著者

山元 孝広

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.51, no.4, pp.257-271, 2006-08-31

被引用文献数

3

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The Sashikiji 2 (S_2) Member in the products of Izu-Oshima volcano was formed by an explosive eruption accompanied with caldera depression. This member is characterized by breccia called as a "low-temperature pyroclastic flow deposit". In this paper, the S_2 breccia is re-examined based on stratigraphy, grain fabrics, grain-size distributions and modal compositions. The S_2 Member is divided into six units from S_2-a to S_2-f in ascending order. The S_2-a unit consists of scoria, bomb and aa lava flows from flank fissures. The S_2-b unit is made up of well-bedded ash and fine-lapilli from the summit. The S_2-c unit is composed of matrix-supported breccia, locally filling valley bottoms and containing abundant deformed soil fragments and woods. The S_2-d unit consists of reverse to normal grading, clast-supported breccia with ash matrix, covering topographic relief in the whole island. The S_2-e unit is composed of dune- to parallel-bedded lapilli and ash in the proximal facies. The S_2-f unit is clast-supported breccia with and without ash matrix. New ^<14>C ages of wood fragments in the S_2 Member have been determined as about Cal AD 340. Although the S_2-c and -d units are previously interpreted to the low-temperature pyroclastic flow deposit, these units are quite different in sedimentological features as follows. The grain fabric measurements have revealed that the S_2-d unit has a-type imbrication showing the longest axis of grains parallel to the flow direction. On the other hand, the S_2-c has random fabric of grains. The grain size distribution of the S_2-d unit shows a bimodal nature having subpopulations at phi -1.0 to 1.0 and coarser than phi -2.5. The bimodal nature and a-type imbrication suggest that the two transport processes overlap; the load of a turbulent suspension is not all in true suspension as the coarser population may travel in a cast-dispersion mass flow. The S_2-c unit shows a polymodal grain size distribution with multi subpopulations from coarse to fine. The poor sorting, massive appearance, valley-confined distribution, and random grain fabric of the S_2-c unit are characteristic of deposition from a cohesive flow without formation of traction-related bedforms or sorting of different grain sizes by turbulence. The modal composition measurements have indicated that the S_2-c and -d units lack essential scoriceous or glassy fragments. This evidence indicates that both units are derived from steam explosions due to outburst of highly-pressurized geothermal fluid within the edifice. The S_2-c unit was plausibly generated by remobilization of phreatic debris around the summit caused by ejection of condensed water from a plume or heavy rainfall. The S_2-d unit was a pyroclastic density current deposit resulted from collapse of a highly-discharged phreatic plume. Estimated velocities of the current are 150 to 30m/s based on suspended grain sizes.

著者

高橋 正樹

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.39, no.4, pp.191-206, 1994-09-20

参考文献数

66

被引用文献数

6

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Late Quaternary polygenetic volcanoes in Japanese islands can be classified into two types based on the distance of vent migration : the stable and unstable vent types. The eruptive center of unstable vent type has migrated longer distance (exceeding 1.5 km within 10,000 years) than that of the stable one. The stable vent type constructs a symmetrical cone, contrarily the unstable vent type shows an elongated cluster of small stratovolcanoes. The unstable vent type is generally present in the regional or local extensional stress field, because independent open fractures are easily developed under the extensional stress regime ; the occurence of each type also depends on the crustal strain rate and/or strength of the basement. The eruption rate of stable vent type is larger than that of the unstable one ; it is probably because the high efficiency of magma transport to the surface can be realized by the stable vent.

著者

田沢 堅太郎

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.36, no.4, pp.419-430, 1991-12-25

被引用文献数

1

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Studies on coastal deposits and geomorphology of Oshima island have revealed some uplifts and subsidences associated with the volcanic activity of Oshima volcano. Uplifts and subsidences of the island are inferred by subtracting the sea levels at the time from the heights of the past sea surface recorded on coastal deposits. Two series of large-scale eruptions starting at the ages of O_<41> (7,780±160 yB.P.) and S_2 (1,360±40 yB.P.) have similar characteristics as follows. (1) At the ages of O_<42> (8,200-7,800 yE.P.) and O_1(1,500-1,400 yE.P.) just before the two series of large-scale eruptions, lagoons were formed along the west coast of Oshima island, suggesting the subsidence of the island. (2) At the beginning in each series, a large-scale steam explosion occurred at the summit crater to generate a debris flow. (3) In each series, the volume of the eruptive materials rapidly increased at the beginnings and gradually decreased after those times. (4) Volcanic activities were explosive at the beginnings in the series and gradually changed to effusive. (5) Oshima island was remarkably and rapidly uplifted at the beginnings of the two series of the large-scale eruptions and was gradually subsided. (6) In the stage of uplifling of the island, the scale of eruptions had been enlarging, and in that of subsiding, the scale of eruptions had been getting smaller.

