Izu-Mariana Arc Research Articles

東北日本弧・伊豆−マリアナ弧の会合部における火山体直下の基盤構造

東北日本弧・伊豆−マリアナ弧の会合部における火山体直下の基盤構造

プロト伊豆‐マリアナ島弧の衝突付加テクトニクス―レビュー―

This paper reviews the geology, petrology, and tectonics of the Proto-Izu-Mariana arc against the Honshu arc, Japan, since 15 Ma, to formulate current topics related to the Pacific-type orogeny. Since the first pioneering work by Sugimura (1972), which placed the plate boundary on-land Japan north of the Izu peninsula collision-accretion tectonics of the Proto-Izu-Mariana arc against the Honshu arc started to be extensively investigated through multi-disciplinary methods. Aoike (1999) proposed a comprehensive scenario of five successive accretions of Proto-Izu to the present, with all bounded by top- and bottom-faults since 15 Ma. Oroclinal bending of the Honshu arc began simultaneously with the collision and indentation of the Proto-Izu arc, as is well documented by the Neogene paleomagnetic declination of sedimentary rocks in the Kanto district (Niitsuma et al., see summary, 1989). The entire Izu-Mariana arc is subducting without any accretion now under Honshu, as is well documented by seismic tomography imaging by Hasegawa et al. (see Hasegawa et al., 2010). Tanzawa Tonalite–Trondhjemite–Granodiolite (TTG) pluton intruded into the already-accreted Tanzawa Group at 4 Ma (Tani et al., 2010), resulting in a contact metamorphic aureole ranging from amphibolite through greenschist to zeolite facies at a depth of ca. 10 km, and tectonically denudated along the Kan-nawa fault to be exposed on the surface by 1.0 Ma. The Proto-Izu-Mariana arc was built on the Pre-Paleogene, presumably Cretaceous MORB crust on which boninitic and fractionated silicic lava flows accumulated since 48 Ma, which is older than the bending of the Hawaii–Emperor seamount chain at 43 Ma. Hence, we need another plate off Proto-Ogasawara arc, called North New Guinea plate (Seno, 1985), and ridge subduction underneath Ogasawara arc to initiate subduction magmatism. Both calk-alkali rock series (CA) and coeval blueschist–eclogite rocks are exposed along the inner wall of the Izu–Mariana trench. This strongly suggests that extensive tectonic erosion occurred, resulting in the coexistence of two rock units that were formed 100-150 km from each other horizontally, i.e., rocks on the volcanic front and rocks on the descending oceanic slab. Petrogenesis of juvenile crust and TTG rocks are being debate with two models: basaltic arc magma fractionation (Kawate and Arima, 1998) and delaminated mafic lower crust (Nakajima and Arima, 1998). In the latter model, a difficulty is derived from density at the Moho depth and small size of delaminated residue. Instead, extensive tectonic erosion that transports not only the upper crust but also the lower mafic crust together with parts of the descending slab could be more plausible than these two models in order to present a thinner mafic lower crust.

Eocene ostracodes from Hahajima Island of the Ogasawara (Bonin) Islands, northwestern Pacific, and their paleobiogeographic significance in the West Pacific

During the Eocene, Hahajima Island of the Ogasawara (Bonin) Islands of the Izu-Mariana Arc in the North Pacific was located near the present-day position of western Pacific Micronesia. We identified 17 Eocene ostracode species from the island, of which are new species and likely endemic to this island or to the West Pacific. These new species are systematically described herein and include:Costa matsumaruin. sp.,Cytherelloidea ogasawaraensisn. sp.,Havanardia ujiiein. sp.,Jugosocythereis nishiin. sp.,Neonesidea boninensisn. sp.,Schizocythere antiquimicronesianan. sp., andUroleberis hahajimaensisn. sp. The Eocene ostracode fauna has a higher rate of endemism than does the modern Micronesian fauna and contains Tethyan genera. These findings contradict a hypothesis that the Pacific fauna had a higher endemism rate after the Paleogene. The findings support another hypothesis that some genera invaded the Pacific from Tethys during the Paleogene. They suggest that West Pacific species did not disperse to the periphery of the Indo-West Pacific region during the Middle–Late Eocene. The “center of origin” hypothesis cannot be applied to Middle–Late Eocene marine ostracode biogeography.

