Precambrian Era

Precambrian time, interval of geologic time from 3.8? billion years ago, the age of the oldest known rocks, to 540 million years ago, the beginning of the Cambrian Period. 

The Precambrian is divided into two eons, the Archean and the Proterozoic, with the time boundary between them at 2.5 billion years ago. Precambrian rocks are exposed typically in vast areas several hundreds or even thousands of kilometres across; examples of such include the Canadian, Baltic, Indian, and West Australian shields, as well as the Ukrainian shield. Because there are few definitive fossils in Precambrian rocks, dating is provided by the isotopic analysis of pairs of radioactive elements. 


In the early Precambrian, heat production by the breakdown of radioactive isotopes was several times higher than it is today. Just as it does today, however, the bulk of the heat must have escaped from the Earth's mantle through oceanic ridges in connection with very rapid seafloor spreading and consequent subduction by some form of primitive plate tectonics. (Subduction is the process in which one lithospheric plate descends beneath another at regions where the two plates converge.) 

The result of these processes was the formation of three distinctive Archean rock groups: (1) voluminous island arcs, called greenstone belts, such as those in western Australia, Zimbabwe, India, and southern Canada, where the rocks are the host of many economic mineral deposits (e.g., gold, chromium, nickel, copper, and zinc); (2) rare slabs of ophiolite-like ocean floor that were thrust over the arc rocks, as in the Barberton Mountains of South Africa and in the Yellowknife district of northwest Canada; and (3) abundant tonalites and granites that are chemically comparable to Mesozoic equivalents (i.e., those formed between about 245 and 66 million years ago) in the Andes. 

These tonalites and granites were overthrust by the greenstone belts and thus were pushed down into the deep continental crust, where they were highly deformed and metamorphosed to become granulites and gneisses. Uplifted representatives of these rocks can be seen today in western Greenland, northern Finland, and southern India.  

The Archean was a time of rapid crustal growth and thickening. This activity resulted in uplift and erosion, which in turn gave rise to deposition of clastic sediments in major sedimentary basins, such as the late Archean Witwatersrand basin in South Africa and the early Proterozoic Huronian basins near the Great Lakes of North America. 

The Archean-Proterozoic boundary marked an important turning point in continental evolution, for it shows evidence of the beginning of what might be termed "modern-style" plate tectonics. The Archean magmatic arcs had coalesced to form the first major continent or continents around which shelf-type sediments were subsequently deposited; also, ocean floor was subducted, giving rise to Andean-type granitic batholiths along active continental margins. 
Consequently, continents were able to drift and collide with one another to form the first linear Himalayan-type mountain belts. Examples of these early Proterozoic collisional orogens are the Wopmay, Wollaston, and Labrador belts in Canada, the Ketilidian in southwest Greenland, and Svecofennian in Finland. It is likely that the formation of so many collisional orogenic belts at this time gave rise to a supercontinent by about 1.5 billion years ago. 
The breakup of this enormous landmass resulted in the formation of individual continents and new oceanic crust, the subduction of which produced new collisional orogenic belts, as, for example, the Grenville, which extends along the eastern side of North America and which persisted from 1.5 to 1 billion years ago. The remnants of many island arcs 1.1 billion to 500 million years old have been found in Saudi Arabia and Egypt. 
Such island arcs resemble those in Indonesia today. By the end of the Precambrian a new supercontinent had probably formed. On and around it there existed many sedimentary basins (e.g., the Sinian in China) that were filled with conglomerates and sandstones derived through erosion of the surrounding mountainous collisional belts. 

Certain types of sediment bear testimony to the climatic conditions that prevailed during the Precambrian. Most important are the tillites (glacial sediments left behind by retreating glaciers), especially those deposited in North America and South Africa 2.3 billion years ago. The most extensive glaciation in Earth history occurred between 1 billion and 600 million years ago, when tillites were laid down in most continental areas. 

The Precambrian was originally defined as the era that predated the emergence of life in the Cambrian Period. It is now known, however, that life on Earth began by the early Archean and that fossilized organisms became more and more abundant throughout Precambrian time. Both centimetre-size stromatolites (sheetlike mats deposited by algae) and millimetre- to micrometre-size carbonaceous spheroids occur in well-preserved sediments as old as 3.5 billion years. 
Archean organisms were prokaryotes that could survive the high radiation levels in the early anoxygenic atmosphere that had no ozone screen, and they were succeeded in the Proterozoic by eukaryotes that used oxygen for their growth in the increasingly oxygenic atmosphere. Soft-bodied organisms without skeletons began to appear toward the end of the Precambrian; they were the forerunners of the metazoans (multicelled organisms whose cells are differentiated into tissues and organs) that proliferated throughout the Phanerozoic and eventually gave rise to primates and the human species. 

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