著者

横山 泉

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.58, no.1, pp.91-102, 2013-03-29

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桜島火山には多くの寄生火口が地質及び地形の面から認められている.また,その噴火史において寄生火口の噴火がしばしば記録されている.寄生火口の分布パターンを調べるのに,色んな方法が提案されてきたが,ここでは,火山中心から半径方向の密度分布(km^2当たり)を調べた.一般論として,火山の下に点力源を仮定して,地表面で直応力の分布と水平差応力の分布を考え,岩石の強度を考慮すると,寄生火口の生ずる地点の見当がつく.それは,地表で力源を伏角51°で見る山腹の地点で,火山中心に対して対称な2点である.多くの火山では,対で生ずることは少ない.桜島火山の寄生火口の火道が主火道から分岐する深さを求めると,深さが3kmと10kmの2群となる.これらの深さと既に推定されているマグマ溜まりとの関連について触れた.桜島火山の歴史時代の(1471年以降の3)回の大噴火は総て,山頂に対称的に対をなして形成された.このことは力学的には正常であるが,事例としては例外である.ただ,分岐の深さが10kmの場合(1779〜80年噴火),山頂に対して対称位置に寄生火口が生じていない.この例外的な事例は,桜島地下で,浅部と深部で地殻構造が異なることに起因するのかも知れない.更に,寄生火口が再噴火しない機構について仮説を述べた.次の桜島火山の噴火地点は何処であろうか.山頂火口か,それでなければ,寄生噴火である.その場所は統計的に,山体の中心軸から約2.5km或は8.5kmの円環上で,かって噴火したことのない地点が考えられる.寄生火口の火道が主火道から分岐する機構が未解明である限り,これ以上のことは言えない.

著者

鹿野 和彦 大口 健志 林 信太郎 宇都 浩三 檀原 徹

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.47, no.5, pp.373-396, 2002

参考文献数

82

被引用文献数

3

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An alkali-rhyolite tuff-ring is newly identified in the western end of the Oga Peninsula and named as Toga volcano in this paper. The existence of this maar-type volcano at the Toga Bay has been suspected for a long time because of the elliptical embayment reminiscent of a maar and the distribution of the Toga Pumice localized along the bay coast. The Toga Pumice is cornposed mainly of pumice and non- to poorly-vesicular glass shards, but many pumices of lapilli size are rounded and fines are poor giving a sandy epiclastic appearance to the deposit. In our latest survey along the bay coast, the Toga Pumice is found to be in direct contact with the basement rocks. The contact steeply inclines at 40-50° and envelopes an elliptical area 2.0 km×2.4 km covering the bay and bay coast to form a funnel-shape structure. The basement rocks at the contact are brecciated to a depth of several tens of centimeters, or collapsed into fragments to be contained in the Toga Pumice. The beds inside the inferred crater incline toward the center of the crater at 10-30° or much smaller angles, presumably reflecting a shallow concave structure infilling the more steeply sided crater. The deposit is thinly to thickly bedded to be parallel- to wavy- or cross-stratified, inversely to normally graded with many furrows, rip-up clasts and load casts, and is sorted as well as fines-depleted pyroclastic flow deposits and/or pyroclastic surge deposits. These features are characterisitic to turbidites and indicate the place of emplacement was filled with water. Constituent glass shards are, however, commonly platy or blocky and likely to be phreatomagmatic in origin, and pumice lapilli are interpreted to have been originally angular but rounded by repeated entrainment and abrasion in multiple phreatomagmatic eruptions and succeeding emplacement in the crater lake. A pyroclastic surge deposit (Oga Pumice Tuff) correlative in composition and age to the Toga Pumice occurs at Anden and Wakimoto, 11 km and 15 km east of Toga, respectively. The juvenile pumice lapilli are angular to subrounded, in contrast with the pumice lapilli of the Toga Pumice.

