著者

石川 晃

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.120, no.6, pp.1026-1034, 2011-12-25 (Released:2012-03-05)

参考文献数

26

被引用文献数

2 3

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西太平洋に現存するオントンジャワ海台は,白亜紀に生じた地球史上最大規模の火成活動の産物であるが,その成因についてはいまだ不明な点が多い。海台縁辺部が水上に露出しているソロモン諸島・マライタ島には,海台形成後に生じたキンバーライト類似のマグマ活動により,海台直下に由来する多様なマントル捕獲岩が表層にもたらされており,大規模融解現象の発生機構やその起源物質を解明するための有用な情報源となる。地質温度圧力計の適用によると,各種捕獲岩の由来深度はモホ面直下(約60km)からリソスフェア最下部(約120km)に至ることがわかる。また由来深度に対応して,卓越する岩相や化学組成が系統的に変化する観察事実から,海台下リソスフェアの構造としてジュラ紀の海洋プレート直下に海台玄武岩を抽出した融け残りマントルが底付けされている様子が復元される。このことは,マライタ島に産する一部の捕獲岩はかつて地球深部に存在しており,温度・組成異常をあわせもつ「マントルプルーム」の構成物質として地表近傍へ上昇してきたことを示唆している。

著者

財城 真寿美 三上 岳彦

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.122, no.6, pp.1010-1019, 2013-12-25 (Released:2014-01-16)

参考文献数

25

被引用文献数

4 6

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Climate variations in Tokyo based on reconstructed summer temperatures since the 18th century and instrumental meteorological data from the 19th century to the present are discussed. During the Little Ice Age, especially in the 18th century, remarkably cool episodes occurred in the 1730s, 1780s and 1830s. These cool conditions could be a significant reason for severe famines that occurred during the Edo period. Around the 1840s and 1850s near the end of the Edo period, it was comparatively warm which could correspond to the end of the Little Ice Age in Japan. Although there was a low-temperature period in the 1900s, a long-term warming trend could be seen especially in winter temperatures and daily minimum temperatures throughout the 20th century. While annual precipitation has been increasing during the last 30 years, relative humidity has been decreasing. This could result from a saturated vapor pressure rise due to warming and from a loss of water bodies due to urbanization. During the last century, not only warmer conditions but also wetter conditions in summer and autumn and drier conditions in winter and spring were documented by analyzing hythergraphs.

著者

河本 大地

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.117, no.3, pp.617-636, 2008-06-25 (Released:2010-05-14)

参考文献数

45

被引用文献数

1

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The aim of this study is to examine to what extent the introduction of organic farming methods, typical post-productivism in agriculture, affects changes in plantation agriculture, the typical productivism in the Global South. A case study was conducted on the tea industry in Sri Lanka. The method was intensive interviews with key persons at the headquarters of plantation companies, management sections of tea estates, relevant organizations, and with some tea estate residents, as well as collecting relevant documents in July, August, and December, 2005. The findings are as follows. Firstly, the tea industry in Sri Lanka, particularly the plantation estate sector, has structural problems in terms of environment, economy, and social sustainability. These problems have been mainly caused by decreasing consumption of tea in industrialized countries and former policies of nationalizing Sri Lankan tea estates. Secondly, organic farming methods in tea plantation estates have been introduced mainly in disadvantaged mountainous areas, characterized by many abandoned tea areas and old tea bushes. Organic farming is an alternative strategy for developing the estates in such areas. These organic tea estates are trying to brand themselves, diversify crops, promote biodiversity, conduct eco-tours of estates, enhance social welfare system for residents, and introduce fair-trade certifications, as well as obtain organic certifications. Thus, organic farming in the tea estates can be understood as a strategy of diversified management, which is a combination of the elements of post-productivism. However, the cost of production (COP) is higher in organic tea estates than in conventional estates, especially the cost of composting and weeding. Due to their labor-intensive character, organic estates are affected enormously by increasing labor costs, and in the near future the outflow of labor might also be a problem. These issues are common in the Sri Lankan tea industry. Moreover, marketing to acquire new consumers in industrialized countries is not easy for organic estates, and the bought price is not increasing. This trend will not lead to the establishment of more organic tea estates. Some existing organic estates are also on the verge of closure under these circumstances. To ensure the sustainability of organic tea estates, it is necessary to expand their social welfare policies especially for younger residents, and communicate these efforts to society along with environmental conservation policies. Moreover, consumers in industrialized countries need to understand and support the challenges affecting disadvantaged estates.

