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An ophiolite is a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. Ophio is Greek for ''snake'' (ὄφις), and lite means ''stone'' from the Greek lithos (λίθος), after the often-green-color rocks (spilites and serpentinites) that make up many ophiolites. The term ophiolite was originally used by Alexandre Brongniart 〔Brogniart, A. (1813)〕 for an assemblage of green rocks (serpentine, diabase) in the Alps; Gustav Steinmann〔Steinmann, G (1927)〕 later modified its use to include serpentine, pillow lava, and chert ("Steinmann's trinity"), again based on occurrences in the Alps. The term was little-used in other areas until the late 1950s to early 1960s, with the recognition that this assemblage provided an analog for oceanic crust and the process of seafloor spreading. This recognition was tied to two events: (1) the observation of magnetic anomaly stripes on the seafloor, parallel to oceanic ridge systems, interpreted by Frederick Vine and Drummond Matthews 〔Vine F.J. and Matthews D.H. (1963)〕 to represent the formation of new crust at the oceanic ridge and its subsequent symmetric spreading away from that ridge, and (2) the observation of a sheeted dike complex within the Troodos ophiolite (Cyprus) by Ian Graham Gass and co-workers,〔Gass, I.G. (1968)〕 which must have formed by repetitive extension of crust and intrusion of magma, resulting in a formation consisting of 100% dikes with no older wall rocks preserved within the complex. Moores and Vine〔Moores E.M. and Vine, F.J. (1971)〕 concluded that the sheeted dike complex at Troodos could form only by a process similar to the seafloor spreading proposed by Vine and Matthews.〔 Thus, it became widely accepted that ophiolites represent oceanic crust that had been emplaced on land. Their great significance relates to their occurrence within mountain belts such as the Alps or the Himalayas, where they document the existence of former ocean basins that have now been consumed by subduction. This insight was one of the founding pillars of plate tectonics, and ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts. ==Stratigraphy and definition== The stratigraphic sequence observed in ophiolites corresponds to the lithosphere-forming processes at mid-oceanic ridges: * Sediments: muds (black shale) and cherts deposited since the crust formed. * Extrusive sequence: basaltic pillow lavas show magma/seawater contact. * Sheeted dike complex: vertical, parallel dikes that fed lavas above. * High level intrusives: isotropic gabbro, indicative of fractionated magma chamber. * Layered gabbro, resulting from settling out of minerals from a magma chamber. * Cumulate peridotite: dunite-rich layers of minerals that settled out from a magma chamber. * Tectonized peridotite: harzburgite/lherzolite-rich mantle rock. The Penrose field conference on ophiolites in 1972 redefined the term ''ophiolite'' to include only the igneous rocks listed above, excluding the sediments formed independently of the crust they sit on. This definition has been challenged recently because new studies of oceanic crust by the Integrated Ocean Drilling Program and other research cruises have shown that in situ ocean crust can be quite variable, and that in places volcanic rocks sit directly on peridotite tectonite, with no intervening gabbros. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ophiolite」の詳細全文を読む スポンサード リンク
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