Monogenetic Volcano Group Research Articles

B54 時空的に近接する独立単成火山群と複成火山のマグマ活動 : 五島列島北方の場合

B54 時空的に近接する独立単成火山群と複成火山のマグマ活動 : 五島列島北方の場合

中部北海道，滝川地域の中新世―鮮新世玄武岩のＫ‐Ａｒ年代と主成分化学組成 東北日本弧―千島弧，島弧会合部の玄武岩単成火山群

Cenozoic volcanism in the Takikawa district of the central Hokkaido, located at the junction of the Kuril and the northeastern Japan arcs, is unique in arc-trench system. It consists of Takikawa monogenetic volcano group (TMV) and four polygenetic volcanoes (Ofuyu-dake, Etai-dake, Shokanbetsu and Irumukeppu volcanoes). TMV are widely distributed in the district and mainly made up of alkali olivine basalt and olivine tholeiitic basalt. Whereas, voluminous polygenetic volcanoes associated with the TMV are mainly composed of calc-alkaline andesite and dacite. New data of K-Ar ages for the rocks show that the volcanic activity of TMV continued from late Miocene to early Pleistocene, with coeval polygenetic volcanism during Pliocene. The presence of TMV and the geological evidence for the development of sedimentary basin suggest that the junction area of the two arcs has been under extensional stress field during late Miocene and Pliocene. The SiO2 contents of nearly primitive basalts (FeO/MgO<1.3) in the Takikawa district exhibit an ellipse shaped zonation in space. More silica-saturated basalts are distributed in the center of the district. This feature is not concordant with across-arc compositional zonation in arc volcanics. Major element chemistry and their spatial zonation of these rocks from Takikawa district suggest that the magmatism should not be explained by the steady state subduction-related one but by mantle upwelling under extensional stress field in the arc-arc junction.

Width change of a planar magma path: implication for the evolution and style of volcanic eruptions

The width change of a planar path filled with magma is analyzed, assuming that the process is controlled by laminar flow of an incompressible viscous magma between movable parallel walls of country rock. The analysis shows that the width of a planar path changes at a rate is proportional to the effective opening pressure, which is defined as the pressure of the penetrating magma plus the tensile stress component stretching the fluid interface. When the opening pressure is a positive constant, the fissure widens at an accelerating rate and finally diverges at a critical time constant, which depends on the initial size of the fissure and the viscosity of magma as well as on the opening pressure. In contrast, the fissure gradually narrows under a negative opening pressure. This result is applicable to a variety of magmatic processes. First, the orientation of a volcanic fissure or a dike is such that the tensile stress component is maximized, because a fissure with this orientation becomes wider than those at other orientations, as it has the minimum value of critical time constant. Second, local irregularities in the geometrical and mechanical conditions of the magma sheet lead to significant variations in fissure width in the widening process. This nonuniform increase in width is further exaggerated by mechanical and thermal erosion of the wall rock, which makes the irregular surfaces irrecoverable. In this manner, a planar magma path tends to be split into isolated cylindrical conduits when the magma pressure is reduced and the fissure closes as a whole. Some actual fissure eruptions, such as those at Kilauea and Izu-Oshima, show evolutionary processes consistent with this prediction. Another application of the analysis is to explain how the style of a volcanic eruption is determined by the tectonics. Specifically, extensional tectonics, which gives a positive opening pressure for any magma pressure, allows magma to ascend dispersively along various paths, and produces a group of small monogenetic volcanoes. In contrast, contractional tectonics, in which new fissures can be made only for a very high magma pressure, concentrates magma into a restricted number of vents and leads to the growth of large polygenetic volcanoes.

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.

Why Does the Higashi-Izu Monogenetic Volcano Group Exist in the Izu Peninsula?: Relationships between Late Quaternary Volcanism and Tectonics in the Northern Tip of the Izu-Bonin Arc.