著者

下司 信夫

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.118, no.6, pp.1254-1260, 2009-12-25 (Released:2010-03-23)

参考文献数

10

被引用文献数

11 14

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A large pyroclastic eruption occurred around 7.3 ka from the Kikai caldera about 30 km north of Yakushima Island. Its pyroclastic flow and fall deposits covered the entire area of Yakushima Island and may have influenced the evolution of unique floras and faunas of Yakushima Island. Detailed field survey revealed that the Koya pyroclastic flow deposit spread from NW to SE, covering almost the entire area of Yakushima. A part of the southern coastal area remained from the pyroclastic flow due to local alignment of topographic ridges and valleys, which acted as barriers to the pyroclastic flows. Possible tsunami deposits associated with the Kikai-Akahoya eruption were discovered in the area below ca. 50 m above sea level along the northern coasts of Yakushima and Kuchinoerabujima Islands.

著者

長谷川 昭 中島 淳一 内田 直希 弘瀬 冬樹 北 佐枝子 松澤 暢

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.190-204, 2010-04-25 (Released:2010-07-06)

参考文献数

44

被引用文献数

13 10

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A dense nationwide seismic network recently constructed in Japan has been yielding large volumes of high-quality data that have made it possible to investigate the seismic structure in the Japanese subduction zone with unprecedented resolution. We introduce a precise configuration of the Philippine Sea and Pacific plates subducting beneath the Japanese Islands, which was recently obtained by seismic tomographic imaging, precise earthquake hypocenter determinations, and focal mechanism studies. Seismic tomographic studies show that the Philippine Sea plate subducting beneath southwest Japan is continuous throughout the entire region, from Kanto to Kyushu, without disruption or splitting even beneath the area north of the Izu Peninsula. The estimated geometry of the subducted Pacific and Philippine Sea slabs shows a broad contact zone between the two slabs located directly beneath the Kanto plain. It further shows the wavy configuration of the Philippine Sea slab subducting beneath the entire region of southwestern Japan. Contact between the Philippine Sea plate and the Pacific plate causes anomalously deep interplate and intraslab earthquake activity in Kanto. Moreover, the interplate coupling coefficient estimated from repeating earthquake data shows a distinct change across the northeastern edge of this slab contact zone, suggesting that the overlying plate controls large-scale interplate coupling. High-resolution studies of spatial variations of intraslab seismicity and the seismic velocity structure of the slab crust strongly support the dehydration embrittlement hypothesis for the generation of intraslab earthquakes.

著者

河江 肖剰 亀井 宏行

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.120, no.5, pp.864-868, 2011-10-25 (Released:2012-01-17)

参考文献数

7

被引用文献数

1 1

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三大ピラミッドが建つギザ台地はおもに新生代始新世時代に遡るモカッタム累層と呼ばれる浅海成石灰岩の岩盤からなっている。古代の巨石建造プロジェクトにおける最優先事項の一つは採石場の選択であるが,この累層の石灰岩は硬い層と柔らかい層の交互層からなるため採石に適していた。古代人は柔らかい層を除去し,硬い層からピラミッドや神殿建設のための巨大な岩塊を採石した。 2,590,000立方メートルの石材からなるクフ王の大ピラミッドの採石場は,南約300メートルに近在しており,そこから約276,000立方メートルの石灰岩が切り出されたと概算されている。第2ピラミッドの造営者カフラー王もギザの地形を考慮しながら自らのピラミッド複合体の建造を推進し,プロジェクト最後にはモカッタム累層の岩盤を生かし,スフィンクスを巨大な彫像としてつくりあげた。王家最後の巨石建造物は,ケントカウエスと呼ばれる女王が建てさせた彼女の2段式の墓で,独特な形をもつ。墓はもともとクフ王の石切場の中央に,おそらく石材量を量るために,残されていた岩塊だった。三大ピラミッド建造後の限られた空間のなか,彼女は使用可能な最後の土地を巧みに利用し自らの墓を建てさせた。これによってギザ台地の巨石建造プロジェクトは終焉を迎えた。

著者

吉山 昭 柳田 誠

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.104, no.6, pp.809-826, 1995-12-10 (Released:2009-11-12)

