著者

加藤 祐三

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.33, no.1, pp.21-30, 1988-04-01 (Released:2018-01-15)

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Since the late of May, 1986, many pumices have drifted to the Ryukyu Islands. These pumices are characterized by their size, color and components. The pumices vary in size: some are about 5 mm and the maximum size is about 20 cm. Most of these pumices are gray and a few of them are drak- or light-gray. The pumices contain several percent of essential black xenolithes which vary in size from 1 mm to 5 mm and rarely 2 cm. The rock name of pumices is olivine-augite-bearing trachyte and that of xenolithes is forsterite chromian diopside trachybasalt. These characteristics of the pumices correspond with those of pumices erupted from Fukutoku-oka-no-ba in the north of the Mariana Islands in January 18-21, 1986. Moreover, the chemical composition of host pumice (free from xenolith), bulk composition (including xenolith) and mineral chemistry of phenocryst also correspond with those of the pumices effused from Fukutoku-oka-no-ba. Based on these facts, it is evident that the pumices drifted to the Ryukyu Islands had their origins in Fukutoku-oka-no-ba. Judging from the data concerning the current and wind around the area through Fukutoku-oka-no-ba toward the Ryukyu Islands, it is confirmed that the pumices reached the Ryukyu Islands from Fukutoku-oka-no-ba after four months under the effect of current and wind which run both to the direction of west.

著者

鈴木 隆介

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:04534360)

巻号頁・発行日

vol.13, no.2, pp.95-108, 1968

被引用文献数

1

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One of the fundamental differences between volcanic cone and non-volcanic mountains such as folded mountains is that the latter itself is a part of the earth's crust, while the former is taken as a heavy load which is laid upon the pre-existing earth's crust within a short geological time and is durable for several tens of thousands of years. In this respect, a volcanic cone resembles an ice sheet, a huge building and a large dam. It is, therefore, postulated that volcanic cone settles down by its own weight. From this point of view, characteristics of the subsidence of some strato-volcanic cones in Japan and Indonesia (Table 1) are comprehensively discussed in this paper. The results are summarized as follows. The settlement of volcanic cone causes various deformations at the foot of volcanic cone such as ring fault, thrust and the circular anticlinally uplifted ridge, all of which tend to encircle the volcanic cone settled. Based on the modes of these deformations at the foot, the settlement of volcanic cone is classified into three types ; 1) fault type, 2) fold type, and 3) mixed type. They are schematically shown in Fig. 5. Which type among the three takes place seems to depend on the nature of the basal rocks beneath the volcanic cone (Table 1 and Fig. 5). The fault type occurs in the case where Pliocene sedimentary rocks are thinner than about two hundreds meters in thickness and also most of the basal rocks are composed of Tertiary sedimentary rocks older than Pliocene. On the contrary, in the case where Pliocene sedimentary rocks are thicker, generally several hundreds to thousands meters, the fold type or the mixed type results. Magnitude of settlement is of order of one to two hundreds of meters in the depth settled. Rate of settlement of Iizuna volcano, which belongs to the fold type, is estimated to be of order of about four millimeters per year. Distance from the center of volcanic cone to the circular deformed feature (D), which is thought to show the magnitude of deformation originated by the settlement, is proportional to the relative height of volcanic cone (H), which can be taken as the substitute for the weight of volcanic cone (Fig. 6). Such relationship between D and H is also found in the case of guyot, which is surrounded by circular moat or ridge (Fig. 5), but not found in the case of collapse calderas as shown in Fig. 6.