Uplifting age of the Sengen-yama hills, Yamakita, Kanagawa, Japan: a constraint for the active tectonics of the northeastern border of the Izu-Mariana arc

The Sengen-yama hills, located at Yamakita town, Kanagawa Prefecture, Japan, represent the upthrown block along the E-W trending Hinata fault. The roughly 5-km long Hinata thrust fault is a segment of a long bell-shaped splay of faults (the Tanna, Hirayama, Hinata and Kozu-Matsuda faults) developed at the collision zone of Honshu island and Izu-Mariana arc. We found old fluvial gravel deposits and an overlying pumice fall deposit near the top of the hills. The pumice fall deposit is identified to be the Fuji-Yoshioka tephra (F-YP; 70-80 ka) on the basis of phenocryst assemblages, refractive index of the heavy minerals and facies association. Based on geology and tephra correlation, the onset of emplacement of the Sengen-yama hills is determined to be 70-80ka. According to previous studies, the initiation of the faults occurred about 100 ka. Close spatial correlation and close onset ages of these faults suggest that they were formed and developed in the same tectonic regime.

Origin of anorthite and olivine megacrysts in island-arc tholeiites: petrological study of 1940 and 1962 ejecta from Miyake-jima volcano, Izu-Mariana arc

Although aphyric tholeiites were discharged from nearly the same fissures during 1940 and 1962 eruptions of Miyake-jima volcano, some of the 1940 rocks are characterized by the presence of anorthite (to 3 cm) and olivine (to 4 mm) megacrysts. We focus on the assemblage and composition of crystal-clots to discuss magmatic processes, because minerals in the same type of clots must have crystallized at the same time. Three types of clots are identified, megacryst (M), basaltic (B; 1190–1175°C) and andesitic (A; 1150–1080°C). The M-type crystal-clots are characterized by anorthite (An 87–96) and olivine (Fo 78–86) megacrysts. Major mafic minerals of the B-type and A-type crystal-clots are olivine-clinopyroxene without magnetite, and pyroxenes with magnetite, respectively. The 1940 megacryst-bearing rocks contain all the three types of clots, whereas 1940 megacryst-free rocks contain only A-type and B-type. However, megacrysts and M-type clots show petrographic features such as kink-bands of olivine, spherical olivines in anorthite, and wide homogeneous cores of anorthite, suggesting that these minerals may not be comagmatic phenocrysts but xenocrysts. Chemical compositions and crystal size distribution plots of these crystals are identical to those of plutonic xenoliths erupted from one of the 1940 fissure. Thus, we concluded that megacrysts and M-type clots are xenocrysts derived from the same sources as plutonic xenoliths. We consider that the magma plumbing system of the 1940 eruption consists of a deeper basaltic magma storage system and a shallower andesitic one. The plutonic body might be located under the northeastern part of the volcano between two magma storage systems. The basaltic magma captured plutonic xenoliths, and then mixed with the andesitic magma during the 1940 eruption. Whether erupted rocks contain xenocrysts or not must depend on the spatial relationship between ascending basaltic dykes and location of the deep-seated plutonic body. On the other hand, the 1962 ejecta contain only A-type crystal-clots, indicating that only shallower andesitic magma was discharged. Although anorthite megacrysts are often recognized in island-arc tholeiites, calcic plagioclase could crystallize not from nearly dry tholeiites but from H 2O-saturated conditions. This could be achieved by closed system differentiation and/or addition of H 2O from the crust, such as plutonic conditions.