著者

横山 勝三

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.45, no.4, pp.209-216, 2000-08-28

被引用文献数

1

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The Ito ignimbrite, the product of a big eruption at Aira caldera about 24,500 y B. P., is distributed very extensively around the caldera in south Kyushu. The region within about 70 km from the center of the caldera was the previously known extent of distribution of the ignimbrite. Recent field research revealed, however, Iocal but extensively-scattered distribution of the ignimbrite in many places beyond the previously known extent of distribution northwest to northeast of the caldera. The farthest site of distribution of the ignimbrite is located about 90 km north of the caldera, indicating that the Ito pyroclastic flow originally spread at least 20 km farther than the previously known extent. The ignimbrite in the remote region is characteristically fine-grained compared with the one near the source. Both pumice and lithic fragments in the ignimbrite decrease, as a whole, in size with distance from source. However, the size of lithic fragments increases in the mountainous area beyond 70 km from source. This is because lithic fragments were incorporated into the pyroclastic flow from local land surface probably due to increased turbulence of pyroclastic flow during the passage on the irregular basal relief. The most remote ignimbrite, at a site 90 km from source, attains to about 35 m in thickness and contains abundant lithics of 5-15 cm in diameter, suggesting that the Ito pyroclastic flow spread farther beyond.

著者

津久井 雅志 中野 俊 齋藤 公一滝

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.53, no.2, pp.79-91, 2008-04-30

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Based on geological and archaeological data as well as historic documents, we review eruptions and earthquakes which have occurred during the 9th century in central and east Japan. The results reconfirm vigorous activities on Izu Arc, Izu-Oshima(〜838AD < N_3, N_2, N_1 < 886AD), Niijima(〜857AD and 886AD), Kozushima(838AD), Miyakejima(832AD and 850AD), and at Fuji volcano(800AD, 838AD < < 864AD, 864AD) during the 9th century. Beside these events, a big eruption of Niigata Yakeyama volcano had likely occurred in 887AD. Chokai volcano also erupted in 871AD, and 810-823AD. Collapse of Yatsugatake volcano took place in 887, probably invoked by a strong shock in 887. In addition, earthquakes with a magnitude from 7 to 8 had taken place along the Itoigawa-Shizuoka active fault system (in 841 or 762AD), Nagano fault system(887AD), Echigo plains(863AD), Shonai plains(850AD), Akita plains(830AD) and Nankai trough(887AD). As a result, we can point out a linkage of big eruptions and seismic activities in the 9th century over 800km long crossing Japan Arc. Geologically this seems to be a surface expression of East-West compression along eastern margin of Amurian Plate over 800km which was driven by the eastward motion of the plate. It is noteworthy to mention that the very similar volcanic and seismic activities have been occurring in the last 50 years to those happened in the ninth century, in their source areas and manners.

著者

野上 健治 吉田 稔 小坂 丈予

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.38, no.3, pp.71-77, 1993-08-15

参考文献数

10

被引用文献数

6

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薩摩硫黄島東温泉沿岸では温泉水と海水とが様々な比率で混合した結果,極微細なSiO_2-Al_2O_3-Fe_2O_3-H_2O系の低結晶質沈殿物が生成しており,海面が様々な色調を呈している.これらの変色海水について,母液及び沈殿物のSi-Fe-Al_3成分の化学組成及び生成条件,特にpHとの関係について検討を行った.その結果,沈殿物の化学組成は温泉水と海水との混合溶液のpHに強く依存し,pHが2前後では沈殿物中のFeの割合は低いが,pHが3〜5の範囲ではFeの割合が相対的に高くなる.更にpHが上昇するとAlの割合が相対的に高くなる.また変色海水の色調はpHが2前後の時は透明から乳白色であるが,pHが3〜5の時には黄褐色である.更にpHが上昇すると色調は再び白色系になり,沈殿物中のFeの割合によって色調が変化する.これらに対して,各採取地点における沈殿物と母液との混合物の化学組成は東温泉から湧出している温泉水のそれと殆ど同じであり,見かけ上沈殿の生成過程においてSi,Fe,Alの3成分は分別していない.