著者

早川 由紀夫

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.28, no.1, pp.25-40, 1983-04-01 (Released:2018-01-15)

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The Hachinohe ash is a widespread pyroclastic-fall deposit erupted from the Towada caldera about 13, 000 years B. P. Along the dispersal axis, the thickness is 150cm 50km away from source, and the estimated volume is 14km3 It is composed of alternating beds of fine ash (65%) and pumice lapilli (35%). No erosional break is observed and the contacts between beds are gradational. The fine ash beds have two components : a dominate component of grain-supported accretionary lapilli and a subordinate component of fine ash-coated pumice; an ash matrix is lacking. The maximum grain size of accretionary lapilli does not decrease systematically away from source. The size population of constituent ash particles shows a small degree of fractionation with distance from source; the grain size class 1mm to 1/4mm increases while the class finer than 1/16mm decreases. Pumice beds are composed primarily of sub-angular to sub-rounded pumice fragments coated with fine ash and a subordinate amount of lithic fragments and accretionary lapilli. Maximum pumice size and maximum lithic size systematically decrease away from source. The beds show bimodal grain size distributions and contain more than 10 weight percent fine ash. An individual fine ash particle has too low a terminal velocity to fall out as a separate grain near the source area. It is certain that, throughout the Hachinohe ash eruption, fine ash continued to fall in the form of accretionary lapilli and/or attached to pumice fragments. The fine grained nature and wide dispersal indicate that the Hachinohe ash is representative of the phreatoplinian deposit formed by the interaction of water and silicic magma during explosive eruptions. At times when the proportion of erupted magma to lake water gaining access to the vent became sufficiently high, violet eruptions took place and deposited pumice fragments and accretionary lapilli simultaneously at the same place. Examples of phreatoplinian deposits are also reported from the Kutcharo and Hakone calderas, in addition to two other deposits from the Towada caldera. Such deposits are used as a possible indicator of source environment.

著者

中村 一明

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.25, no.4, pp.255-269, 1980-12-01 (Released:2018-01-15)

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Rift zones are characteristic features of Hawaiian volcanoes. They are long narrow zones of flank fissure eruptions but are distinct from ordinary flank eruption sites on stratovolcanoes in that eruptions, and therefore dike intrusions, occur repeatedly at the same general place for a long time and thus cause a considerable amount of lateral spreading. This spreading should somehow be accomodated. Moreover, the stress field should remain the same after accomodation in order for a new dike to intrude in the same orientation. The current spreading episode in Iceland (BJORNSSON et al., 1979) between North American and European plates revealed that the sequence of events in the spreading process is similar to that observed for Hawaiian volcanic activities. This implies that the process of plate separation and accretion is nothing but the activity of rift zones. Constructional plate boundaries may be regarded as composed of a chain of rift zones and associated feeding polygenetic centers. Room necessary for repeated dike intrusion is supplied in the case of spreading centers, by the lateral motion (separation) of lithosphere over asthenosphere. In the case of Hawaii, sliding of the volcanic edifice over a deep sea sediment layer may be the analogous mechanism such as appears to have occurred during the 1975 Kalapana earthquake, as studied by ANDO (1979) and FURUMOTO and KOVACH (1979). Kalapana earthquake had been anticipated by SWANSON et al. (1976) as one of the repeated steps as the east rift zone has continuously dilated. Thus, the primary cause for the long, well developed rift zones of Hawaiian volcanoes may be in the existence of thick enough oceanic sediments serving as a potential sliding plane beneath the volcanic edifices. Lack of rift zones in Galapagos shields which grew over the young ocean floor with rough topography is consistent with this view.

