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In geology, a rift is a linear zone where the Earth's crust and lithosphere are being pulled apart〔(Rift valley: definition and geologic significance ), Giacomo Corti, The Ethiopian Rift Valley〕〔(Decompressional Melting During Extension of Continental Lithosphere ), Jolante van Wijk, MantlePlumes.org〕 and is an example of extensional tectonics.〔(Plate Tectonics: Lecture 2 ), Geology Department at University of Leicester〕 Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a rift valley, which may be filled by water forming a rift lake. The axis of the rift area may contain volcanic rocks, and active volcanism is a part of many, but not all active rift systems. Major rifts occur along the central axis of most mid-ocean ridges, where new oceanic crust and lithosphere is created along a divergent boundary between two tectonic plates. ''Failed rifts'' are the result of continental rifting that failed to continue to the point of break-up. Typically the transition from rifting to spreading develops at a triple junction where three converging rifts meet over a hotspot. Two of these evolve to the point of seafloor spreading, while the third ultimately fails, becoming an aulacogen. ==Geometry== Most rifts consist of a series of separate segments that together form the linear zone characteristic of rifts. The individual rift segments have a dominantly half-graben geometry, controlled by a single basin-bounding fault. Segment lengths vary between rifts, depending on the elastic thickness of the lithosphere. Areas of thick colder lithosphere, such as the Baikal Rift have segment lengths in excess of 80 km, while in areas of warmer thin lithosphere, segment lengths may be less than 30 km. Along the axis of the rift the position, and in some cases the polarity (the dip direction), of the main rift bounding fault changes from segment to segment. Segment boundaries often have a more complex structure and generally cross the rift axis at a high angle. These segment boundary zones accommodate the differences in fault displacement between the segments and are therefore known as accommodation zones. Accommodation zones take various forms, from a simple relay ramp at the overlap between two major faults of the same polarity, to zones of high structural complexity, particularly where the segments have opposite polarity. Accommodation zones may be located where older crustal structures intersect the rift axis. In the Gulf of Suez rift, the Zaafarana accommodation zone is located where a shear zone in the Arabian-Nubian Shield meets the rift. 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「Rift」の詳細全文を読む スポンサード リンク
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