Precambrian era

Precambrian name of a major division of geologic time (see Geologic Timescale , table), from c.5 billion to 570 million years ago. It is now usually divided into the Archean and Proterozoic eons. Precambrian time includes 80% of the earth's history.

Precambrian rocks are mostly covered by rock systems of more recent origin, but where visible they commonly display evidence of having been altered by intense metamorphism . Precambrian rocks often occur in shields, which are large areas of relatively low elevation that form parts of continental masses. One of the largest exposed areas of early Precambrian rocks is the Canadian Shield, where geologist Sir William Logan did his pioneer work. It covers most of Greenland, extends over more than half of Canada, and reaches into the United States as the Superior Highlands and the Adirondack Mts.

The rocks of this region, and of the early Precambrian as a whole, are generally granite , schist , or gneiss . The most notable formations are the Keewatin and Coutchiching of Minnesota and the adjoining part of Canada; the Grenville of Ontario, which, however, may be late Precambrian; and the widely distributed Laurentian. The Keewatin series of rocks is composed chiefly of metamorphosed lava, with some sediments; the Coutchiching series is chiefly of sedimentary gneisses and schists. The Grenville limestone, marble, gneiss, and quartzite are predominantly metamorphosed sediments; the Laurentian gneiss and granite are probably younger than the other series, having been forced up through the Grenville as igneous rock. After the appearance of the Laurentian, the Temiskaming, or Sudburian, sediments were deposited, and a second series of gneisses and granites, the Algoman, was formed.

Elsewhere in North America, early Precambrian rocks are exposed in the Grand Canyon of Arizona and in the Teton Range of Wyoming. Among the other shield areas composed of early Precambrian rocks are the Angara Shield in Siberia, the Australian Shield, the Baltic Shield in Europe, the Antarctic Shield, and the African Shield comprising most of the African continent. In South America, the Amazon River basin separates the Guiana and the Brazilian shields. Fossils have been reported from this era, but few have been found in strata universally acknowledged to be early Precambrian. Evidence such as bacteria and algallike spheroids, supports the belief that rudimentary life existed. During the early Precambrian, radioactive heat from the new planet may have been so great that little permanent crust could survive.

By the latter Precambrian, heat dissipated enough to allow the continental crust to form; crustal rifting, mountain building, and volcanic activity then dominated, as did sedimentation. The life of the late Precambrian is poorly represented by fossils, but a few invertebrates including creatures resembling jellyfish and worms have been discovered. The best evidence that there probably were numerous forms of life is the variety and complexity which suddenly appears in Cambrian fauna. Mineral deposits associated with Precambrian rocks have yielded most of the world's gold and nickel in addition to large quantities of copper, silver, radium, and uranium.

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.

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.