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Precambrian
The Precambrian or Pre-Cambrian; sometimes abbreviated pЄ is the largest span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale. It spans from the formation of Earth about 4.6 billion years ago (Ga) to the beginning of the Cambrian Period, about million years ago (Ma), when hard-shelled creatures first appeared in abundance. The Precambrian is so named because it precedes the Cambrian, the first period of the Phanerozoic Eon, which is named after Cambria, the classical name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of geologic time.
==Overview==
Relatively little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history, and what is known has largely been discovered from the 1960s onwards. The Precambrian fossil record is poorer than that of the succeeding Phanerozoic, and those fossils present (e.g. stromatolites) are of limited biostratigraphic use.〔James Monroe and Reed Wicander, ''The Changing Earth'', 2nd ed, (Belmont: Wadsworth Publishing Company, 1997), p. 492.〕 This is because many Precambrian rocks have been heavily metamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.〔
It is thought that the Earth itself coalesced from material in orbit around the Sun roughly 4500 Ma, or 4.5 billion years ago (Ga), and may have been struck by a very large (Mars-sized) planetesimal shortly after it formed, splitting off material that formed the Moon (see Giant impact hypothesis). A stable crust was apparently in place by 4400 Ma, since zircon crystals from Western Australia have been dated at 4404 Ma.〔(【引用サイトリンク】 title=Zircons are Forever )〕
The term ''Precambrian'' is recognized by the International Commission on Stratigraphy as a general term including the Archean and Proterozoic eons.〔http://www.stratigraphy.org/upload/bak/chron.htm Ch. 9 "Chronostratigraphic Units," note C7 and Table 4〕 It is still used by geologists and paleontologists for general discussions not requiring the more specific eon names. It was briefly also called the ''Cryptozoic'' eon.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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