A synthesis of available geological, petrological, and geophysical data was made to construct a model for explaining the relationships between the late Quaternary volcanism (the Higashi-Izu monogenetic volcano group, HIMVG) and tectonics in and around the Izu Peninsula, Japan. The Izu Peninsula and adjacent areas can be divided into three tectonic blocks: the western Izu (W-Izu), eastern Izu (E-Izu), and Oshima-Zenisu blocks. The vents or intrusions of the HIMVG are formed as open cracks aligning with the nearly NW-SE azimuth of horizontal maximum compression of regional stress in the E-Izu block. This stress field is mainly controlled by the NW-SE compression caused by the strong mechanical coupling of the buoyant Izu-Bonin arc with the Japan arc. In spite of this compression, the upper crust of the E-Izu block is under weak lateral constraint on northeastward extension. Both the plate geometry involving an intra-plate fracture, the West Sagami Bay Fracture, and the existence of block rotations in the northern E-Izu block probably enable the northeastward upper crust extension in the E-Izu block, and make a monogenetic volcano field, where a new dike is formed in the upper crust at each eruptive or intrusive event.

Diversity of Parent Magmas of Higashi-Izu Monogenetic Volcano Group.

Systematic correlations are recognized between spinel and olivine in basalts and andesites, and the whole rock compositions of the Higashi-Izu monogenetic volcano group (HMVG), Izu Peninsula. Low-Cr spinel coexists with low-Ni, high-Mg olivine, both of which have crystallized from type A magmas with low Fe/Mg and Ni/Mg ratios. High-Cr spinel and high-Ni olivine have crystallized from type B magmas higher in SiO2, content, Fe/Mg and Ni/Mg ratios than the former. Mixing of type A and B magmas would yield transitional magma type AB. Type C magmas have crystallized extremely high-Cr spinel and nickeliferous olivine. The activity of type B magmas has continued since the earliest stages of the volcanic history of the HMVG, but is restricted in the northeast of the volcanic region. On the contrary, type A magmas have erupted since the last 22, 000 years and are mainly concentrated in the southwest. Type AB magmas occur in the transitional area between type A and B magmas. The secular and spatial variations in mineral and whole rock chemistry suggest that type B magmas were produced by assimilation of tonalitic upper crust.

A51 応力場と火山活動 : 伊豆半島の衝突と単成火山群

A51 応力場と火山活動 : 伊豆半島の衝突と単成火山群

P22-03 近畿北部, 神鍋単成火山群の火山層序

P22-03 近畿北部, 神鍋単成火山群の火山層序

Geology of the Kannabe Monogenetic Volcano Group, Southwest Japan

Geology of the Kannabe Monogenetic Volcano Group, Southwest Japan

東伊豆単成火山群の岩石学 捕獲結晶の意義と噴出物の時空的変化

Petrography and bulk rock chemistry of volcanic rocks and plutonic xenoliths from HigashiIzu monogenetic volcano group (HIMVG), the Izu Peninsula, central Japan, revealed a bimodal volcanic activity-high alumina basalts to calc-alkaline andesites (SiO2=48-59 wt%) vs. calcalkaline dacites (SiO2=68-73 wt%)-. The samples include 7 rock types based on bulk compositions and phenocrysts assemblages; ol + pl ± aug basalts, ol + pl ± aug andesites, ol + pl ± aug + hyp andesites and pl + ho + hyp ± (ol + aug) dacites. Two types of basalts and andesites with different modal compositions of xenocrystic minerals (corroded quartz and sodic resorbed plagioclase) are found in HIMVG, i.e. X-poor type (less than 1 vol% xenocrysts) and X-rich type (more than 1 vol% xenocrysts). The X-poor type includes mostly of basalts and shows iron enrichment on the SiO2 vs. FeO*/MgO diagram. On the other hands, the X-rich type with more than 1 vol% xenocrysts includes mostly andesites and does not show iron enrichment. In the X-poor type, modal composition of phenocrysts of olivine and chromian spinel, and bulk MgO, Ni, and Cr contents decrease abruptly with an increasing SiO2, whereas in the X-rich type, contents of these elements are variable and decrease moderately. Partialy fused granitic xenoliths from Omuroyama scoria cone contain quartz, partialy resorbed plagioclase, and glass. Bulk rock composition of these xenoliths are similar to that of dacites. In the HIMVG area, a concentric zonal pattern of rock types and phenocryst assemblages are observed. The basalts erupted at outer zone, while andesites and dacites, at inner zone. The volcanic products had been basaltic and andesitic untill the eruption of Kawagodaira, after which they became andesitic and dacitic.

