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The Pacific Ocean evolved in the Mesozoic from the Panthalassic Ocean, which had formed when Rodinia rifted apart around 750 Ma. The first ocean floor which is part of the current Pacific Plate began 160 Ma to the west of the central Pacific and subsequently developed into the largest oceanic plate on Earth. The tectonic plates continue to move today. The slowest spreading ridge is the Gakkel Ridge on the Arctic Ocean floor which spreads at less that 2.5 cm/yr while the fastest, the East Pacific Rise near Easter Island, has a spreading rate of over 15 cm/yr. ==History== The Pacific Ocean developed from the Panthalassa, a vast global ocean, following the breakup of the supercontinent Pangaea in the Mesozoic Era, gradually forming Gondwana and Laurasia, which in turn divided.〔(Carmen Arias, "Palaeoceanography and biogeography in the Early Jurassic Panthalassa and Tethys Oceans" ), Universidad Complutense de Madrid. Retrieved 25 June 201.〕 There is no firm date for when the changeover occurred, as the replacement of the sea bed is a continuous process, though reconstruction maps often change the name from Panthalassic to Pacific around the time the Atlantic Ocean began to open. The Panthalassic Ocean first opened around 750 million years ago at the breakup of Rodinia,〔 but the oldest Pacific Ocean floor is only around 180 Ma old. Any older Pacific floor crust has been subducted. During the Jurassic period, four tectonic plates developed in the Pacific Basin: the Kula and the Farallon in the north, the large Pacific Plate in the centre and south and the Phoenix Plate in the far south. The Kula Plate was subducted under the eastern and south-eastern Asian landmass. The northern part of the Farallon was subducted under North America while its southern section, together with the Phoenix Plate, was subducted under South America and the Antarctic. During the late Tertiary period, the Farallon plate broke up, leaving the Juan de Fuca Plate in the north, the Cocos Plate off today's Central America, and the Nazca and Phoenix Plates in the southern Pacific. Charles Darwin had proposed a theory of the slow subsidence of the ocean floor based on his studies of Pacific oceanic islands, seamounts (underwater volcanoes) and guyots (flat-topped seamounts). His observations have been verified and expanded in the development of plate tectonic theory.〔 The tectonic plates continue to move today. The slowest ridge spreading rate is across the Gakkel Ridge in the Arctic Ocean which separates at less than 2.5 cm/yr while the fastest, across the East Pacific Rise near Easter Island, has a spreading rate of over 15 cm/yr.〔("Understanding plate motions" ), USGS. Retrieved 26 June 2013.〕 The Pacific Plate moves generally towards the northwest at between 7 and 11 cm/yr while the Juan De Fuca Plate has an east-northeasterly movement of some 4 cm/yr.〔("Plate Tectonics" ), Pacific Northwest Seismic Network. Retrieved 26 June 2013.〕 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「Geology of the Pacific Ocean」の詳細全文を読む スポンサード リンク
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