From Academic Kids

Precambrian/Infobox The Precambrian or Cryptozoic is the period of the geologic timescale from the formation of Earth (around 4500 million years before the present [BP]) to the evolution of abundant macroscopic hard-shelled fossils, which marked the beginning of the Cambrian, some 542 million years BP. Remarkably little is known about the Precambrian, despite it making up roughly seven-eighths of the earth's history, and what little is known has largely been discovered in the past four or five decades.

It is thought that the Earth itself coalesced from material in orbit around the sun roughly 4500 million years BP and may have been struck by a very large (Mars-sized) planetesimal shortly after it formed, splitting off material that came together to form the Moon (see Giant impact theory). A stable crust was apparently in place about 4400 million years BP, since the oldest dated terrestrial rocks are about 4400 million years old. It is not known when life originated, but carbon in 3800 million year old rocks from islands off western Greenland may be of organic origin. Well-preserved bacteria older than 3460 million years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the Precambrian.

Excepting a few dubious reports of much older forms from Texas and India, the first complex multicelled lifeforms seem to have appeared roughly 600 million years BP. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 542 and 600 million years BP. These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that timespan. A very diverse collection of forms appeared around 544 million years BP, starting in the latest Precambrian with a poorly understood "small shelly fauna" and ending in the very early Cambrian with a very diverse, and quite modern "Burgess fauna," the rapid radiation of forms called the "Cambrian explosion" of life.

Details of plate motions and such are only hazily known in the Precambrian. It is generally believed that most of the Earth's landmasses collected into a single supercontinent around 1000 million years BP. The supercontinent, known as Rodinia, broke up around 600 million years BP. A number of glacial periods have been identified going as far back as the Huronian epoch, roughly 2200 million years BP. The best studied is the Sturtian-Varangian glaciation, around 600 million years BP, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth".

The atmosphere of the early Earth is poorly known, but it is thought to have been smothered in noxious gases, containing very little free oxygen. The young planet had a reddish tint, and its seas were thought to be olive green. Many materials with insoluble oxides appear to have been present in the oceans for hundreds of millions of years after the Earth's formation. When evolving life forms developed photosynthesis, oxygen began to be produced in large quantities. The oxygen was immediately tied up in chemical reactions, primarily with iron, until the supply of oxidizable surfaces ran out. After that the modern high-oxygen atmosphere developed. Older rocks contain massive banded iron formations that were apparently laid down as iron and oxygen first combined.

A diverse terminology has evolved covering the early years of the Earth's existence, but it is tending to fall out of use as radiometric dating allows plausible real dates to be assigned to specific formations and features. The terms Archean (older than about 2500 million years BP), Proterozoic (2500-600 million years BP), and Neoproterozoic (600-542 million years BP) still have some general currency. Some additional terms are included in the geological time line.

As originally used, everything prior to the Cambrian boundary, which has been placed at various times by various authors (but has now been settled at 542 million years BP). Modern usage would often be as described in the previous paragraph, 600-2500 million years BP.
Roughly from the Cambrian boundary back to about 900 million years BP. Modern usage tends to represent a shorter interval : 542-600 million years BP. The Neoproterozoic corresponds to "Precambrian Z" rocks of older North American geology.
In March 2004, the International Union of Geological Sciences officially defined the term to describe this geologic period. The period begins at the time of deposition of a particular stratigraphic boundary, about 620 million years BP. The period ends at the beginning of the Cambrian, 542 million years BP.
Roughly from 900-1600 million years BP. Corresponds to Precambrian Y rocks of older North American geology.
Roughly from 1600-2500 million years BP. Corresponds to Precambrian X rocks of older North American geology.
Roughly from 2500-3800 million years BP.
Prior to 3800 million years BP. This term was probably intended originally to cover the period before any preserved rocks were deposited. A very few old rock beds seem to be slightly older than 3800 million years.

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External links

et:Eelkambrium fr:Prcambrien it:Precambriano ja:先カンブリア時代 pl:Prekambr pt:Pr-Cambriano zh:隐生宙


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