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The supercontinent cycle is the quasi-periodic aggregation and dispersal of Earth's continental crust. There are varying opinions as to whether the amount of continental crust is increasing, decreasing, or staying about the same, but it is agreed that the Earth's crust is constantly being reconfigured. One complete supercontinent cycle is said to take 300 to 500 million years. Continental collision makes fewer and larger continents while rifting makes more and smaller continents. ==Description== The most recent supercontinent, Pangaea, formed about 300 million years ago. There are two different views on the history of earlier supercontinents. The first proposes a series of supercontinents: Vaalbara (~3.6 to ~2.8 billion years ago); Ur (~3 billion years ago); Kenorland (~2.7 to ~2.1 billion years ago); Columbia (~1.8 to ~1.5 billion years ago); Rodinia (~1.25 billion to ~750 million years ago); and Pannotia (~600 million years ago), whose dispersal produced the fragments that ultimately collided to form Pangaea. The second view (Protopangea-Paleopangea), based on both palaeomagnetic and geological evidence, is that supercontinent cycles did not occur before about 0.6 Ga (during the Ediacaran Period). Instead, the continental crust comprised a single supercontinent from about 2.7 Ga until it broke up for the first time, somewhere around 0.6 Ga. This reconstruction〔Piper, J.D.A. “A planetary perspective on Earth evolution: Lid Tectonics before Plate Tectonics.” Tectonophysics. 589 (2013): 44-56.〕 is based on the observation that if only small peripheral modifications are made to the primary reconstruction, the data show that the palaeomagnetic poles converged to quasi-static positions for long intervals between about 2.7-2.2, 1.5-1.25 and 0.75-0.6 Ga, .〔Piper, J.D.A. “Continental velocity through geological time: the link to magmatism, crustal accretion and episodes of global cooling.” Geoscience Frontiers. 4 (2013): 7-36.〕 During the intervening periods, the poles appear to have conformed to a unified apparent polar wander path. Thus the paleomagnetic data are adequately explained by the existence of a single Protopangea-Paleopangea supercontinent with prolonged quasi-integrity. The prolonged duration of this supercontinent could be explained by the operation of ''lid tectonics'' (comparable to the tectonics operating on Mars and Venus) during Precambrian times, as opposed to the plate tectonics seen on the contemporary Earth.〔 The kinds of minerals found inside ancient diamonds suggest that the cycle of supercontinental formation and breakup began roughly 3.0 billion years ago. Before 3.2 billion years ago only diamonds with peridotitic compositions (commonly found in the Earth's mantle) formed, whereas after 3.0 billion years ago eclogitic diamonds (rocks from the Earth's surface crust) became prevalent. This change is thought to have come about because the process of subduction and continental collision introduced eclogite into subcontinental diamond-forming fluids. The hypothesized supercontinent cycle is complemented by the Wilson cycle named after plate tectonics pioneer J. Tuzo Wilson, which describes the periodic opening and closing of ocean basins. Because the oldest seafloor material found today dates to only 170 million years old, whereas the oldest continental crust material found today dates to at least 4 billion years old, it makes sense to emphasize the much longer record of the planetary pulse that is recorded in the continents. 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「Supercontinent cycle」の詳細全文を読む スポンサード リンク
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