著者

井口 正人

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.39, no.2, pp.49-67, 1994-05-20

参考文献数

27

被引用文献数

10

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代表的な安山岩質火山である桜島の高周波B型地震,低周波B型地震および爆発地震を,波動の特性,震源位置およびモーメント加速度・テンソルについてA型地震と比較検討することにより,火山性地震の発生機構を論じ,それを火山特有の構造である火道と関連させて考察した.高周波B型地震,低周波B型地震および爆発地震は,火口直下の半径約200mの円筒状の領域に分布し,上下方向のダイポール成分が卓越する体積膨張型の力源をもつことから,マグマに満たされた火道内において発生していると推定され,その原因として,火道に沿ったガス相の膨張が考えられる.A型地震は,これら3種類の地震の震源域の周囲に分布し,ダブル・カップル成分が大きいことから,火道周辺の岩石のせん断破壊によって発生すると推定される.高周波B型地震,低周波B型地震および爆発地震のスペクトルの相違は,震源過程の違いによるものと考えられる.これらの地震発生に伴う地盤変動,表面現象および地震発生の時系列から考えると,震源過程の相違は火道上部の閉塞状態やマグマの物性の違いに起因していると推定される.

著者

小山 真人 早川 由紀夫 新井 房夫

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.40, no.3, pp.191-209, 1995-07-31

被引用文献数

4

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We reveal the eruptive history of the Higashi Izu monogenetic volcano field by tephrochronology and loess-chronometry and describe mainly volcanoes older than 32,000 years ago. The eruption of the volcano field started at about 150,000 years ago. Twelve tephra from volcanoes outside the Izu Peninsula were used as key beds. We found four eruptive fissures ; Takatsukayama-Sukumoyama, Kadono-Umenokidaira, Koike-Oike, and Numanokawa fissures, along each of which two to five monogenetic volcanoes erupted simultaneously. Interbedding of a distal wide spread tephra between eruptive deposits proper to the volcano field (the KIP-4 pumice between the Takatsukayama-Sukumoyama tephra, and the Hakone Da-4 pumice between the Kadono-Umenokidaira tephra) means that an eruption of a volcano outside the Izu Peninsula occurred simultaneously with the eruption of the volcano field. Vent locations of the volcano field were limited to the northern half of the present distribution during 80,000-150,000 years ago and were expanded in the later stage. Average frequency of eruption in the volcano field is calculated to be one/7,900 years for the period of 40,000-150,000 years ago, and one/2,500 years for the past 40,000 years. Average discharge rate of magma is calculated to be 0.64 kg/s before 40,000 years ago, and 2.5 kg/s for the past 40,000 years. Thus, both the average frequency of eruption and the magma discharge rate are higher for the past 40,000 years than those in the earlier stage. Many of the eruptions of andesite magma occurred later than 14,500 years ago, and the ones of dacite-rhyolite magma occurred only for the past 3,200 years. The cumulative pattern of the discharge mass of magma from the whole volcano field against time shows no clear predictability.

著者

吉永 秀一郎

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.40, no.3, pp.153-166, 1995-07-31

被引用文献数

11

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Successive terrestrial coverbeds called "volcanic ash soils" are widely distributed in Japan. In this paper, the origin of parent material of volcanic ash soils is discussed from the stratigraphy and the mineralogy of volcanic ash soils to clarify the accumulation of volcanic ash soils. In the Hakkoda Mountains and in the southern part of Tokachi Plain, thickness of tephra interbeds recognized within peaty soils are almost the same as those within the volcanic ash soils. No more distinct tephra interbed is observed in the volcanic ash soils with the exception of the marker tephra interbeds. This fact indicates that the accumulation of the volcanic ash soils have occurring through the periods without tephra deposition. Moreover, the existence of soil developed on the latest Hokkaido-Komagatake tephra of 1929, also suggests that the accumulation of volcanic ash soils is almost independent of the tephra deposition. Mineralogical composition of the fraction coarser than 0.02 mm in the volcanic ash soils are mainly derived from the underlying tephra interbeds. On the other hand, the finer fraction contains amorphous clay minerals and non-volcanogeneous materials such as quartz and illite. The former is weathering products of tephra and the latter would be derived from eolian dust. In particular, several studies on oxygen isotope composition have revealed that the origin of fine quartz is the eolian dust. Accumulation rate of fine materials in the volcanic ash soils ranges from 10^<-1> to 10^0 mg cm^<-2> y^<-1>. It agrees well with those of paleosols intercalated into sand dunes along the coast of the Japan Sea, of pelagic sediment of the Japan Sea, and of eolian dust fall calculated from atmospheric dust density. From the evidence mentioned above, parent material of volcanic ash soils is originated from the reworked materials of tephra with the fine materials of the eolian dust.

