著者

松岡 憲知 藁谷 哲也 若狭 幸

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.126, no.3, pp.369-405, 2017-06-25 (Released:2017-08-03)

参考文献数

221

被引用文献数

3 6

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Physical rock weathering has been studied through laboratory experiments, field observations, and numerical modeling, but linking these approaches and applying the results to weathering features in the field are often problematic. We review recent progress in three weathering processes—frost shattering, thermal fracturing, and lightning strikes—and explore better approaches to linking weathering processes and products. New visual and sensor technologies have led to great advances in field monitoring of weathering of fractured bedrock and resulting rockfalls in cold mountains. Laboratory simulations successfully produce fractures resulting from segregational freezing in various intact rocks. Modelling approaches illustrate the long-term evolution of periglacial slopes well, but improvements are required to apply laboratory-derived criteria to frost weathering. The efficacy of thermal weathering, which has long been under debate, is now partly supported by laboratory and field evidence that cracking takes place when wild fires or artificial explosions lead to thermal shock. Rock fracturing due to strong radiation is also reevaluated from the presence of large cooling/warming rates and meridian cracks in rocks exposed to arid environments. Linking laboratory simulations and natural features, however, needs further field-based observations of thermal fracturing. Irregular fractures formed in boulders are often attributed to lightning strikes, despite rarely being witnessed. Artificial lightning in the laboratory produces radial cracks, marking the first step toward interpreting irregular fractures in the bedrock that are unlikely to originate from other weathering processes. Identifying the origins of fractured rocks in the field requires distinguishing between fracture patterns derived from these weathering processes.

著者

菅沼 悠介 石輪 健樹 川又 基人 奥野 淳一 香月 興太 板木 拓也 関 宰 金田 平太郎 松井 浩紀 羽田 裕貴 藤井 昌和 平野 大輔

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.129, no.5, pp.591-610, 2020-10-25 (Released:2020-11-13)

参考文献数

101

被引用文献数

1

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The Antarctic Ice Sheet (AIS) is one of the largest potential contributors to future sea-level changes. Recently, an acceleration of AIS volume loss through basal melting and iceberg calving has been reported based on several studies using satellite observations, including radar altimetry, interferometer, and gravity measurements. A recent model that couples ice sheet and climate dynamics and incorporates hydrofracturing mechanism of buttressing ice shelves predicts a higher sea-level rise scenario for the next 500 years. However, the calibration and reproducibility of the sea-level rise projection from these models relies on geological sea-level reconstructions of past warm intervals. This suggests that a highly reliable reconstruction of the past AIS is essential for evaluating its stability and anticipating its contribution to future sea-level rise. In particular, a relative sea-level reconstruction in East Antarctica is the key to solving the problems and refining future projections. The current understanding of sea-level change along the East Antarctic margin is reviewed, including Glacial Isostatic Adjustment (GIA) effects, and a new strategy is proposed to address this topic based on seamless sediment coring from marine to lake in the East Antarctic margin. This project will provide essential data on AIS change since the last interglacial period.

著者

小元 久仁夫 大村 纂

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.124, no.1, pp.127-135, 2015-02-25 (Released:2015-03-11)

参考文献数

22

被引用文献数

2

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The Rhone Glacier is the best-documented case in the Alps of a rapidly retreating glacier. Photographs and other documents show the history of the changing glacier since the Little Ice Age. Fluctuations of the glacier tongue reflect decadal temperature fluctuations during the observation period. The last quarter century witnessed the most spectacular retreat, resulting in the development of a glacier lake. The pro-glacial lake has increased rapidly in size to the extent that the Swiss Federal Office of Cartography and Survey has given it the official geographical name of “Rhonesee”. The major stages of the Rhone Glacier are presented in this article with selected photographs and figures.

