Most of the lavas of this type bear a negligible amount of small olivine phenocrysts, though in some this mineral is absent. Hypersthene and augite phenocrysts are either lacking or present only in small quantities. The type which the writer describes under the new name "miharaite" constitutes the central cone. It does not differ much from the preceding in its petrographic characters but is characterized by an entire absence of olivine. Basalt is a variety comparatively rich in phenocrysts of olivine and pyroxene. This constitutes the northwestern domolished igneous bodies, but is very rarely found as the lavas of the somma. All of these rocks are strongly basic. The chemical analyses made of the first two types of the above three-basaltic bandaite and miharaite -, which exceed the third in mass, show a remarkable character in that the amount of silica, in spite of its low percentage, is not only enough to form the highest silicates but it is present in excess. This is one of the common characteristics of the most widely spread recent volcanic rocks of Japan, but the rocks of Oshima are peculiar in their more femic and less quaric natures. (4) The physiographic history of the volcano since its birth is summarized as follows: - The body which now constitutes the somma is the earliest of those so far seen. It was very gradually built up by repeated volcanic actions without any prolonged intervals of quietude. During the development of the somma, but far before its skirt reached the present state of development, extruded lavas comparatively rich in olivine formed small igneous bodies in the north-western part of the present Oshima, isolated at that time from the main body by the sea. These soon expired and were destroyed by the marine erosion. When the development of the somma was nearly completed, the steep slope running in the direction N.-S. along the east coast of the island was formed by the depression of the ground. Vulcanism displayed its energy not only at the centre of the volcano but also on the flanks of the somma at several spots during various but comparatively later stages in the volcanic history. The products of these flank eruptions are especially abundant in the southeastern part of the island. The depression of the summit of the somma resulted in the formation of a huge caldera whereupon the encircling wall was badly broken in its northeastern quarter. The central homate appeared in the caldera and its products not only filled the inside of the caldera but spread down to the sea shore, especially abundantly in the east through the northeastern gap of the ring-wall. In historic times the volcano continued frequently to display its activity, and eruptions in the following years are on record: 684, 1112, 1416, 1421, 1600-1601, 1612-1613, 1636-1637, 1684-1690, 1777-1778, 1803, 1822, 1846, 1870, 1876-1877, 1912-1914, and 1915. (5) Turning to the problem of the magma, the original magma is supposed to be basaltic, highly fluidal and with a specific gravity nearly equal to that of the calcic plagioclase crystals. The magma cooled gradually, separating out the crystals of olivine, plagioclase, and pyroxene, in order as shown in Fig. 40 (p. 117), its temperature becoming at last decisively lower than 1470℃. and probably as low as 1200℃- 1300℃. During the course of the cooling of the magma, the separated crystals were affected by gravitation. Thus, plagioclase, matching the magma closely in density, remained practically suspended in the magma basin; while olivine and pyroxene, being much heavier than the magma, subsided in it toward the bottom as they crystallized out. As a result of the separation and the subsidence of the crystals the originally homogeneous magma became heterogeneous as time passed, the heavy crystals being relatively concentrated at the bottom. All the rocks of Oshima represent, according to the writer's belief, certain stages after the above-mentioned magmatic temperature had been reached and the subsidence of the heavy crystals had gone on to a fair extent. The rock-type rich in phenocrysts of mafic minerals - basalt - may have been extruded from the lower portion of the magma basin where the mafic minerals were relatively concentrated; while most of the somma lavas in which olivine crystals are either absent or only very sparingly found - basaltic bandaite - are believed to have come from the upper part of the magma basin at the stage when the olivine crystals had half sunk; the central cone lavas which are entirely free from olivine - miharaite - are probably of a still later stage when the subsidence of olivine crystals had progressed still further so that they had no opportunity of entering the lavas as they were extruded. The effect likely to be brought about by the cooling of the magma was considered and it was concluded that the magma would change from basic to acidic if the cooling proceeded. All the lavas of Oshima now seen represent only one stage of the above course, - in other words the long physiographic history of Oshima, so far as it can be traced back from structural study, is represented by only one stage in the cooling history of the magma. (6) Future activity of the volcano Oshima. The volcanic activity may depend upon many factors, of which the temperature condition of the magma is no doubt most important. Other conditions being equal, a rise of the magmatic temperature would naturally bring about a rise in the volcanic activity, while a fall in temperature would lead to its decay. In the volcano Oshima, as already stated, the magma appears to have maintained the same temperature throughout all the stages in the physiographic development so far as is traceable. moreover, the structural feature shows that there has been no prolonged period of quietude in its life history, there being many historical records of violent outbursts, of which the latest one that lasted from 1912 to 1914 is still very fresh in our memory. All in all, the volcano shows no sign of decay in its activity and appears likely to retain for some time the same energy which it has displayed in the past.