DEFINITION OF PRECAMBRIAN

Comprising nearly 90 percent of geological

time, the Precambrian eon includes the time interval in which

all rocks older than 544 million years old formed. The Precambrian

is preceded by the Hadean eon, representing the period

during which the Earth and other planets were accreting and

no rocks are preserved and is succeeded by the Cambrian, the

dawn of advanced life on Earth. It is divided into two eras

including the Archean, ranging in age from the oldest known

rocks at about 4.0 billion years old, to 2.5 billion years ago,

and the Proterozoic, ranging from 2.5 billion years ago until

540 million years ago. The Archean is further divided into the

Early (4.0 Ga–3.0 Ga) and Late (3.0 Ga–2.5 Ga), and the Proterozoic

is divided into the Early or Paleoproterozoic (2.5

Ga–1.6 Ga), Middle or Mesoproterozoic (1.6 Ga–1.3 Ga), and

Late or Neoproterozoic (1.3 Ga–0.54 Ga).

Most Precambrian rocks are found in cratons, areas of

generally thick crust that have been stable since the Precambrian,

and exhibit low heat flow, subdued topography, and

few earthquakes, and many also preserve a thick lithospheric

keel known as the tectosphere. Exposed parts of Precambrian

cratons are known as shields. Many of the rocks in cratons

are preserved in granite-greenstone terrains, fewer are preserved

as linear high-grade gneiss complexes, and still fewer

form relatively undeformed platformal or basinal volcanosedimentary

sequences resting on older Precambrian rocks. Platformal

sequences form a thin veneer over many older

Precambrian terrains, so geological maps of cratons and continents

show many essentially flat-lying platformal units, but

these are volumetrically less significant than the underlying

sections of the crust. Many other areas of Precambrian rocks

are found as linear tectonic blocks within younger orogenic

belts. These probably represent fragments of older cratons

that have been rifted, dispersed, and accreted to younger orogens

by plate tectonic processes, some traveling huge dis-

tances from the rocks adjacent to them which they initially

formed in their primary tectonic settings.

The Precambrian is the most dramatic of all geological

eons. It marks the transition from the accretion of the Earth

to a planet that has plate tectonics, a stable atmosphereocean

system, and a temperature range all delicately balanced

in a way that allows advanced life to develop and persist on

the planet. The planet has been cooling steadily since accretion

and was producing more heat by radioactive decay in the

Precambrian than it has been since. However, it is uncertain if

this greater amount of heat significantly heated the mantle

and crust, or if this additional heat was simply lost faster by

the present style of plate tectonics. It is likely that more rapid

seafloor spreading or a greater total length of oceanic ridges

with active volcanism was able to accommodate this higher

heat flow, keeping mantle and crustal temperatures close to

what they have been in the Phanerozoic.

Understanding of the development of life in the Precambrian

has been undergoing rapid advancement, and the close

links between life, atmospheric chemistry, plate tectonics, and

global heat loss are only recently being explored. Many mysteries

remain about the events that led to the initial creation

of life, its evolution to more complex forms, and the eventual

development of multicelled complex organisms at the end of

the Precambrian.

See also ARCHEAN; CONTINENTAL CRUST; CRATONS;

GREENSTONE BELTS; HADEAN; LIFE’S ORIGINS AND EARLY EVOLUTION;

PROTEROZOIC; TECTOSPHERE.

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