尾瀬地方に分布する第四紀火山岩の岩石記載および全岩化学組成

Compositions of Quaternary volcanic rocks from Oze area, central part of Honshu arc show trends inconsistent with the general trend of the arc. Analyzed volcanic samples include those from Pre-Nikura, Ooeyama, Chouzugatama volcanoes of shield volcano group, those from monogenetic volcano group, and those from Nikura volcano of stratovolcano group, younging in this order. They are mainly andesites but minor augite basalts are also found in Pre-Nikura volcano. Dacitic andesites of stages Nikurayama and Shiraoyama of Nikura volcano contain basic inclusions and show disequilibrium features such as quartz with pyroxene-jacket, plagioclase with dusty-zone and coexistence of magnesian olivine and quartz. Most samples from Nikura volcano and monogenetic volcano group plot in the field of highalmina basalt series in the alkali vs. silica diagram. Whereas, samples of stage Nikurayama of Nikura volcano and shield volcano group plot in the field of tholeiite series. Samples of all the groups show higher K2O/Na2O ratios than those of Nasu volcanic zone to the north, reflecting a regional character. Interestingly they are richer in K2O content than those from Niigata prefecture, to the side farther from the trench. When major element compositions of lavas are estimated at 60% silica for each volcano of central part of Honshu by linear regression, they show increase of K2O content and K2O/Na2O ratio from Akagi volcano to Oze-Hiuchigatake volcano. In contrast, the Na2O and CaO contents slightly decrease in the same direction.

31B. Stratigraphy of Kannabe monogenetic volcano group.

31B. Stratigraphy of Kannabe monogenetic volcano group.

Textural and compositional evidence for magma mixing and its mechanism, Abu volcano group, southwestern Japan

Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO*/MgO vs SiO2 diagram. One trend is characterized by FeO*/MgO-enrichment with a slight increase in SiO2 content (Fe-type trend), whereas the other shows a marked SiO2-enrichment with relatively constant FeO*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.

56A. Zonation in the Higashi-Izu monogenetic volcano group(Abstracts of Papers Presented at the Spring Meeting of the Society)

56A. Zonation in the Higashi-Izu monogenetic volcano group(Abstracts of Papers Presented at the Spring Meeting of the Society)

Paleomagnetic study of the Higashi-Izu monogenetic volcano group and pyroclastic flow deposits in Kagoshima Prefecture. Paleosecular variation during the last 40,000 years in Japan.

Paleomagnetic study of the Higashi-Izu monogenetic volcano group and pyroclastic flow deposits in Kagoshima Prefecture. Paleosecular variation during the last 40,000 years in Japan.

Sr/Ca-Ba/Ca systematics in Higashi-Izu monogenetic volcano group, Izu Peninsula, Japan.

Ca, Sr and Ba contents of various volcanic rocks (basalts, andesites, dacites and a granitic xenolith) from the Higashi-Izu monogenetic volcano group, the Izu Peninsula, Japan, have been determined by an ICP-OES method. The results indicate that the monogenetic volcano group makes a Sr/Ca-Ba/Ca systematics which is similar to that defined by a stratovolcano. The SB systematics suggests that there are two different high alumina basalt magmas in the Higashi-Izu monogenetic volcano group: one (basalt I) is a primitive basalt magma derived from the mantle with rather small degree of partial melting and the other (basalt II) is a mixture of the primitive basalt magma and a cumulus phase enriched in plagioclase. The high alumina basalt series (basalt (I)-andesite-dacite series) is considered to be established, not by crystal fractionation process in magma chamber, but by assimilation process of the primary basalt magma with the granitic crust. The crust under the Izu Peninsula may have a chemical composition similar to that of the dacite(I) from Kawagodaira.

Petrology of Abu Monogenetic Volcano Group : on the significance of magma mixing(Abstracts of Papers Presented at the Spring Meeting of the Society, 1982)

Petrology of Abu Monogenetic Volcano Group : on the significance of magma mixing(Abstracts of Papers Presented at the Spring Meeting of the Society, 1982)

29. Petrology of the Higashi-Izu monogenetic volcano group(Abstracts of Papers Submitted for the Meeting of the Society)

29. Petrology of the Higashi-Izu monogenetic volcano group(Abstracts of Papers Submitted for the Meeting of the Society)