特集　火山活動と地殻応力場　中央海嶺のマグマ供給システム

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  • 海野 進
    静岡大学理学部地球科学教室 Department of Geology, Australian National University

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タイトル別名
  • Volcanism and Crustal Stress Field. Magma Plumbing System beneath Mid-Ocean Ridges.
  • チュウオウ カイレイ ノ マグマ キョウキュウ システム

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抄録

Recent progress in studies on magma plubming system beneath mid-ocean ridges is reviewed. Mid-ocean ridges are not continuous, homogeneous series of crests, but are segmented by various topographical offsets of several orders of magnitude. Such topographical ridge segmentation is a surface manifestation of along-strike variations in the pattern of mantle convection and the supply of magma.<BR>Upwelling of the mantle material beneath the spreading center takes the form of a diapir, rather than sheet-like flow. Such diapiric upwelling has an along-axis dimension of several hundred to several tens of kilometers, which corresponds to first-and second-order segments, but some are as small as the scale of third-order segments. Where magma upwelling is intense, hot lithosphere and high magmatic productivity produces thick crusts, resulting in low gravity anomaly.<BR>When the spreading rate is low, mantle flow is cooled on route to the bottom of the lithosphere and ceases to melt at depths. Thus magmas produced beneath slow spreading ridges have low degrees of melting. On the contrary, fast spreading ridges are followed by intense mantle upwelling which melts significantly the mantle column from deeper part of the upper mantle up to just beneath the crust.<BR>Magmatic budget is so large at fast spreading ridges as to maintain large, steady-state magma chambers along the ridge axes with extensions comparable to second-and third-order segments. Such magma chambers have a very small melt pocket one to two kilometers wide and up to several hundred meters thick, underlain by a large mush of crystals and melt. Cumulate layers on the top of the mush sink and deform to S-shaped or downwarping layers as seen in large layered plutonic bodies in some ophiolites. However, low spreading ridges do not have enough supply of magma but only possess short-lived small magma chambers consisting of crystal mush.<BR>Volcanic landforms consist of major topographic features associated with small volcanic edifices : the former is a large shieled volcano several tens of kilometers long and is probably formed by episodic eruptions every several tens of thousand years ; the latter is a small conical lava cone comprising pillow flows which is constructed during a short-period eruption but some are active as long as 1.8 million years which are unequivocally central volcanoes.<BR>Rifting episodes at spreading axes depend on the spreading rate and the width of the volcanic zone. Slow spreading ridges such as Iceland have rifting every 100 years, but fast spreading ridges such as East Pacific Rise rift every year or two. Such rifting episodes continue for several years and are associated with multiple dikes with an injection interval of a month up to a year.<BR>Diapiric mantle upwelling, structure of magma chambers and volcanic landforms seen in mid-ocean ridges resemble those in some large ophiolites such as the Troodos, Cyprus and the Semail, Oman, and post-Teritary volcanism in Iceland.

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  • 地学雑誌

    地学雑誌 103 (5), 498-521, 1994

    公益社団法人 東京地学協会

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