Diffuse emission of CO 2 from Miyakejima volcano, Japan

Two soil gas surveys were carried out in May and September 1998 at Miyakejima volcano, in the Izu Mariana arc, Japan. CO 2 flux values for May and September surveys ranged from 0.1 to 18,150 g m −2 day −1 and from 0.1 to 9685 g m −2 day −1, respectively. Statistical graphical analysis showed three overlapping populations. The spatial distribution of these emissions correlated quite closely with the geothermal and geological characteristics of the studied area. The structure releasing higher CO 2 is the summit cone Oyama and surrounding areas, where the most obvious geothermal features occur. A total output about 100–150 t day −1 is estimated from this area. A good correlation was observed between soil CO 2 flux and soil temperature at the summit caldera indicating extensive condensation of fumarolic steam within the upper part of Miyakejima. Carbon isotopic analysis of selected samples inside the summit caldera ( δ 13C–CO 2=−0.90‰ to −5.70‰) suggests a mixing of carbon derived from marine limestone and magmatic CO 2 while a clear biogenic origin ( δ 13C–CO 2=−14.76‰ to −25.52‰) is observed for the diffuse degassing of CO 2 outside summit caldera.

三宅島火山の形成史

Miyakejima Volcano is located 200 km south of Tokyo, Japan, and is one of the active volcanoes situated on the Izu-Mariana Arc. The main cone of Miyakejima has two nested calderas : outer Kuwanokitaira Caldera, 4 km in diameter, and inner Hatchodaira Caldera, 1.8 km by 1.6 km across. The central cone, Oyama, grew in the Hatchodaira Caldera.A short recurrence time of about 22 year-period and continuous crustal inflation in the recent years suggested that the volcano was proceeding to the next eruption. Activity began on June, 26, 2000, resulting in subsidence to form and the forming of a new caldera 1.6 km in diameter. The new Hatchodaira Caldera almost overlaps the site and size of the former Hatchodaira Caldera. This paper presents the volcano-history of Miyakejima during the last 10000 years.Based on the eruption style, together with whole-rock bulk chemistry, the development history during the last 10000 years is divided into four stages. These are 1) the Ofunato Stage of 10000-4000 y.B.P., 2) the Tsubota Stage of 4000-2500 y.B.P., 3) the Oyama stage, from 2500y.B.P. to the early 15th century, and 4) the Shinmio stage, 1469 AD to the present. Since 1085 AD, at least 14 eruptions are documented. The last four eruptions occurred in 1874, 1940, 1962, and 1983.The Ofunato stage is characterized by porphyritic basalts. These lavas and pyroclastics contributed the growth of the main cone and filling up of the Kuwanokitaira Caldera. The Tsubota Stage, reopened after a 3000-year repose, is distinguished by andesitic products from lateral and central eruptions. The Oyama Stage began with the formation of the Hatchodaira Caldera, which resulted from the discharge of ca. 0.37km3 scoria, explosion breccia, and accretionary lapilli. Subsequent products from central and lateral eruptions filled the caldera. It is noteworthy that phreato-magmatic eruptions from the central vent prevail over dry magmatic eruptions. Overflows of lavas from the rim of the Hatchodaira Caldera occurred in 9th century. At the Shinmio stage, 12 documented eruptions, without exception, took place from lateral fissures, with some accompanied by central eruption.The variation of Mg# (=Mg/ (Mg+Fe) × 100) versus erupted age show an abrupt increase of the ratio 2500, 1300, and 500 y.B.P. with a mild decrease. This pattern suggests that relatively undifferentiated magma was supplied to the magma plumbling system underneath the Miyakejima Volcano, which was slowly differentiating.

Experimental constraints on differentiation and H2O abundance of calc-alkaline magmas