参考文献数

63

被引用文献数

20 27 36

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In this paper the new methodology for quantitative estimation of uplift rates at inland region was discussed using fluvial terrace surfaces and deposit bases. Previous studies have revealed that river profiles regularly changed according to the cyclic fluctuation of climate and sealevel ; therefore, in the interglacial ages (the marine isotope stage 5e and the present) similar profiles with large concavity formed, and in the glacial ages (stage 6 and 2) linear profiles appeared on the contrary. In upper reaches of rivers, stage 5e valley bottoms were filled with the deposit accumulated in the subsequent glacial stages. Based on these observations, the value of relative height between the stage 5e burned valley bottom and the present river floor (BV value) is considered to indicate uplift during those stages (120-130 ky). Similarly, the value of relative height between the stage 6 terrace and the stage 2 terrace (TT value) can be used as an indicator of uplift during the period between those two stages.The BV values and TT values distributed along Japanese major rivers were reviewed on the basis of previous studies on terrace development and tephro-chronology. In middle reaches of most of rivers, the TT and BV values at the same sites are concordant to each other. The BV values remarkably decrease upstream in spite of no reduction of the TT values in the upper reaches or branches. This implies that present river profiles have not completely been in equilibrium and are now degrading in the upper streams and branches. As an indicator of long-term uplift rate, the TT values have the advantage of applicability to upper reaches and availability of field data in comparison to the BV value. However, it is impossible to apply these methods in the upper reaches or branches where fluvial terraces are not well developed.Up until now, the uplift rates in mountainous region were presumed from the outlined altitude of mountains or the amount of eroded material trapped in reservoirs, but have not been examined by direct data. The uplift rates obtained by using TT values and BV values are 0.10.8 mm/year in large number of sites, which are not located in very high mountainous area. Large uplift rates more than 1 mm/year were estimated from TT values at the Kurobe river basin, in the northern part of Chubu mountainous region.

著者

大森 房吉

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.19, no.4, pp.251-257_1, 1907-04-15 (Released:2010-10-13)

著者

本井 達夫 鬼頭 昭雄 緑川 貴 荒川 理 笹井 義一 陳 永利

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.117, no.6, pp.1015-1028, 2008-12-25 (Released:2010-04-26)

参考文献数

43

被引用文献数

1 1

---

Climate model experiments are carried out to understand the relationship between large-scale topography and climate variation. Mountain uplift experiments show that sea surface temperature, surface wind fields, precipitation and sea surface salinity are strongly influenced by mountain uplift. An enhanced Asian monsoon due to mountain uplift causes stronger seasonal coastal upwelling in the Indian Ocean and freshening in the Bay of Bengal, Yellow Sea and East China Sea. Mountain uplift experiments using a higher resolution atmospheric general circulation model reveal that the spatial pattern of precipitation becomes finer as resolution increases, and that there is a sharper contrast in the salinity distribution near coastal regions. Experiments in which the Panamanian Gateway is closed, opened and re-closed suggest that reorganization of the ocean current due to closure of the Panamanian Gateway induces a cooler and drier climate with a permanent halocline and sea ice in the subarctic Pacific.

著者

吉田 英嗣

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.568-578, 2010-06-25 (Released:2010-08-30)

参考文献数

112

被引用文献数

2 2

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The Japanese Islands are tectonically active and humid. Thus, erosion and deposition are the primary processes controlling geomorphic development. Catastrophic sector collapses at volcanoes should be considered significant in this context. This study examines the geomorphological role of volcanic sector collapses in Japan, introducing 58 cases with their respective occurrence ages and volumes (≥ 1×108 m3). We find that the frequency of sector collapses becomes exponentially higher as the collapse magnitude decreases. The total volume of the dissected volcanic edifice caused by catastrophic collapses amounts to ca. 6.4 km3 (640×108 m3) during the last 500 years. This value can be translated into an annual denudation rate of 0.53 mm/y per unit area of the Quaternary volcanoes (ca. 24000 km2), which is comparable to the contemporary denudation rate of non-volcanic mountains in Japan. Therefore, although volcanic sector collapses occur intermittently, we have to consider them as sediment sources that are indispensable to an understanding of geomorphology in Japan.

著者

木村 政昭 笠原 順三 麻植 誠二郎

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.4, pp.217-238, 1982-08-25 (Released:2009-11-12)

参考文献数

22

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Spatial and temporal relations between large interplate earthquakes and eruptive activities along Northeast Japan was studied by means of a two-dimensional model using the finite element method. Stress fields were calculated using seismological data in Northeast Japan. The stress value for each earthquake was normalized by using the seismic moment Mo instead of earthquake magnitude M. Stress differences before and after great earthquakes show close relationships with eruptive activities in Northeast Japan. The results suggest that eruptive activities increase under the increased compressive strain of pre-seismic conditions and their activities abruptly decrease under post-seismic conditions.