著者

井田 喜明 山岡 耕春 渡辺 秀文

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集

巻号頁・発行日

vol.33, pp.S307-S318, 1988

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Various features of the eruption that began on November 15, 1986 in Izu-oshima volcano are examined to infer the underground magmatic activities and the mechanism of the eruption. A massive dike intrustion that is assumed to have happened at the same time as the fissure eruption cannot explain the seismicity, deformation and other evidence consistently. Another preferable model that gives a systematic explanation of the available data is proposed as follows. A magma reservoir was situated at a depth of about 5 km below a NW (north-western) flank of Izu-Oshima volcano. This magma reservoir supplied magma to the summit crater through a well-developed vent, and caused the first summit eruption under an enhanced NW-SE compressive component of the tectonic stress. As the magma reservoir deflated with the discharge of magma in this summit eruption, a compressive stress increased in the neighboring area centered at the northern rim of the caldera. The stress finally fractured this area with intense seismicity, and produced fissures. Along these fissures, the magma that had penetrated into interstital space between rocks effused explosively with bubbling of steam. The magma had been more or less cooled and chemically differentiated in the interstitial space so that the ejecta from this fissure eruption was more felsic. The deflation of the magma reservoir due to the summit and fissure eruption resulted in a significant subsidence of the NE part of the island. Strain associated with opening of the volcanic fissures was transmitted through a strike-slip fault to the SE part of the island, and caused a local extensional stress and graben there.

著者

荒牧 重雄

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.8, no.3, pp.110-113, 1963-12-25 (Released:2018-01-15)

著者

早川 由紀夫 荒牧 重雄 高田 亮 浅岡 伸之 金子 隆之 津久井 雅志 岸 智 山下 茂 野原 状 平田 由紀子 小山 真人 白尾 元理 川辺 禎久

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.32, no.3, pp.259-262, 1987-10-15 (Released:2018-01-15)

著者

中村 一明

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:04534360)

巻号頁・発行日

vol.20, pp.229-240, 1975

被引用文献数

14

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Volcanoes are generally classified into monogenetic and polygenetic types. Monogenetic volcanoes erupt only once to form smaller volcanoes, such as maars, pyroclastic cones and lava domes. Polygenetic volcanoes erupt repeatedly from the same general vents (summit or main crater) for up to 10⁵ years to form larger volcanoes such as strato-volcanoes (composite volcanoes of Macdonald, 1972) and shield volcanoes of Hawaiian type. Monogenetic volcanoes tend to occur in clusters as flank and post-caldera cones. Some of the clusters are however, independent of polygenetic volcanoes and appear to be equivalent to them. The essential part of the conduit of a monogenetic volcano is inferred to be a simple dike, intruded into a newly formed crack, whereas a long endured pipe-shaped conduit may exist under a polygenetic volcano. The common occurrence of xenoliths in the eruptive products of monogenetic volcanoes may be related to this difference. Various lines of evidence, indicating the existence, depth, shape, volume and internal structure, of magma reservoirs are tabulated. A shallow magma reservoir appears to exist beneath polygenetic volcanoes with one to one correspondence, which is not the case for monogenetic volcanoes. Most flank volcanoes are monogenetic, thus indicating dikes within the polygenetic volcanic edifice. Dike formation is understood as a magma version of hydraulic fracturing. For the dike to intrude and propagate, would require either the increase of differential stress due to a decrease of minimum compression or increase of pore pressure over the sum of the minimum compression and the tensile strength of the rocks. Earthquakes are understood as the generation of elastic waves associated with an acute release of tectonic stress due to faulting. Accumulation of tectonic stress and strain prior to earthquakes is, then, a necessary part of earthquake phenomena in a broad sense, as well as their release after the event. Based on the above-stated understanding, possible mechanical correlations between volcanic eruptions and earthquake occurrences have been studied. Contractional strain around the magma reservoir can cause the squeezing up of magma within an open conduit causing a summit eruption on the one hand, and dike formation resulting in a flank eruption through the increase of pore pressure, on the other. Second boiling triggered by both the magmatic pressure decrease caused by dilatational strain and the dynamic excitation due to seismic waves might have the same effect as contraction. Decrease of minimum compression causing the increase of differential stress leading to dike formation will also contribute to the liklihood of flank eruptions. Both volcanic eruptions and earthquake occurrences can precede each other depending on geographical location in terms of faulting-related stress-strain changes which are calculated by the fault model of earthquakes. Actual possible examples of volcanic eruptions and earthquakes which are allegedly mechanically related are given. In order to demonstrate which mechanism is responsible for the correlation of the two phenomena, continuous strain measurement on and around volcanoes is necessary together with the observation of changes in the level of magma in crater bottoms.