著者

井村 隆介

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.40, no.3, pp.119-131, 1995-07-31

被引用文献数

5

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Sedimentological study of the recent deposit of Sakurajima Volcano has exposed some properties of the deposit accumulated by small-scale eruptions. Fall-out deposit of long-term such eruptive activity occur in any nearly concentric circular area around the source. Change of the small-scale eruptive activity manifests as the facies, bulk density and carbon content of the deposit, because such characters of the deposit are reflections of the accumulation rate of pyroclastics. Gray laminated or massive volcanic sand layer indicates a period of highly active volcanism ; brownish loamy volcanic sand layer a less active one and brownish loam or humic soil a dormant or inactive one. The duration of those periods can be estimated from the facies, thickness and bulk density of the deposit and recent fall-out ash data.

著者

大木 章一 村上 亮 渡辺 信之 浦部 ぼくろう 宮脇 正典

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.50, no.5, pp.401-410, 2005-10-31

被引用文献数

11

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The Asama volcano (elevation 2568m), central Japan, erupted on September 1, 2004. Since then thick volcanic fumes prevented monitoring of the topography on the summit crater floor by standard optical methods. To detect geomorphic changes of the volcano, Airborne Synthetic Aperture Radar (AirSAR) measurements including interferometry were repeatedly carried out on September 16, October 22, December 15, 2004 and March 10, 2005. The comparison of AirSAR result in September with a digital elevation model (DEM) by laser profiling measurements in October 2003 revealed that a pancake shape lava mound (volume=0.9×10^6m^3) was formed on the floor. The subsequent AirSAR measurement on October 22 indicated that the pancake shape mound had disappeared and a small pit hole was formed. The volume of ejecta amounted to 2.1×10^6m^3. The AirSAR results in December 2004 and March 2005 suggested that the shape of the crater floor stayed generally unchanged during that time window. From these, we infer that lava extrusion during the 2004 Asama eruption occurred within a short period around September and October.

著者

宮縁 育夫 星住 英夫 高田 英樹 渡辺 一徳 徐 勝

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.48, no.2, pp.195-214, 2003-05-20

被引用文献数

12

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Aso central cones located within Aso caldera, central Kyushu, southwestern Japan, initiated their activity soon after the formation of the caldera (ca. 90 ka). The cones have produced voluminous airfall tephra layers and lava flows. Most of the tephra layers distributed in and around Aso caldera are andesite to basaltic-andesite scoria-fall and ash-fall deposits. Their stratigraphy is very complicated because it is difficult to distinguish between scoria-fall layers in the field. However, dacite to rhyolite pumice-fall deposits from some central cones interbedded between the tephra layers are very useful to correlate stratigraphic units at separated localities. Therefore, we used the pumice-fall deposits in order to construct the tephrostratigraphy and eruptive history of Aso central cones during the past 90,000 years. Thirty-six pumice-fall deposits were identified including eleven major key beds. In ascending order they are Nojiri pumice (NjP), Ogashiwa pumice (OgP), Yamasaki pumice 5 (YmP5), Sasakura pumice 2 (SsP2) and 1 (SsP1), Aso central cone pumice 6 to 3 (ACP6-ACP3), Kusasenrigahama pumice (Kpfa) and Aso central cone pumice 1 (ACP1). Phenocrystic minerals of most pumice are plagioclase, ortho- and clino- pyroxene and magnetite, but NjP, ACP5, ACP3 and ACP1 include biotite, and NjP and SsP2 contains hornblende phenocrysts. On the basis of several ^<14>C ages of buried soils just below pumice (above Kpfa) and stratigraphic position eruption ages for the eleven major pumice are estimated as follows: NjP (85 ka), OgP (80 ka), YmP5 (69 ka), SsP2 (57 ka), SsP1 (56 ka), ACP6 (52 ka), ACP5 (45 ka), ACP4 (40 ka), ACP3 (39 ka), Kpfa (31 ka) and ACP1 (4ka). During the past 90,000 years Aso central cones produced pumice-fall deposits at an interval of about 2,500 years. Many of the sources of the pumice appear now buried under the present Aso central cones.