著者

神保 小虎

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.3, no.1, pp.36-38, 1891-01-25 (Released:2010-10-13)

著者

宮縁 育夫

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.125, no.3, pp.421-429, 2016-06-25 (Released:2016-07-14)

参考文献数

21

被引用文献数

1 18

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The Kumamoto earthquake (Mj 7.3) on April 16, 2016 triggered numerous landslides in and around Minamiaso Village, which is located at the western part of Aso caldera, southwestern Japan. The landslides were divided into two types: landslides occurring at steep caldera walls and landslides generated on the slopes of post-caldera central cones of Aso Volcano. Several landslides occurred on slopes steeper than 25° at the northwestern to western caldera walls, which comprise pre-Aso volcanic rocks (lavas and pyroclastics). The largest landslide (ca. 300 m high, 130–200 m wide) occurred on the western caldera wall, damaging National Route 57 and the Hohi line of the Japan Railway. Because a clear rupture surface could not be observed, unstable blocks which had been divided by cracks, were likely to collapse due to the intense earthquake on April 16. At the post-caldera central cones of Aso Volcano, earthquake-induced landslides were generated not only on steep slopes but also on slopes gentler than 10°. They occurred in unconsolidated superficial tephra deposits overlying lavas and agglutinates, and the thickness of the slides usually ranged from 4 to 8 m. The sliding masses traveled long distances (<600 m), compared to small differences in elevation. The deposits were composed of tephra blocks of a few meters and there was no evidence that they were transported by water. These facts suggest that some landslides mobilized rapidly into debris avalanches, traveling a few hundred meters. The associated debris avalanche resulted in five casualties and severe damage to houses at the foot of the Takanoobane lava dome. The characteristics of the April 16, 2016 earthquake-induced landslides differ from those of rainfall-induced landslides in July 2012, June 2001, and July 1990 at Aso Volcano, and provide important information for preventing or mitigating future landslide disasters in the Aso caldera region.

著者

早川 裕一 松倉 公憲

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.112, no.4, pp.Plate5-Plate6, 2003-08-25 (Released:2009-11-12)

著者

齋藤 めぐみ 山田 和芳 リチャード スタッフ 中川 毅 米延 仁志 原口 強 竹村 恵二 クリストファー ラムジー

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.122, no.3, pp.493-501, 2013-06-25 (Released:2013-07-08)

参考文献数

23

被引用文献数

2 6

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Sediment cores from Lake Suigetsu, Japan, reveal the absence of seawater intrusion into the lake caused by the historically documented tsunami of AD1586 (Tensho Tsunami). A high-precision chronology of the cores established by Bayesian modeling radiocarbon determinations enables us to ascertain the precise position of the historical event in the sediment depth. Diatom analysis of the core shows that a diatom assemblage dominated by freshwater taxa persisted through the period. This makes a clear contrast with the core section around AD1664 when the lake was artificially connected to the sea by a channel, and the subsequent intrusion of seawater was clearly recorded in the sediment cores by the occurrence of marine diatom fossils.

著者

澤木 佑介 佐藤 友彦 藤崎 渉 上田 修裕 浅沼 尚 丸山 茂徳

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.128, no.4, pp.549-569, 2019-08-25 (Released:2019-09-20)

参考文献数

61

被引用文献数

4 4

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The evolution of eukaryotes is one of the most important issues in the history of life. Paleontological studies discovered the oldest eukaryotic fossil from the Paleoproterozoic Francevillian Group in Gabon. To clarify specific features of ancient sedimentary basins in Gabon, geologic evidence for the Francevillian Group is summarized and a new geotectonic model is proposed. The model gives much weight to the upwelling of mantle plume, which can explain why the Francevillian basin only hosted natural reactors. The Great Oxidation Event in the Paleoproterozoic played an important role in not only the evolution of eukaryotes but also in the formation of natural reactors. Reductive weathering of the continental crust, which was affected by plume-related volcanisms, transported Uranium-rich minerals into sediments of the Francevillian Group without dissolution of Uranium. The subsequent oxidation event enabled uranyl ions to accumulate within an oxic failed rift basin, and settled large quantities of organic matter on the seafloor. Hydrothermal circulation within the Francevillian Group precipitated highly Uranium-rich ores, which became natural reactors at approximately 2.0 Ga, and might have influenced the evolution of eukaryotes in this basin. In a large sense, the degree of oxidation of the ocean-atmosphere system has been linked to the amount of sedimentary rocks. In that way, mantle overturn in 2.7 Ga, which created a large continental crust, was one of the crucial events that affected the evolution of eukaryotes.