著者

金 允圭 吉田 武義 李 大声

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.31, no.2, pp.144-145, 1986-07-01 (Released:2018-02-13)

著者

中村 一明

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:04534360)

巻号頁・発行日

vol.16, no.2, pp.63-71, 1971

被引用文献数

1 11

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A model is presented which explains the temporal relation between an eruption and a succeeding earthquake, taking a basaltic stratovolcano, Izu-Oshima volcano, as an example. In the model, volcano is assumed to consist of an underground reservoir and a long pipe connecting the reservoir to the surface. As the compressional crustal strain is gradually stored toward the earthquakes to occur, the volcano, located near the potential fault, is also deformed and contracts to some degree. Then the magma in the reservoir is squeezed up through the pipe. The rise of the magmatic head above a certain level in the pipe causes an eruption, which, once started, may proceed as a self-moving machine. Later, when the earthquake occurs, the strain that squeezed up the magma is released. And the head of the magma falls off resulting in the end of the eruption, in case it has still continued. The bottom of the summit crater of Oshima volcano showed remarkable rise and fall in this century amounting to some 400 meters. The bottom can be regarded as the head of the magma column, since red hot glow was frequently observed during the period. There were two maxima of the height of the bottom, January 1923 and June 1951. Shortly after each of the maxima, occurred great earthquakes with magnitude larger than 8, September 1923 and November 1953 along the Sagami trough which runs some 20km northeast of the volcano toward northwest, branching off from the Japan trench. The area including the volcano has been under compressional tectonic stress with the maximum pressure axis in a horizontal N30°W direction, during at least these hundreds of thousand years. On the other hand, recent fault-model studies of the 1923 earthquake indicate that the fault trace of the earthquake almost coincides with the Sagami trough and that the slip vector of the southwestern block, in which the volcano is located, is toward northwest almost horizontal with slight down going component. This tectonic situation implies that the strain which had been accumulated prior to the occurrence of the great earthquakes along the Sagami trough was caused by the same origin, probably the motion of the Philippine sea plate against the Japanese plate, with what has produced the compressive stress field of the volcanic area. The model appears to be successfully applied for the interpretation of the relation between the eruption of Akita-Komagatake volcano which started on September 17, 1970 and the October 16 earthquake with the magnitude of 6.2 at the epicentral distance of 55km. The frequency of explosion discontinuously dropped down to one half or lower level, three days after the earthquake together with the cessation of Strombolian type of eruption. The preliminary mechanism study of the earthquake showed that there is some component of thrust motion indicating the accumulation of contractional strain prior to the earthquake. The volcano to which the proposed model is applied is thus able to be regarded as a sensitive natural indicator of tectonic crustal strain, and also at the same time as being in a near critical condition ready to erupt.

著者

中村 一明

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:04534360)