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 (ISSN:04534360)

巻号頁・発行日

vol.40, pp.S1-S15, 1995-12-25

被引用文献数

2

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Age of a tephra can be determined by simple stratigraphy, if adequate number of time-markers are provided. Eleven master tephras are chosen as the time-markers for the last one million years. They are Kikai-Akahoya (7.330 ka), Aira-Tanzawa (26.00 ka), Daisen-Kurayoshi (50.00 ka), Aso-4 (87.00 ka), Ata-Torihama (250.0 ka), Kakuto (340.0 ka), Suiendani-TE5 (420.0 ka), Kobayashi-Sakura (540.0 ka), Kaisyo-Toriitoge (650.0 ka), Shishimuta-Azuki (870.0 ka), and Shishimuta-Pink (1000 ka). The present earth surface and Bruhnes/Matuyama boundary (780.0 ka) play a same role as master tephras. Ages of some master tephras are assigned rather arbitrarily, however, it is productive to affix them once to a specific value. A tephra sandwiched between two master tephras is afforded its age by interpolating the thicknesses of loess between them. This technique, loess-chronometry, has the advantage of ability to measure an interval of tens to thousands years in the geologic past, over radiometric dating. More than 900 tephras are presently recorded and linked each other in a computer database including name, source volcano, age, magnitude, stratigraphy, and remarks. An updated version is listed in WWW at "http://www.la.gunma-u.ac.jp/〜hayakawa/English.html".

著者

中川 光弘 丸山 裕則 船山 淳

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.40, no.1, pp.13-31, 1995-03-10

被引用文献数

10

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Hokkaido island is situated at the junction of northeastern Japan and Kuril arcs. Recent K-Ar age dating of Cenozoic volcanics in the island revealed that the distribution of Quaternary volcanoes (<1.7Ma) is remarkably different from previously accepted one. On the basis of the newly established distribution of the volcanoes, spatial variations in SiO_2-normalized major element values (SiO_2=60%) of Quaternary andesitic rocks (SiO_2=56-64wt%) are investigated. The Na_2O, CaO and P_2O_5 values vary systematically in the across-arc direction irrespective of the tectonic setting and/or the geological structure. On the contrary, the variations in FeO, Al_2O_3 and K_2O values seem to be largely correlated with the tectonic setting and the Bouguer anomaly. The distribution of the volcanoes and the mode of spatial variations in major element chemistry suggest the presence of three Quatemary volcanic fields, the southwestern Hokkaido (SWH), Taisetsu-Tokachi-Shikaribetsu (TTS) and Akan-Shiretoko (AKS) regions. Modes of the spatial variations in normalized major element values in SWH and AKS regions resemble each other and are nearly the same as those of northern Honshu arc; chemical zonation of volcanoes is sub-parallel to the trench axis in both regions. On the contrary, the spatial variations in FeO and K_2O values in TTS region are different from those in SWH and AKS regions. The mode of the variations in FeO and K_2O values is thus discontinuous in Hokkaido island. The values of Bouguer anomaly at TTS region (-40 to 0 mgal) are much lower than those at other two regions (+40 to +100 mgal). Because crustal thickness under the regions is nearly identical (25-34km), the regional variation in the values of Bouguer anomaly suggests that crustal materials of TTS region are less dense than those of SWH and AKS regions. These different crustal materials, which have been produced under the different tectonic settings, are believed to have resulted in mode of spatial variations in major element chemistry of Quaternary andesites.

著者

宮縁 育夫 池辺 伸一郎 渡辺 一徳

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.50, no.4, pp.227-241, 2005-08-31

被引用文献数

6

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Ash emissions occurred from a hot water pool of one of the craters of Nakadake, the only active central cone of Aso Volcano, southwestern Japan, on July 10, 2003 and January 14, 2004. The former ash was dispersed throughout an area extending 14km east-northeast of the vent, whereas the latter ash extended 8km to the east-southeast. Based on isopleth maps, the total weight of ash discharged by each event was 41tons and 32tons, respectively. Both ash-fall deposits were composed of fine-grained (<1mm) glass shards, crystals and lithic fragments, and contained neither lapilli nor blocks. Some glass shards appear fresh and are probably juvenile material. We therefore believe that these phenomena represent non-explosive gas-and-ash emissions through the hot water pool. The ash particles emitted from the hot water pool of the Nakadake crater were aggregated at a size of 1-3mm and deposited because of a high content of water. The ash-fall deposits in proximal areas (<1.5km from the crater) were finer grained and more poorly sorted than in distal areas (>5km). Moreover, the deposit volume obtained from actual data in the proximal area was much larger than the volume extrapolated only from the isopleth data of the distal deposits. These phenomena suggest that the proximal deposits were emplaced mainly by another fine-grain-rich column different from the main ash plume that extended to distal areas, or that some parts of the column detached from the main plume that emplaced the proximal deposits.

