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.
It is thought that the Earth itself coalesced from material in orbit around the sun roughly 4500 Ma 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 by 4400 Ma, since zircon crystals from Western Australia have been dated at 4404 Ma.
The term Precambrian is somewhat dated, but is still in common use among geologists and paleontologists. It was briefly also called the Cryptozoic eon. It seems likely that it will eventually be replaced by the preferred terms Proterozoic, Archaean, and Hadean, and become a deprecated term. (See geologic time scale.)
Excepting a few contested reports of much older forms from USA and India, the first complex multicelled life forms seem to have appeared roughly 600 Ma. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 542 and 600 Ma. 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 Ma, 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.
The atmosphere of the early Earth is poorly known, but it is thought to have been smothered in reducing gases, containing very little free oxygen. The young planet had a reddish tint, and its seas are thought to have been 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, causing an ecological crisis sometimes called the Oxygen Catastrophe. 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.
It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by GSSPs. The Precambrian could be divided into five "natural" eons, characterized as follows.