巻号頁・発行日

vol.14, no.1, pp.8-20, 1969

被引用文献数

2

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Examples are presented in which arrangement of lateral and post-caldera cones indicates the probable regional stress field in the late Quaternary. Izu-Oshima, Hakone and Fuji are among the larger stratovolcanoes to the southwest of Tokyo, the former two having collapse calderas on their summit. Parasitic and post-caldera cones and craters of the three volcanoes are following a trend of similar direction, some of them being produced by fissure eruptions on their flank. The zones trend in the direction of about N 35° W at Fuji, N 45° W at Hakone and N 30° W at Oshima. Dikes are also found in these zones running mostly parallel to them. The trend of fissures of fissure eruptions on the flanks of Fuji and Oshima is also similar to the zones of recent activity. Because sites of flank eruption are regarded as points where radially formed dikes around the central magma column have penetrated the flank, the above described distribution of craters in these volcanoes would indicate a concentration of radial dikes in a specific direction at the three adjacent volcanoes. Considering that dikes are fossil tension cracks formed perpendicularly to the axis of the minimum principal compressional stress, the concentration of parasitic craters can be explained by the stress field caused by the pressure increase in the magma column superimposed on the preexisting regional field with the maximum principal stress axis in a NW direction. The nearly identical, preexisting stress field in the three adjacent volcanoes suggests that the field is part of a more regional one including the area of the volcanoes. This suggestion is strongly supported by the presence of active, conjugate strike-slip faults in the same general area, i.e. on the Izu-peninsula. The maximum compressonal axis indicated by the faults is again in a direction of about N 30° W and oriented horizontally. The last movement of the fault system was observed in 1930, at the time of the Kita-Izu earthquake, whose magnitude was 7.0.

著者

巽 好幸 柵山 雅則 福山 博之 久城 育夫

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.27, no.1, pp.45-65, 1982-04-30 (Released:2018-01-15)

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The compositions of the primary tholeiitic, high-alumina and alkali-olivine basalt magmas (THB, HAB and AOB magmas, respectively) which are derived directly from the upper mantle beneath the volcanic arcs, are obtained by calculating the average compositions of liquid in equilibrium with the mantle peridotite, which can produce basalts in NE Japan arc through olivine maximum fractionation. Anhydrous high-pressure melting experiments on these three basalts indicate that the AOB and HAB magmas coexist with olivine, orthopyroxene and clinopyroxene at 1360℃ and 17.5 kbar and at 1340℃ and 15 kbar, respectively. The THB magma, on the other hand, coexists with olivine and orthopyroxene at 1320℃ and 11 kbar. The volcanic arc magmas are believed to contain significant amounts of water which affect the P-T conditions of the phase equilibria at high temperatures and pressures. However, the detailed petrographic studies on the rock suites in volcanic arcs revealed that the island arc primary basalt magmas contain water not more than 3 wt.% at generation in the upper mantle. Combining this with the experimental results, the THB, HAB and AOB magmas are suggested to segregate from the mantle at temperatures of about 1300℃ and at pressures of 11 kbar (THB), 16 kbar (HAB) and 20 kbar (AOB), respectively. As the temperatures of segregation of the magmas given above are too high for a stable mantle geotherm, the mantle diapir is the most probable mechanism for magma generation under the volcanic arcs. Due to the heat of formation of liquid in the diapir, the temperature of the diapiric mantle must be higher at deeper levels. The required temperature of the upper mantle is 1400℃ at a certain depth between the descending slab and depth of approximately 70km.

著者

中村 一明

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.17, no.1, pp.33, 1972-06-01 (Released:2018-01-15)

著者

関谷 慱

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.12, no.2, pp.90, 1967-08-01 (Released:2018-01-15)

著者

荒牧 重雄 藤井 敏嗣

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.33, no.SPCL, pp.S297-S306, 1988-06-30 (Released:2018-01-15)