著者

LONDONO B John Makario 小林 芳正

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.39, no.2, pp.31-47, 1994-05-20

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1990年8〜9月,ネバド・デル・ルイス(NRV)火山でいくつかの高周波火山性群発地震が,5カ所におかれた可搬式3成分デジタル地震計で記録された.この期間に3つの群発地震が記録されたが(8月28日,9月11日,9月17日),それぞれの群の震源は,活動している火口周辺の,互いに異なる領域内に決定された.スペクトル解析と波形の観察から,同一の群に属する多くの地震は,同一観測点ではよく似たスペクトルと波形を示すが(ほとんど相似),群が変わると非常に違った形状になることがわかった.これは,一つの群発地震中,震源時間関数がどの地震でも互いに似ているためと考えられる.また,同一群の地震でも観測点が変わるとスペクトルが異なっており,伝播経路の効果が強いと考えられる.これらの群発地震と比較するため,9月11日の群発地震の震源領域に近い領域で起こったいくつかの孤立的な地震を同様に解析した.これらの孤立的地震のスペクトルと波形は,同一の観測点でも,異なる観測点間でも互いによく似ていた.このことは,孤立的地震では経路効果はあまり強くないことを意味し,孤立的地震の経路が,経路効果の強い群発地震とは,性質の異なる領域を通っているためと考えられる.今回の解析から,NRVの高周波群発地震はある決まった領域内に起こること,それぞれは特有のスペクトルと波形を持つことがわかった.孤立的地震は均一な領域などで不均質な領域で起こり,この不均質のために観測点ごとに経路効果が異なるものと考えられる.NRV周辺各点のスペクトル間の相似性からNRV地域は3つの地帯に分けられそうである.

著者

宇井 忠英 隅田 まり 大学合同観測班地質班 荒牧 重雄 大島 治 鎌田 桂子 小林 武彦 小屋口 剛博 佐藤 博明 中川 光弘 中田 節也 藤井 敏嗣 藤縄 明彦 古山 勝彦 三宅 康幸 横瀬 久芳 渡辺 一徳

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.38, no.2, pp.45-52, 1993-07-01

被引用文献数

2

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Small-scale pyroclastic flows due to the collapse of the lava dome have been frequently generated during the 1991-93 eruption of Unzen Volcano. We have recorded video footages which show the generation of pyroclastic flows during January-March 1992. Two types of phenomena have been observed : deformation of the lava dome due to flowage ; and a sudden discharge of gas and ash through fractures and peeling-off of rock fragments from the surface of cooling lava blocks. Pyroclastic flows were generated only in places on the lava dome where these precursory phenomena were frequently observed.

著者

ペレツ ネメシオ ヘルナンデス ペドロ カストロ ローデス サラサー ホセ 野津 憲治 森 俊哉 岡田 弘

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.47, no.4, pp.347-351, 2002-09-17

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有珠山2000年噴火の18月前の1998年9月に, 山頂域, 麓域の150箇所で採取した土壌中のホウ素とアンモニアの定量を行った. その結果, 土壌中のホウ素含有量は1,300μg/kgに達し, アンモニア含有量は14mg/kgに達した. ホウ素含有量の空間分布は, 900μg/kg以上の極めて高い領域が高温噴気活動が見られる山頂カルデラ内のほかにも, 2000年の噴火地点に近い北西山麓に存在していた. また, アンモニア含有量の高い領域は, ホウ素含有量の高い領域に一致し, さらに昭和新山でも見られた. 土壌中のホウ素, アンモニアの高濃度異常域は, 二酸化炭素の土壌からの放出量が高い領域 (Hernandez et al.,2001) とよく合っており, これらの成分が共通の起源をもつことを示唆している. このことは,土壌中のホウ素やアンモニアが, 二酸化炭素と同様に,火山体における揮発性物質の挙動を知る上で有用な指標となることを示している.