著者

松本 良 奥田 義久 蛭田 明宏 戸丸 仁 竹内 瑛一 山王 梨紗 鈴木 麻希 土永 和博 石田 泰士 石崎 理 武内 里香 小松原 純子 Antonio Fernando FREIRE 町山 栄章 青山 千春 上嶋 正人 弘松 峰男 Glen SNYDER 沼波 秀樹 佐藤 幹夫 的場 保望 中川 洋 角和 善隆 荻原 成騎 柳川 勝則 砂村 倫成 後藤 忠則 廬 海龍 小林 武志

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.118, no.1, pp.43-71, 2009-02-25 (Released:2010-04-05)

参考文献数

46

被引用文献数

65 58

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A number of extensive methane plumes and active methane seeps associated with large blocks of methane hydrates exposed on the seafloor strongly indicate extremely high methane flux and large accumulations of methane hydrate in shallow sediments of the Umitaka spur and Joetsu knoll of the Joetsu basin 30 km off Joetsu city, Niigata Prefecture. Crater-like depressions, incised valleys, and large but inactive pockmarks also indicate methane activities over the spur and knoll. These features imply strong expulsions of methane gas or methane-bearing fluids, and perhaps lifting and floating-up of large volumes of methane hydrate to the sea surface. High heat flow, ∼100 mK/m, deposition of organic-rich strata, ∼1.0 to 1.5%TOC, and Pliocene-Quaternary inversion-tectonics along the eastern margin of the Japan Sea facilitate thermal maturation of organic matters, and generation and migration of light-hydrocarbons through fault conduits, and accumulation of large volumes of methane as methane hydrate in shallow sediments. Microbial methane generation has also contributed to reinforcing the methane flux of the Joetsu basin. Regional methane flux as observed by the depth of the sulfate-methane interface (SMI) is significantly high, < 1 m to 3 m, when compared to classic gas hydrate fields of Blake Ridge, 15 to 20 m, and Nankai trough, 3 to 15 m. δ13C of methane hydrate and seep gases are mostly within -30 to -50‰, the range of thermogenic methane, while dissolved methane of the interstitial waters a few kilometers away from seep sites are predominated by microbial with δ13C of -50 to -100‰. Seismic profiles have revealed fault-related, well-developed gas chimney structures, 0.2 to 3.5 km in diameter, on the spur and knoll. The structures are essential for conveying methane from deep-seated sources to shallow depths as well as for accumulating methane hydrate (gas chimney type deposits). The depth of BSR, which represents the base of gas hydrate stability (BGHS), on the spur and knoll is generally 0.20 to 0.23 seconds in two-way-travel time, whereas the BSRs in gas chimneys occur at 0.14 to 0.18 seconds, exhibiting a sharp pull-up structure. The apparent shallow BGHS is due to the accumulation of large volumes of high-velocity methane hydrate in gas chimneys. The depth to BGHS is estimated to be 115 m on an experimentally determined stability diagram, based on an observed thermal gradient of 100 mK/m. Then the velocity of the sediments on the Umitaka spur is calculated to be 1000 m/s, which is anomalously low compared to normal pelagic mud of 1600-1700 m/s. This exciting finding leads to the important implication that sediments of the Umitaka spur contain significant amounts of free gas, although the sediments are well within the stability field of methane hydrate. The reasons for the existence of free gas in the methane hydrate stability field are not fully explained, but we propose the following possible mechanisms for the unusual co-existence of methane hydrate and free-gas in clay-silt of the spur. (i) High salinity effect of residual waters, (ii) degassing from ascending fluids, (iii) bound water effect and deficiency of free-waters, and (iv) micro-pore effect of porous media. All of these processes relate to the development of gas hydrate deposits of the Umitaka spur.(View PDF for the rest of the abstract.)