著者

諏訪 彰

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.64, no.4, pp.169-172, 1955-12-30 (Released:2009-11-12)

著者

鈴木 力英

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.556-561, 2010-06-25 (Released:2010-08-30)

参考文献数

10

被引用文献数

1 1

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In mid-November, 2009, a breach of the server of the Climatic Research Unit (CRU) of the University of East Anglia, UK, occurred, and more than 1,000 e-mails were copied and disseminated over the Internet. CRU is one of the leading climatology institutes, and has constructed a long-term world temperature database that was referenced in the assessment report of the Intergovernmental Panel on Climate Change (IPCC) as evidence of global warming. The words “trick” and “hide the decline” in an e-mail, which mentioned the process for preparing the temperature time series, generated the allegation; those words demonstrated that scientists at CRU intended to falsify data to exaggerate warming. However, it can be regarded that the allegation was a manufactured controversy originating from irrelevant interpretations of personal e-mails at CRU. Scientists are now being requested to ensure that scientific knowledge and understanding are convincing not only to scientists but also to the general public.

著者

村田 昌則 小林 淳 青木 かおり 高橋 尚志 西澤 文勝 鈴木 毅彦

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.130, no.3, pp.379-402, 2021-06-25 (Released:2021-07-20)

参考文献数

41

被引用文献数

2

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Kozushima is a volcanic island, located in the northern part of the Izu Islands, approximately 170 km SSW of central Tokyo. The volcanism of Kozushima Volcano started at 80-60 ka and formed monogenetic volcanoes. The latest eruption occurred in AD 838, and was associated with the formation of the lava dome and pyroclastic cone of Tenjo-san. In order to contribute to the long-term forecasting of volcanic eruptions on the Izu Islands off Tokyo, the tephrostratigraphy and eruption history of Kozushima Volcano during the last 30,000 years is reconstructed. According to the geological surveys, the widespread Kikai-Akahoya (K-Ah; 7.3 cal ka) and Aira-Tanzawa (AT; 30.0 cal ka) tephras from southern Kyushu, and Niijima-Mukaiyama (Nj-My; AD 886), Niijima-Shikinejima (Nj-Sk; 8 cal ka) and Niijima-Miyatsukayama (Nj-Mt; 12.8 cal ka) tephras from the Niijima Volcano 20 km NNE of Kozushima are recognized. Tephras that also erupted from Kozushima Volcano are Kozushima-Tenjosan (Kz-Tj; AD 838), Kozushima-Ananoyama (Kz-An; 7-9 c), Kozushima-Chichibuyama-A′ (Kz-CbA′; 14-12.8 cal ka), Kozushima-Chichibuyama-A (Kz-CbA; 30-22 cal ka), and Kozushima-Chichibuyama-B (Kz-CbB; ca. 30 ka). Kz-An is formed by the activity of the northern volcanic chains (Kobe-yama–Anano-yama–Hanatate) just before the Tenjo-san eruption. Kz-CbA′ is distributed in southern Kozushima. Source vent and distribution of Kz-CbA′ have not yet been identified. The eruption history of Kozushima volcano over the last 30,000 years is as follows. At ca. 30 ka, Kz-CbB erupted in the central Kozushima, and the Nachi-san dome and Takodo-yama dome formed. At 30-22 ka, the Kz-CbA eruption in the southern Kozushima and the formation of the southern volcanic chain occurred. After the eruption of Kz-CbA′, the formation of the northern volcanic chain was followed by the eruption of Tenjo-san volcano. In addition, the eruption rate of Kozushima volcano is estimated during the last 30,000 years to be approximately 0.06 km3/1000 y in DRE.

著者

松本 至巨 尾方 隆幸 内川 啓

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.115, no.2, pp.221-235, 2006-04-25 (Released:2009-11-12)

参考文献数

28

被引用文献数

1 1 3

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The Northern Japanese Alps (Chubusangaku National Park) has experienced a dramatic increase in the number of visitors since the boom referred to as “the Japanese hundred mountains”. The Ushiro-Tateyama Range, located in the northernmost area of the Japanese Alps, provides various alpine landscapes such as cirques, asymmetrical ridges, patterned ground, perennial snow patches and alpine plant communities. Data from 6922 trekkers indicate that the distribution of mountain huts and accessibility to trails control their trekking courses, and that the trekkers concentrate in three mountain areas : Shirouma-mountain area, Goryu-mountain area and Kashima-mountain area. These areas are divided by a landform called kiretto, where a col with steep rockwalls lies along a main ridge, operating as a natural obstacle. Such a concentration suggests that human impacts on mountain geoecosystem occur locally, but intensively.

