Precambrian

In geologic time , Precambrian time encompasses the time from Earth's formation, approximately 4.5 billion years ago, until the start of the Cambrian approximately 540 million years ago (mya). Because the Precambrian is not a true geologic eon, era, period, or epoch, geologists often refer to it as Precambrian time (or simply, Precambrian). Precambrian time represents the vast bulk of Earth's geologic history and covers nearly 90% of Earth's history.

Although scientists do not yet know all the exact steps by which the earth formed, cooled, and took on its approximate shape and physical characteristics, a good deal of reliable evidence can be inferred from studies that concentrate on the formation of landmass, oceans , and atmosphere. Astrophysical data—and theories of physics that explain the evolution of physical law and nucelosynthesis—make these studies of Earth's formation both possible and reliable because the same laws of physics and chemistry that exist now operated during the formation of Earth's solar system .

Radiological dating provides overwhelming evidence that dates known terrestrial (Earth origin) rock specimens to more than 3.6 billion years old. Earth and lunar meteorites date to 4.5 billion years.

Precambrian time is subdivided into Hadean time (4.5 billion years ago to 3.8 billion years ago); Archean time (3.8 billion years ago to 2.5 billion years ago; Paleoproterozoic time (2.5 billion years ago to 1.6 billion years ago); Mesoproterozoic time (1.6 billion years ago to 900 million years ago); and Neoproterozoic time (900 million years ago to 540 mya).

Hadean time represents the time during which the solar system formed. During the subsequent course of Precambrian time, Earth's lithospheric plates formed and the mechanisms of geologic change described by modern plate tectonic theory began to occur. During Precambrian time, life arose on Earth. The oldest known fossil evidence (fig tree group fossils in what is now Africa ) dates to early in Archaean time. During the Paleoproterozoic, Earth's primitive atmosphere made a transition to an oxygen rich atmosphere. Soon thereafter in geologic time, i.e. within a few hundred million years, there is evidence of the earliest appearance of eukaryotes (organisms with a true nucleus containing DNA). Evidence of the oldest fossilized animal remains dates to the end of Neoproterozic time.

The extensive debris field that existed in the early solar system assured frequent bombardment of Earth's primitive atmosphere by asteroids and comets . Despite the consuming effects of geological weathering and erosion , evidence of Precambrian time impacts dating almost 2.0 billion years ago have been found in what are now South Africa and Canada.

See also Cambrian Period; Cenozoic Era; Cretaceous Period; Dating methods; Devonian Period; Eocene Epoch; Evolution, evidence of; Fossil record; Fossils and fossilization; Historical geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Miller-Urey experiment; Miocene Epoch; Mississippian Period; Oligocene Epoch; Ordovician Period; Paleocene Epoch; Paleozoic Era; Pennsylvanian Period; Phanerozoic Eon; Pleistocene Epoch; Pliocene Epoch; Precambrian; Proterozoic Era; Quaternary Period; Silurian Period; Tertiary Period; Triassic Period

Precambrian Precambrian rocks are those formed before the beginning of the Cambrian period, about 590 million years ago. The oldest known are dated at rather more than 3800 Ma. They represent about 85 per cent of all geological time. Most are igneous or metamorphic crystalline rocks; many of them have been severely deformed, commonly several times. Their principal outcrops constitute the great Precambrian shields of continental crust upon which later formations were deposited. They provide evidence of the evolution of the Earth and of early life upon it (Fig. 1). Nevertheless, many Precambrian rocks are difficult to interpret, and it is clear that processes no longer operative were once important.

Regional classifications, such as those for the Canadian shield, the Baltic area, or South Africa, identify Precambrian rock units, orogenies, and events including deep burial and metamorphism. Precambrian correlation, classification, and geochronology are primarily based upon radiometric (isotope) dating. Precambrian time is often referred to as the Cryptozoic (hidden life) Eon, as distinct from the Phanerozoic (apparent life) Eon beginning with the Cambrian Period.

The subdivision of Precambrian time is difficult since the basic principles of stratigraphy (including biostratigraphy) used for the Phanerozoic Eon are difficult to apply. Convention has distinguished two Precambrian eras, the older being the Archaean, extending back to the consolidation of the planet, and the younger being the Proterozoic, which began about 2500 million years ago. However, several specialists recognize also an initial eon, the Hadean or Priscoan, for the 1000 million years or so between the consolidation of the planetary mass and Archaean time around 4000 million years ago, while life on earth began.