著者

柵山 徹也

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.224-234, 2010-04-25 (Released:2010-07-06)

参考文献数

57

被引用文献数

8 6

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Cenozoic western Pacific is characterized by numerous subduction volcanisms, opening of back arc basins, and within-plate volcanisms in backarc regions. These phenomena suggest that thermal disturbances induced by mantle flow caused mantle melting and eruption of magmas in back-arc regions, although diverse modes have been proposed for the mantle condition. To constrain the origin of the magmatism, it is important to clarify both the geodynamics and physicochemical conditions of the mantle in the arc-back arc system. Cenozoic back-arc volcanisms and tectonics in northern Kyushu are key objectives. This paper focuses on the dynamics of the upper mantle in relation to the tectonics of the area. Cenozoic volcanism of the northern Kyushu district is characterized by eruptions of a number of alkaline volcanisms. This volcanic province extends some one hundred kilometers along the Japan Sea coast and includes the Chugoku area. Although the volcanism has been regarded as a large single volcanic province, the northern Kyushu activity is distinguishable from that in Chugoku by geochemical characteristics. Cenozoic basalt volcanism in northern Kyushu occurred mainly in the Tertiary sedimentary basins, which is common to Cenozoic volcanism in the Japan Sea Basin. The sedimentary basin deposits underwent intensive folding with extensive inversion tectonics occurring mainly around the end of Miocene. Studies on dykes and thrust faults also indicate a compressional stress field dominated in northwestern Kyushu during the late Miocene. Initiation of the basaltic activity took place in Kita-Matsuura area and expanded outwards. Basaltic rocks from the Kita-Matsuura are mostly hypersthene-normative and sub-alkalic, whereas rocks from peripheries, such as North Kyushu and Goto Islands, are predominantly less normative-hypersthene and thus are alkalic. From spatial and temporal variations and genetic conditions of basalt magmas, a most plausible source was a small-scale mantle diapir with a potential temperature of more than 1400°C. Combined with an inversion tectonic background after the Japan Sea opening event, the origin of the back-arc magmatism was unrelated to large-scale mantle upwelling. Fluid-fluxed melting of the mantle is not supported by the chemistry. My proposal is active upwelling of a small-scale mantle diapir, probably from the depth of stagnant slab or mantle transition zone.

著者

矢田 俊文

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.91, no.6, pp.489-495, 1982-12-25 (Released:2009-11-12)

参考文献数

6

被引用文献数

1

著者

新井 智一

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.114, no.5, pp.767-790, 2005-12-25 (Released:2009-11-12)

参考文献数

47

被引用文献数

5 1

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This study examines the interaction between the politics in Fussa City and the Yokota Air Force Base, that is “the politics of place, ” from a range of political, economic, and cultural processes which maintain the existence of the base.Military bases are generally established for global geopolitical purposes. However, the establishment of a military base has cultural implications for the local communities where the base is located, i.e., increased concerns about crimes committed by seavicepersons and noise by aircrafts. Although these military bases have some serious impacts, especially in Japan, geographers have not yet examined the issues regarding a particular military base and the resultant politics in the city.On the contrary, since the 1980s, Anglo-American political geographers have paid more attention to “the politics of place” This refers to the local politics that occur due to the interaction between a structural constraint and the individuality of a particular place.Therefore, this study examines “the politics of place” on the Yokota Air Base in Fussa City, Tokyo, by using resources from the local newspapers, novels, magazine articles, and council proceedings.The results are as follows : (1) Owing to the independence between the U.S. Air Force and local political and economic actors, an urban structure and local economy that depended on the Yokota Air Force Base were constructed in Fussa Town after the establishment of the base in 1945; (2) because of this structure, the local economy became to depend on Air Force personnel as consumers; (3) however, due to a shift to the floating exchanging rate and a reduction in the population of the base in the first half of the 1970s, there was a decrease in the influence that Air Force personnel had on the local economy; (4) since the 1980s, an economic agent utilized the “atmosphere” and “image” adjacent to the base to revitalize the local economy; (5) furthermore, the mass media represented Fussa City as a “base town” and conducted a review of the city in the 1960s, thus contributing to its revitalization; (6) during the economic slump in the 1970s, some local political agents were against the existence of the base. However, successive mayors of Fussa City have accepted the existence of the base, and utilized the subsidies it receives from the national government in order to construct the urban infrastructure.