著者

神谷 泉 小荒井 衛 関口 辰夫 佐藤 浩 中埜 貴元 岩橋 純子

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.122, no.5, pp.854-874, 2013-10-25 (Released:2013-10-31)

参考文献数

31

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SAR interferometry is widely used for dense measurements of surface displacements caused by earthquakes, but the method cannot be applied if displacements are too large. The near-epicentral area of the Iwate–Miyagi Nairiku Earthquake in 2008 is an inapplicable case. Therefore, we applied photogrammetry to measure surface displacement caused by the earthquake. The maximum horizontal and vertical displacements were found to be 5.3 m and 2.9 m, respectively. We recognized three lines where displacement changes abruptly. The displacement distribution is like that of a reverse fault along the first line (A–B), an east-rising fault along the second line (F–G–H; west of line A–B), and a left-lateral fault along the third line (B–C; between line A–B and line F–G–H). The earthquake source fault reaches or approaches the ground surface at line A–B, with slippage decreasing toward the ground surface. The fault-like large surface deformation found north of the Aratozawa Dam is on the first line. The fault-like deformation was caused by the motion of the earthquake source fault, and the relative displacement of the fault-like deformation was enlarged by local causes. A gravitational mass movement found north of the fault-like deformation is one cause. Because the width of the rising area is small, only 3.5 km, at the southwestern side of line B–C, the slip of the earthquake source fault is mainly distributed near the ground. Line F–G–H suggests the existence of a geological structure that causes the abrupt changes of vertical displacement without a horizontal displacement, for example a high-angle fault.  We assumed: (1) the slip on the main fault is distributed only in a shallow area at the southern part of the main fault and only in a deep area at the northern part; and, (2) the difference of slip caused two lateral faults between southern and northern parts. The assumption qualitatively explains many observation results, such as why there is an abrupt change of horizontal displacement along line B–C and why line F–G–H has a convex part to the east. We found a correlation between the occurrence of large landslides and abrupt changes of displacement, in other words large surface strain. The following mechanisms are possible causes of the correlation: (1) stress from surface strain increased large landslides; (2) faults (not only the main fault) may exist under the focused areas, rupture of faults caused both large surface strain and large seismic motion, and seismic motion induced large land slides. We also found that landslides and slope failures occurred densely over the slipping area on the main fault, based on the assumptions in the previous paragraph. Because photogrammetric measurements need interactive observations, we could avoid observations on possible embanking areas. Because photogrammetry allows intensive measurements at interesting areas, we revealed a two flexure-like distribution of vertical displacement. Therefore, photogrammetry is an effective method for measuring surface displacement caused by an earthquake.

著者

須貝 俊彦 松島(大上) 紘子 水野 清秀

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.6, pp.921-948, 2013-12-25 (Released:2014-01-16)

参考文献数

109

被引用文献数

18 19

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The Kanto Plain, the hinterland of the Tokyo metropolitan area, is the largest plain in Japan and is characterized by marked marine and fluvial terrace levels that developed during Marine oxygen Isotope Stage (MIS) 5. Late Quaternary topographical changes to the plain have been controlled by concurrent tectonic activity and glacio-eustatic sea-level changes. The shoreline at the maximum transgression of MIS 11, 9, 7, 5 and 1 is reconstructed based on the distribution of marine sediments revealed by many geologic columnar sections and marine terrace surfaces. A comparison of the magnitudes of the last five full-interglacial transgressions above shows that magnitude decreased over the long term. This is due probably to changes in the tectonic regime in the Kanto basin, from subsidence to uplift along with the northward migration of the depositional center, probably associated with changes in the motion of the Philippine Sea Plate and the collision with the Izu peninsula. The marine transgression has also been controlled by fluvial processes, especially in the north-western part of the plain because of high sediment inputs from the Tone, Ara, and Watarase rivers. Aggradation coupled with regional uplift since MIS 5.4 limited the MIS 1 marine transgression within the incised valley formed during MIS 2. As a result, the Paleo Tokyo bay, which was connected directly with the Pacific Ocean, disappeared. Instead, a large shallow submarine area of about 10,000km2 emerged. The northern part of the present Tokyo bay is still subsiding and large volumes of water and sediments have been concentrated in the bay area during the Holocene. Such natural environmental conditions enable supplies of natural resources, such as fresh water, fertile soil, and flat land for the development of greater Tokyo.