Partial melting experiments were conducted on a natural high-alumina basalt from the Higashi-Izu volcanoes (47.4 wt.% SiO2; 8.3 wt.% MgO) with 1 and 2 wt.% total H2O at 0.5 and 1 GPa, 1125-875°C. The partial melts produced segregate by capillary action into crimped parts of the capsule, preventing chemical modification of the melt during quenching. Liquid compositions in 1 wt.% total H2O runs at 0.5 GPa mimic the chemical variations of calc-alkaline andesites and dacites associated with the basalt. The residual phase assemblages from 1 wt.% total H2O experiments are consistent with the observed phenocrysts in two pyroxene andesite and hornblende (hb)-orthopyroxene (opx) dacite. In contrast, a clinopyroxene (cpx)-hb rhyolite assemblage is produced in the 2 wt.% total H2O experiments. Although no such cpx-hb rhyolite is found in either Northeast Japan or the Izu-Mariana arc, this type of rhyolite is found in Central America, which is also characterized by high H2O contents in the associated basaltic melts ([1]).The abundance of SiO2 in residual liquids increased from 49 to 54 wt.% in the 25°C interval between 1125° and 1100°C in the 1 wt.% total H2O system, and from 63 to 71 wt.% in the 50°C interval between 975° and 925°C. The first gap between basalt and andesite is generated by a sudden change in the amount of partial melt, and the second gap between andesite and dacite is produced by the formation of hornblende by the consumption of pyroxene, because the SiO2 abundance of hornblende is less than that of pyroxene. These two reaction relations result in the existence of two compositional gaps, between basalt and andesite, and between andesite and dacite, which are recognized in the Higashi-Izu volcano group and many other calc-alkaline volcanoes.

Origin of alkaline volcanic rocks occuring on the volcanic front of the Izu-Mariana arc, northwest Pacific.

Iwojima and adjacent area located in the southern part of the Izu-Ogasawara arc and the northern part of the Mariana arc, alkaline volcanic rocks occurs on the volcanic front. These rocks are characterized by a extreme enrichment in K2O and LILE, relatively constant concentrations of HFSE, and high 87Sr/86Sr and low 143Nd/144Nd ratios compared to tholeiitic rocks occuring along the volcanic front of the northern part of the Izu-Ogasawara arc and the southern part of the Mariana arc. The alkaline magma is interpreted to have been generated by partial melting of a mantle source depleted in HFSE but enriched in LILE. The enriched processes in the source mantle might involve additions of LILE-rich H2O fluids released from a mica+apatite metasomatized mantle.

A57 伊豆-マリアナ弧の火山岩の 234U/238U 放射能比

A57 伊豆-マリアナ弧の火山岩の 234U/238U 放射能比

The 1989 submarine eruption off eastern Izu Peninsula, Japan: ejecta and eruption mechanisms

The submarine eruption of a new small knoll, which was named “Teishi knoll”, off eastern Izu Peninsula behind the Izu-Mariana arc occurred in the evening of 13 July 1989. This is the first historic eruption of the Higashi-Izu monogenetic volcano group. The eruption of 13 July followed an earthquake swarm near Ito city starting on 30 June. There were subsequent volcanic tremors on 11 and 12 July, and the formation of the Teishi knoll on the 100 m deep insular shelf 4 km northeast of Ito city. There were five submarine explosions, which were characterized by intermittent domelike bulges of water and black tephra-jets, which occurred within 10 min on 13 July. Ejecta of the eruption was small in volume and composed of highly crystalline basalt scoria, highly vesiculated “pumice”, and lithic material. Petrographical features suggest that the “pumice” was produced by vesiculation of reheated wet felsic tuff of an older formation. The Teishi knoll, before the eruption, was a circular dome, 450 m across and 25 m high, with steep sides and a flat summit. Considerations of submarine topographic change indicate the knoll was raised by sill-like intrusion of 106 m3 of magma beneath a 30 m thick sediment blanket. This shallow intrusion is assumed to have started on 11 July when volcanic tremors were observed for the first time, but there was no indications of violent interaction between wet host sediments and intruding magma. The submarine eruption of 13 July appears to have been Friggered by a major lowering of the magma-column. The basalt scoria, having crystal-contents of more than 60%, is assumed to be derived from the cooled plastic margin of the shallow intrusive body. However, glassy scoria, which would indicate the interaction between hot fluidal magma and external water, was not observed. A scenario for the 1989 submarine eruption is as follows. When rapid subsidence of the hot interior of the intrusive magma occurred, reduced pressure caused the implosion of cooled plastic magma, adjacent pressurized, hot host material, and wet sediment. The mixing of these materials triggered the vigorous vapor explosions.