著者

大関 仁智 清水 紀和 上松 佐知子 指田 勝男

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.130, no.3, pp.311-329, 2021-06-25 (Released:2021-07-20)

参考文献数

69

被引用文献数

2

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Zircons in Cretaceous sandstones have recently been the focus of a new provenance analysis technique. Microfossil-bearing clasts in conglomerate are also used in this analysis; however, there are few reports on clasts, including microfossils, which are mainly found in the vicinity of Shikoku and Hokuriku districts. Although the Ishido Formation, Sanchu Group, in the Kanto Mountains, is considered to be Barremian in age based on the occurrences of ammonoids, interbedded with conglomeratic beds, there have been no reports of microfossil-bearing clasts from this bed. To confirm whether each fore-arc basin in the whole of southwest Japan had a common hinterland during the Late Mesozoic, microfossils are extracted from gravels in the Ishido Formation to examine their ages. As a result of acidic treatment, Permian and Triassic radiolarians and Triassic conodonts are obtained from eight chert and siliceous mudstone pebbles of the Ishido Formation. On the basis of their lithofacies and ages, these pebbles of this formation are likely to derive from the Chichibu Belt, which constitutes a large part of the Kanto Mountains. These results indicate that, in the Early Cretaceous period, not only granitic rocks but also Jurassic accretionary complexes were exposed as hinterlands of each fore-arc basin in the Shikoku and Kanto districts. Moreover, based on previous reports on the zircon spectra in Cretaceous sandstones and microfossil-bearing clasts in the conglomerates, hinterlands, such as granitic rocks and Jurassic accretionary complexes, of each fore-arc basin in the whole of southwest Japan including the Kanto district, as well as Kyushu and Shikoku districts, indicate common exposure and denudation histories.

著者

LOWE David J. PITTARI Adrian

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.130, no.1, pp.117-141, 2021-02-25 (Released:2021-03-18)

参考文献数

122

被引用文献数

2 7

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西暦232 ± 10年の晩夏にニュージーランド北島タウポ火山で起こった噴火は,過去5,000年間において地球上で起こった噴火のなかでもっとも強力なものであった。噴火は数日から数週間継続し,5つの明確な降下火砕堆積物(ユニットY1~Y5)に続いて,非常に爆発的な噴火による低アスペクト比イグニンブライト(ユニットY6)が堆積した。降下火砕堆積物の内,ユニットY1,Y3およびY4は水蒸気プリニー式噴火によって形成され,Y2とY5はプリニー式噴火であった。Y5とY6は一連の噴火で形成され,非常に強いY5噴火による噴煙柱は高度35-40 kmに達し,それが崩壊することによって非常に高速(600-900 km/h)で高温(最高500°C)の火砕密度流が発生し,ユニットY6が堆積した。このイベントによる堆積物は噴火後十数分で北島中央部の約20,000 km2に及ぶ範囲を覆い尽したと考えられる。また一連の噴火によるマグマ噴出量は約35 km3と見積もられている。この噴火による周辺環境への影響は甚大であり,現代においても農業などの土地利用において火山ガラスを多く含み,コバルトなどの微量元素に枯渇した土壌への対策が必要となっている。

著者

武山 尚生 高橋 佑歌 永田 祥平 澤木 佑介 佐藤 友彦 丸山 茂徳 金井 昭夫

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.129, no.6, pp.899-912, 2020-12-25 (Released:2021-01-18)