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Detailed characterization of the whole rock composition of the ejecta of the 1986-1987 eruption of Izu-Oshima volcano by the XRF technique (FUJII et al., 1988) clearly indicates that the ejecta from the central crater of Miharayama (Crater A) are different from those erupted from the fissures (Fissures B and C) on the caldera floor and on the outer slope of the main stratovolcano. This suggests that the conduits which led the A and B, C magmas to the surface were separated physically from each other down to a certain depth. The ejecta from A crater resemble closely to those erupted during the past 1300 years (Y magmas) while the ejecta from B and C fissures are unique in composition among the Izu-Oshima magmas. The A and Y magmas are Fe-enriched island arc type tholeiites that must have been derived from the primary tholeiite magmas through crystal fractionation of olivine, pyroxenes and plagioclase. The B and C magmas can be derived simply through crystal fractionation of A or Y magmas leading to Fe-enriched basaltic andesites, andesites and dacites. This strongly suggests that an isolated pocket of magma starting with a composition of Y underwent strong fractionation to produce volatile enriched ferroandesitic magma. This body of magma was probably activated by the sudden depressurization caused by shattering and fissuring of the crust to form a 1500 m high fire fountains and to produce a sub-Plinian scoria fall deposit. The vent of A crater must have been stable at least during the last 1300 years and directly, or through the subsidiary magma chamber(s), connected to the main chamber above the Moho, where an extensive crystal fractionation has been taking place to produce Y magma from the parent tholeiitic magma. Marked ground depression and extension and migration of seismicity observed during the eruption suggest a possibility that a substantial amount of additional magma was intruded to form a NW-SE trending dike during the peak phase of the eruption. This is in harmony with the Nakamura’s model of the volcano with the NW-SE trending dike swarm which is controlled by the regional compressional stress field. However, gravity and some other grophysical data suggest that the deformation could have been the result of underground cracking without magma injection. Our model is not conclusive on this matter and the expectation that this eruption will eventually lead to the large-scale activity that has been recurring in every 130±50 years is yet to be tested.

著者

八木 健三

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:04534360)

巻号頁・発行日

vol.16, no.1, pp.28-35, 1971

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Calderas are classified into salic and mafic types according to their association with either salic pyroclastic flows or mafic lava flows. The salic type corresponds to the low gravity anomaly type, and the mafic type to the high gravity anomaly type of YOKOYAMA. When the longer diameters of the two types of calderas in Japan and the world are plotted against the SiO₂ contents of the pyroclastic flows, or lava flows, they are distributed in diverging U-shaped areas, i.e., the size of salic type calderas becomes larger with increasing SiO₂ content, and that of mafic type calderas increases with decreasing SiO₂ content. The colossal amounts of salic magmas and their high explosivity explain the larger size of the more salic type calderas, while the lower viscosity of the more mafic magmas explains the larger size of the more mafic type calderas. The relation between the diameters and the depths of the salic type calderas of Japan is examined. For smaller calderas, less than 7 km in diameter, there is some linear relation between the two parameters, but for the larger ones the depths are nearly constant at 500~700 m, irrespective of their diameters. Comparison is made with artificial explosion and lunar craters, and their genesis is discussed.

著者

宇井 忠英 柴橋 敬一

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.20, no.2, pp.51-64, 1975-08-01 (Released:2018-01-15)

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Volcanic activity at the snow-capped summit of Mt. Chokai was first noticed by the captain of scheduled airline on March 1, 1974. The activity began with swarm of volcanic earthquakes, succeeded by fumarolic activity and finally explosion took place and a few craters were formed at eastern (late February-early March) and western (late April) foot of 1801 lava dome (Shinzan). The ejecta were exclusively fine-grained air-fall ash and accidental blocks. The blocks, formed the mud flow mixed with melted snow, rushed down the slope of volcanic edifice. Essential materials, such as bombs, air-fall scoria, or lava flows were not erupted. Rapid melting of snow was supposed to have been triggered by the formation of fumaroles caused by ascent of hot magma and associated juvenile gas. Thermal energy consumed for melting and evaporating snow is calculated as 3×1021 ergs. Total volume of mud-flow deposit is around 3×104 cubic meters, and that of air-fall ash is an order of 105 cubic meters. The entire area which showed thermal activity is approximately 700×200 meters, elongated in east-west direction. Distribution of earthquake foci was also trending east-west just passing the prehistoric summit and parasitic vents. Direction of vent alignments is the same for the most volcanoes in northeastern Japan, and is supposed to reflect regional stress field.

著者

水上 武

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集

巻号頁・発行日

vol.13, no.1, 1968

著者

久野 久

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集

巻号頁・発行日

vol.12, no.2, 1967

著者

久野 久

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集

巻号頁・発行日

vol.8, no.1, 1963