Crustal Structure of Oceanic Basins and Ridges Surrounding Iwo-jima

The crustal structures of the basins of Shikoku, Parece Vela (West Mariana) and West Philippine are oceanic in character resembling those of the Pacific Ocean basin. And the crustal structures of the ridges of Izu-Mariana Arc, Kyushu-Palau Ridge and Ryukyu Arc are different from those of basins. The Izu-Mariana Arc and the Kyushu-Palau Ridge are different from the Ryukyu Arc in point of lacking a granitic layer.

南部フォッサマグナの湾曲構造と伊豆の衝突

The origin of the northward-convex structure in and around the South Fossa Magna, central Honshu is discussed in relation to the collision of the Izu Block on the Izu-Mariana arc with Honshu. The Izu Block has been pushing Honshu since the middle Quaternary when the collision occurred. The 15-30km contraction between the Izu Block and collided Honshu is presumed to have been consumed in the manner as follows: 1) internal deformation of the Izu Block, 2) thrusting, folding and uplift of the Neogene and Quaternary deposits in the South Fossa Magna, and 3) compressive uplift and strikeslip faulting dominant in central Japan outside the Fossa Magna.Geological evidences suggest that the convex structure of the Neogene of this region was formed, inheriting the pre-existing bend of the pre-Miocene terrains surrounding the South Fossa Magna and that the Quaternary collision of the Izu Block contributed to the compression of the region, though it did not strengthen the northward-convex structure significantly.

Alkali mapping of the japanese quaternary volcanic rocks

Of ca. 200 Quaternary volcanic centers in the Japanese islands, ca. 120 are characterized by K 2O and Na 2O concentrations of their ejecta normalized to SiO 2 = 60% K 2O values give a geographically well-defined trend of regularly increasing away from the volcanic front. Na 2O values show no geographical trend. Relationships between the SiO 2-normalized K 2O and the depth of the deep seismic zone (K- h relation) are different for different arcs. The Southwest Japan arc shows distinctly higher K 2O and less regular geographic trend than the others probably due to crustal contamination. In the normal segments of the Northeastern Japan belt, K 2O increases ca. 2.5 to 3.0% per 100 km depth of the seismic zone. Groups of volcanic centers near the junction between the Northeast Honshu and Izu-Mariana arcs give characteristically lower K 2O while Rishiri volcano far behind the arc junction between the Kurile and Honshu arcs also gives a low K 2O value.

Magnetization of Izu islands with special reference to Oshima volcano

Assuming uniform magnetization and using airborne magnetic total-force survey data, the direction and magnitude of four Izu volcanic islands, Oshima, Miyakejima, Nii-jima and Kozushima, located in the northern part of the Izu-Mariana arc have been calculated. The results showed the following general features: Nii-jima and Kozushima have lower magnetization intensities than those of Oshima and Miyakejima, agreeing with the difference in the petrography of rocks, i.e., the former are rhyolitic and the latter basaltic. The calculated directions of magnetization of these Quaternary volcanic islands are not far from that of the present geomagnetic field, except Oshima. The computed declination of Oshima is deflected eastward by several tens of degrees, disagreeing also with the paleomagnetic data on the surface volcanic rocks. One possible interpretation of this discrepancy is the inhomogeneous magnetization, i.e., the intensity of the eastern part of Oshima is less than that of the other parts. In fact, the magnetization of older volcanic rocks exposed on the east coast of Oshima has been found to have much lower intensity than the rocks in other parts. This interpretation is in agreement with the geology of Oshima which predicts a hidden older volcano, late Tertiary or early Quaternary in age, beneath the eastern part of the island.