参考文献数

56

被引用文献数

2

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The origin of eukaryotic organisms is one of the most important questions in biology. So far, it has been suggested that eukaryotes are phylogenetically related to Archaea. Indeed, recent progress in archaeal genomic biology seems to have accurately determined the exact position of Archaea in the birth of the Eukaryota. In particular, identifying groups of archaeal species, such as the superphylum TACK and the Asgard archaea, has shown that primitive genes for eukaryotic signature proteins (ESP) already existed in the genomes of these archaeal species. Some ESPs are especially important, including actin and tubulin in the cytoskeleton and the ESCRT complex, which is involved in nuclear membrane formation. There have been many reports that eukaryotic intracellular organelles, such as mitochondria and chloroplasts, evolved from specific symbiotic bacteria. Moreover, eukaryotic genes are disrupted by intronic sequences, which must be removed or “spliced” and the exons connected after the primary transcript is generated, to make a mature functional mRNA. Recently, it has been suggested that the self-splicing factor in both bacterial and archaeal genomes, called “group II intron”, may cause gene disruption. In this review, the frontiers of genome biology are summarized in terms of the importance of prokaryotes (both Archaea and Bacteria) for the origin of Eukarya. From an Earth history perspective, how the increase in atmospheric oxygen concentration at 2.4-2.0 billion years ago may have contributed to the rise of the eukaryotes is discussed.

著者

田中 圭 中田 高 松浦 律子 田力 正好 松田 時彦

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.127, no.3, pp.305-323, 2018-06-25 (Released:2018-08-02)

参考文献数

49

被引用文献数

1 1

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Kambara Jishinyama (earthquake-mound) located on the west bank of lower reach of the Fujikawa river, is widely believed to be a mound that was tectonically formed at the time of the 1854 Ansei Tokai earthquake. Using old maps and aerial photogtaphs, geomorphological changes around Kambara Jishinyama before and after the earthquake are examined. The Fujikawa river frequently flooded and the course on its west bank changed especially after construction of the Karigane-zutsumi (big bank) in order to protect farmland on its east bank. The area around the lower reach of the river was surveyed in 1803 for the Dai Nihon Enkai Yochizu large-scale map, which is the so-called Ino-Daizu. On that map, the river was at almost the same location as its present course. The historical road map (Kaido-Ezu) of Tokaido, which was the trunk road connecting Edo and Kyoto, illustrated in the same period as Ino-Daizu, shows that the Fujikawa river shifted its course close to the foot of river terraces at the west bank. Due to lateral erosion of the river, part of the Tokaido between the towns of Iwabuchi and Kambara collapsed several times. Subsequently, the road was diverted to the new route via Shinzaka as shown on the 1:20,000 scale topographic map published in 1890. A micro-landform classification map of the alluvial lowland of the west bank of the Fujikawa river based on interpretations of aerial photographs taken in 1952 and 1953 reveals that Kambara Jishinyama was located on one of the former mid-channel bars in the braided channels of the river before the 1854 Ansei Tokai earthquake. The earthquake caused a large landslide that dammed the Fujikawa river for a short period at the foot of Shiratori-yama to the north of Iwabuchi. The discharged flood water changed the river course close to the present stream. Geomorphic evidence for tectonic uplift does not exist around Kambara Jishinyama. The Koike river, a small stream flowing in the former main stream of the Fujikawa river, abandoned at the time of the Ansei Tokai earthquake, concordantly flows into the present main stream of the Fujikawa river showing that co-seismic uplift did not take place at the west bank. We conclude that Kambara Jishinyama was not tectonically formed by the earthquake, but is a product of the river course change.

著者

横瀬 久芳 佐藤 創 藤本 悠太 Maria Hannah T. MIRABUENO 小林 哲夫 秋元 和實 吉村 浩 森井 康宏 山脇 信博 石井 輝秋 本座 栄一

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.1, pp.46-68, 2010-02-15 (Released:2010-05-21)