D. L. Dineley

Bibliography

Cloud, P. (1988) Oasis in space: Earth history from the beginning. Norton, New York.
Goodwin, A. M. (1991) Precambrian geology: the dynamic evolution of the continental crust. Academic Press, London.
Nisbet, E. G. (1987) The young Earth: an introduction to Archaean geology. Allen and Unwin, Boston.
Plumb, K. A. (1991) New Precambrian time scale. Episodes, 14, 139–40.
Stanley, S. M. (1993) Exploring Earth and life through time. W. H. Freeman, Oxford.

Precambrian A name that is now used only informally to describe the longest period of geological time, which began with the consolidation of the Earth's crust and ended with the beginning of the Cambrian Period 590 Ma ago. The Precambrian lasted approximately 4000 Ma; the rocks of this period of geological time are usually altered and few fossils with hard parts or skeletons have been found within them, although Precambrian limestone rocks in Australia, Siberia, and parts of the USA contain stromatolites, which are believed to have been formed by cyanobacteria; frond-like impressions of a supposed plant (Charnia) have also been found. The Ediacaran fauna is late Precambrian. Precambrian rocks outcrop extensively in shield areas such as northern Canada and the Baltic Sea. In modern usage the Precambrian is subdivided into the Proterozoic, Archaean, and Priscoan.

Precambrian A name that is now used only informally to describe the longest period of geological time, which began with the consolidation of the Earth's crust and ended with the beginning of the Cambrian Period 590 Ma ago. The Precambrian lasted approximately 4000 Ma; the rocks of this period of geological time are usually altered, and few fossils with hard parts or skeletons have been found within them, although Precambrian limestone rocks in Australia, Siberia, and parts of the USA contain stromatolites, which are believed to have been formed by cyanobacteria; frond-like impressions of a supposed plant (Charnia) have also been found. Precambrian rocks outcrop extensively in shield areas such as northern Canada and the Baltic Sea. In modern usage the Precambrian is subdivided into the Proterozoic, Archaean, and Priscoan.

Precambrian Describing the time from the formation of the earth, nearly 5 billion years ago, to the start of the Cambrian period, around 570 million years ago. The term `Precambrian' is no longer used for a specific geological time interval, but remains as a general adjective. Precambrian time is now divided into three eons: Hadean, Archaean, and Proterozoic, the latter extending to the start of the present (Phanerozoic) eon. Fossils are rare, although stromatolites indicate that there were flourishing populations of cyanobacteria and other bacteria. However, subsequent metamorphism of Precambrian rocks makes correlation of rocks and events extremely difficult. The largest areas of exposed Precambrian rocks are the shield areas, such as the Canadian (Laurentian) Shield and the Baltic Shield.

Precambrian A name that is now used only informally to describe the longest period of geological time, which began with the consolidation of the Earth's crust and ended with the beginning of the Cambrian Period 542 Ma ago. The Precambrian lasted approximately 4000 Ma; the rocks of this period of geological time are usually altered and few fossils with hard parts or skeletons have been found within them. Precambrian rocks outcrop extensively in shield areas such as northern Canada and the Baltic Sea. In modern usage the Precambrian is subdivided into the Hadian, Archaean, and Proterozoic.

Precambrian A name now used only informally to describe the Priscoan, Archaean, and Proterozoic, which together comprise the longest period of geologic time that began with the consolidation of the Earth's crust and ended approximately 4000 million years later with the beginning of the Cambrian Period around 570 Ma ago. The rocks of this period of geologic time are usually altered and few fossils with hard parts or skeletons have been found within them. Precambrian rocks outcrop extensively in shield areas such as northern Canada and the Baltic Sea.

Precambrian Oldest and longest era of Earth's history, lasting from the formation of the Earth c.4.6 billion years ago to the beginning of a good fossil record c.590 million years ago. Precambrian fossils are extremely rare, probably because the earliest life forms did not have hard parts suitable for preservation. Also, Precambrian rocks have been greatly deformed. Primitive bacteria and cyanobacteria have been identified in deposits that are more than 3 billion years old.

Pre·cam·bri·an / prēˈkambrēən; -kām-/ • adj. Geol. of, relating to, or denoting the earliest eon, preceding the Cambrian period and the Phanerozoic eon. Compare with Cryptozoic. ∎  [as n.] (the Precambrian) the Precambrian eon or the system of rocks deposited during it.