Evolution of the tertiary volcanic rocks in the Izu-Mariana arc

Petrological evolution of the Tertiary island arc in the Izu-Mariana region has been accompanied by the development of three different volcanic suites: 1) oceanridge basalt now exposed as the metamorphic basement on Yap; 2) island-arc tholeiites of Eocene to early Oligocene age characterized by low contents of incompatible elements at all levels of silica enrichment; and 3) calc-alkalic rocks of late Oligocene to early Miocene age showing higher contents of silica and incompatible elements. All these three suites have primitive, undifferentiated basalts or andesites (boninites) characterized by high Mg/Fe, Cr, and Ni, suggesting that they have been derived from an upper mantle peridotite at relatively high temperatures. The earliest volcanism appears to have occurred at a spreading ridge. Later, as subduction proceeded, the island-arc tholeiite magma may have been produced by the introduction of a smaller amount of water into the locus of fusion from the subducted oceanic crust. An increasingly larger amount of water introduced into the same region could have led to the development of the more siliceous, calc-alkalic magma, as represented typically by the boninite.

Boninite as a possible calc-alkalic primary magma

Boninite is an unusual, plagioclase-free magnesian andesite, occurring as vesicular pillow lavas and hyaloclastites, accompanied by andesites and dacites in Chichi-jima, Bonin Islands. The Bonin Islands belong to the Izu-Mariana arc and consist of dominant volcanic rocks and subordinate sedimentary rocks of late Oligocene-early Miocene age. The chemistry of boninite is characterized by high contents of MgO. Cr and Ni similar to primitive basalts, but apparently in ill accord with its relatively high SiO2 content of ≃ 55%. The relation of SiO2 to total FeO/MgO ratio indicates that boninite belongs to the cale-alkalic rock suite. The mineralogy of boninite consists of olivine (Fo87-90), orthopyroxene (En87-90), clinopyroxene (Wo38-35En37-44Fs25-21), hydrous glass and Cr-spinel, Experimental studies show that the magma of boninite composition could be in equilibrium with upper mantle peridotite at pressures less than 17 kb and temperatures of 1200–1050°C under high PH2O. It is suggested that boninite is a sea-floor quenched product (900°C) of a direct partial melt of the upper mantle. Related andesites and dacites are considered to be probably fractional crystallization products from the same magma.

Relationship between chemical composition of Japanese Island-Arc volcanic rocks and gravimetric data

1670 published chemical analyses from 137 Quaternary eruption centers of Kurile, Japan and Izu—Mariana arcs are collected and K 55 and K 60 values of individual centers are calculated. Along with the well-established trend of increase of K 55 and K 60 values across the arc towards the continent, longitudinal change is also recognizable. Interrelation between K 55 or K 60 value pattern and a map of Bouguer anomaly distribution along the volcanic front is characteristic. To develop one possible simplified model to explain the facts, we assume first that the asthenosphere has a homogeneous thermal state beneath the volcanic zone, second that the thickness of the crust is quite variable areally, and third that the mean density of the crust is constant throughout the area. Thus, the composition of the magma generated along a zone corresponding to equal depths along the Benioff zone should be identical throughout the volcanic arc, and lower values of the Bouguer anomaly should be observed where the crust is thicker. We then assume that magma generated in the asthenosphere ascends into the crust, then cools as fractional crystallization proceeds. Depths of fractional crystallization would be deeper where the crust is thicker. The ratio of the clinopyroxene phase separated from the magma to the plagioclase phase should be larger where the level of fractionation is deeper. Thus, magmatic liquid should be enriched in K where K-poor crystal fractions are subtracted at the deeper level.