参考文献数

80

被引用文献数

7 20 1

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To understand the submarine volcanism surrounding the Tokara Islands, a submarine topographic analysis and 67 dredge samplings were carried out. Prior to the submarine investigations, we reviewed comprehensively geological and geophysical data on this region and confirmed the complexity of both volcanic activity and tectonic setting of the Tokara Islands. In contrast to the homogeneous subaerial volcanic rocks comprising predominantly two-pyroxene andesite lava flows, the dredged samples vary from basaltic andesite to rhyolite in composition. Furthermore, we reveal that dacitic and rhyolitic pumices are abundant and broadly distributed throughout the submarine area. The recovered volcanic rocks were mainly subangular to angular cobble-boulder fragments of lava, scoria, and variably vesiculated pumice. Volcanic rocks with hornblende phenocrysts occur only north of the Tokara strike-slip fault, which is a major tectonic element of volcanism. The pumices can be classified into three categories based on the size and abundance of the phenocrysts: aphyric pumice, fine-grained porphyritic pumice, and coarse-grained porphyritic pumice. Occurrences, such as amount in a dredge, shape without extensive abrasion, large fragment size, and bulk rock chemical compositions of the major pumice fragments suggest that they are in situ, rather than originating as drifted pumice or air fall, exotic pyroclastic fragments derived from the four super-eruptions of Kyushu Island. Because dredged samples contained fresh volcanic glass in the groundmass, and are not covered by iron-manganese oxide crust, they appear to have originated from the Quaternary eruptions. Indeed volcanic islands have developed above the submarine erosional terraces (indicated as knick points at approximately 110 m in depth), which is assumed to have formed during the last glacial age. K-Ar age dating on the representative pumice samples resulted in ages of 0.60 ± 0.20 Ma and < 0.2 Ma, respectively. These newly obtained submarine data support that acidic volcanisms occurred around the submarine calderas during the Mid-Pleistocene age.

著者

瀬野 徹三

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.114, no.3, pp.350-366, 2005-12-25 (Released:2009-11-12)

参考文献数

27

被引用文献数

2 3

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Plate motions provide the most basic information on plate tectonics. I explain how to describe plate motions on the Earth mathematically, and how they can be determined. I review the history of the determination of global plate motions, the most recent of which has been derived by space geodetic techniques.

著者

宮原 ひろ子

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.3, pp.510-518, 2010-06-25 (Released:2010-08-30)

参考文献数

20

被引用文献数

4 3

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The relationship between solar activity and climate change in the past can be examined using proxy records. Variations of solar activity can be reconstructed based on carbon-14 in tree rings, which are produced by galactic cosmic rays modulated by the solar wind, while climate change can be reconstructed from changes of tree-ring growth rate or content of stable isotopes in ice cores from the polar region. A comparison of solar activity and climate change at the Maunder Minimum in the 17th century and the Early Medieval Maximum Period in the 9-10th century suggests that the sun plays an important role in climate change even on a decadal time scale. The characteristic variations detected in climate change suggest the mechanism of solar influence on climate involves galactic cosmic rays. Variable features of eleven-year and twenty-two year cycles of solar activity and consequent variations of cosmic rays are possible origins of complex variations of climate change on decadal to multi-decadal time scales. We summarize variations of solar activity and cosmic rays during the past 1200 years and their possible influence on climate change.

著者

多田 隆治

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.107, no.2, pp.218-233, 1998-04-25 (Released:2009-11-12)

参考文献数

57

被引用文献数

1 1

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Since the ice core records from central Greenland revealed the presence and significance of millennial-scale large and abrupt climatic changes, widely known as Dansgaard-Oeschger [D-O] Cycles, it becomes the major objective of paleoclimatological researches to clarify their extent, nature, propagation mechanism, and driving force. Although the ultimate driving force is not yet understood, results of recent studies suggest 1) D-O Cycles are global phenomena, 2) they involve complicated interactions and feedback processes among the subsystems including atmosphere, cryosphere, hydrosphere, and biosphere, and 3) they seem tohave initiated from changes in atmospheric circulation.Catastrophic surges of Laurentide Ice Sheet called Heinrich Events are closely associated with D-O Cycles. Although Heinrich Events are likely to have been caused by free oscillationsof ice sheet growth and decay, they were probably not a cause of D-O Cycles but the events seem to have been phase-locked by D-O Cycles. Results of numerical modelling suggest the presence of multi-modes for global deepwater circulation. Switching among the modes is most likely caused by slight variation in hydrogical cycles which change the fresh water balance between Atlantic and Pacific.