著者

山本 伸次

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.6, pp.963-998, 2010-12-25 (Released:2011-03-17)

参考文献数

99

被引用文献数

20 20 13

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This is a brief review of tectonic erosion originally proposed by von Huene and Scholl (1991) who have spent most of their academic careers studying marine geophysics along the circum-Pacific subduction zone. Accretionary complex is considered to be formed by trench turbidite resting on the subducting oceanic plate that accretes against a hanging wall at a shallow or deep crustal level due to off-scraping or underplating. This concept was introduced to Japan in the early 1970s and was developed in great detail to propose a new paradigm for accretionary geology that involves ocean-plate stratigraphy. Later, identification of accretionary complex on-land in Japan became the mainstream. A new idea refutes the common occurrence of an on-going accretion process forming accretionary complexes along the circum-Pacific subduction zone. Instead, the concept of tectonic erosion has emerged to explain extensive crustal thinning and subsidence as an on-going process destroying the hanging wall of an older subduction complex or even the basement of the overriding plate at more than half of the active trench. During the past three decades, marine geophysicists and geologists have documented tectonic erosion as a more common process than the formation of an accretional complex in subduction zones, and supeculate that a large volume of the continental crust is subducted into the mantle at both accretionary and erosive convergent margins. A simple calculation of the amount of subducting continental material versus arc producted at the subduction zone suggests a balance, resulting in no growth of continental crust at present (e.g. Clift and Vannuchi, 2004; Scholl and von Huene, 2007). However, considering direct subduction of intra-oceanic arcs and foundering of the continental lower crust into the mantle, we must conclude there is negative growth of the continental crust on the Earth at present.

著者

丸山 茂徳 小宮 剛

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.120, no.5, pp.869-876, 2011-10-25 (Released:2012-01-17)

参考文献数

13

被引用文献数

4 12

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グリーンランド・イスア地域は緑色片岩–角閃岩相程度の広域変成作用を受けてはいるが,見事な枕状溶岩の構造を普遍的に残している。枕の形,急冷縁,マトリックスの構造などが明瞭に残存している。枕状溶岩の存在は38億年前に海洋がすでに存在していたことを示すが,さらに付随する岩石との関係から,典型的な付加体の存在が認められる。海洋プレート層序は付加体が現在の西太平洋地域のような海洋内島弧の一部として形成されたことを示す。付加体の形成は北から南側にプレートが沈み込んだことを示す。さらにCA系列の花崗岩帯が付加体を貫く。これらは太平洋型造山運動が38-37億年前に機能していた証拠である。玄武岩の化学組成から,起源マントルの組成と温度が推定される。海洋の温度を100℃以下とみなすと,38-37億年前の海洋プレートの剛性度とプレートの厚さが求められ,プレートテクトニクスが38億年前にすでに機能していたことがわかる。

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.4, pp.Cover04_01-Cover04_02, 2018-08-25 (Released:2018-10-05)

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長野県・諏訪湖(面積12.8 km2,平均水深4.7 m)では,冬季に全面結氷して数日後に「御神渡」現象が現れることが多い.御神渡は夜間と日中の気温変化で湖氷が収縮・膨張を繰り返すと,亀裂部分の氷がせり上がることで生じる.古来から,その道筋は諏訪大社の男神(上社)が女神(下社)に会いに行った証であるとの伝説がある.また,御神渡が出現すると,その道筋によってその年の農作物の豊凶や景気を占う拝観式が執り行われてきた. こうした諏訪湖の結氷日や御神渡出現日,拝観日の詳細な記録が1444年以降ほぼ連続的に残されており,過去574年間の冬の気候変動を知る貴重な史料として世界的にも注目されている.厳しい寒冬年には12月中に御神渡が出現することがあるが,暖冬年の場合,2月になって出現したり,御神渡ができずに明海(あけのうみ)となる.近年は温暖化の影響で湖が結氷しにくくなっているが,今冬(2017-2018年)は5年ぶりに御神渡が出現し,2月5日に八剣神社の宮坂清・宮司らによって拝観の儀式が執り行われた.写真では,湖の東岸から北岸(赤砂崎)にうねりながら延びる御神渡の様子がよくわかる.(写真:宮坂 清 2018年1月31日撮影;解説:三上岳彦)