Submarine Topography of the Philippine Sea

The Philippine Sea is bounded on the west by the southern half of Japan, the Ryukyu Islands and the Philippines, and on the east by the arcuate seafloor ridge extending from Japan to the Palau Islands through the Mariana Islands. Both arcuate structures are associated with deep sea trench, island arcs, volcanic activity, deep focus earthquakes and negative gravity anomalies. This abyssal deep basin which consists of the oceanic crust is separated into the east and the west basin by the Kyushu-Palau Ridge. The east basin is somewhat shoaler, and can be divided by the central narrow zone at 23°N into two parts, Shikoku Basin and West Mariana Basin, which gradually rise toward the Honshu-Mariana Ridge. There are, however, several seamounts, called the Kinan Seamount Chain, arranged in NNW-SSE direction in the Shikoku Basin. And on its southern extension, there are several troughs, called the West Mariana Trough, arranged is N-S direction. The Shikoku Basin is composed of the smooth floor of the northeastern part and the mountainous area of the southwestern part. The West Mariana Basin is also divided into the abyssal plain on the east and abyssal hill region on the west. And the West Mariana Trough is running on its boundary.The west basin, Philippine Basin, is unusually deep. A region of rugged topography which can be termed the Daito Ridges lies at the northernmost part of the basin. These several parallel ridges elongate from the respective coral islands to southeastward intervening narrow depressions. There are flat plains at the depth of about 1500, 2000 and 3000 m on the northern side of each of the ridges. There is the striking feature, called Central Basin Fault, in the central part of the Philippine Basin. Central Basin Fault which is the low ridge associated with trough in the NW-SE direction separate the Philippine Basin into the north basin and the south basin. On the Philippine Basin floor, the most extensive area is abyssal hill region. Abyssal plains are developed only in limited localities such as the west side of the northern arc of the Kyushu-Palau Ridge.From the description on the submarine topography of the Philippine Sea the author concluded as follows.(1) A few ridges and seamounts not associated with trench are running parallel to the Ryukyu-Philippine Arc and Izu-Mariana Arc : one Kyushu-Palau Ridge, two Kinan Seamount Chain-West Mariana Trough, three Nishi-Shichito Ridge-West Mariana Ridge. These ridges close the space eastward. And basements of basins which intervened between each ridges rise toward the east.(2) There are another ridges having unusual trend. The Daito Ridges have E-W trend and the minor ridges on the Izu Ridge have the NE-SW trend.The numerous minor ridges which construct the abyssal hill region also have the E-W trend on the northern part of the Philippine Sea, although its trend changes to the N-S trend southward gradually. These trends are correspond to the Southwest Japan Arc.(3) Ridges with E-W trend are cut by the ridges with N-S trend, alike the Japan Arc crossed by the Izu-Mariana arc.(4) Kyushu-Palau Ridge consists of two arcs, northern arc parallel to the Ryukyu Arc and southern arc parallel to the Philippine Arc. These two arcs are separated by the Central Basin Fault. Another possible fault runs from the depression in the east side of the northern Kyushu-Palau Ridge to the junction of the Izu-Ogasawara Arc and the Mariana Arc. The southern arc (Mariana) remarkably projects to the southeast along this fault.(5) Abyssal hill provinces in the Philippine Sea occupy the western half of each basins usually. Some of the flat plains occupying the eastern half appear to the archipelagic apron which is constructed by the eruption on the concave side flank of the arcuate ridges.

Mode of eruption of Miyake-jima volcano in historic times

Miyake-jima, an island volcano, resting on the Izu-Mariana arc is a basaltic stratovolcano dotted with many parasitic cones and craters. Literatures tell us thirteen eruptions which took place in A.D. 1085, 1154, 1469, 1535, 1595, 1643, 1712–1714, 1763–1769, 1811, 1835, 1874, 1940 and 1962. The last two eruptions were studied in detail by using modern scientific techniques after the surface activities had almost died down. Of the thirteen eruptions, the earlier five are only described as «eruption» and no detailed accounts are written in a document. The eruptions in and after 1643 all occurred from parasitic craters or fissure vents and are classified into the following two types: Most of these eruptions are simply flank activity, but some are preceded or followed by summit activity. In either type of eruption, the activity begins with the destruction and ejection of pre-existing rocks and then the violent ejection of juvenile scoriae follows. Spindle-shaped bombs seem to be thrown during the later, rather calm stage of the scoria ejection or from lava-filled vent or vents different from the center or centers of the scoria ejection. During the culminating or declining stage of the activity, lava flows down along valley as a torrent and often reaches the sea. In case of the historic eruptions of this volcano, time interval between the start of the activity and the lava outflow is very short, generally less than several hours. The activity abruptly comes to an end soon after the lava outflow. Total volume of the materials erupted in each activity which took place every twenty to seventy years is of the order of 0.01 to 0.02 cubic kilometers. The case history of this volcano as revealed by investigations of the literatures describing the volcanic activity and the basic field survey will give us what a proper means of the prediction of volcanic eruption is.