The Prephanerozoic or Precambrian Time

This includes some 80% of the earth's history from its birth about 4.6 billion years ago (i.e. 4600 x 106 years , or 4600 Ma). The record in the rocks of the Prephanerozoic is much more obscure than for Phanerozoic times because the lack of datable fossil assemblages in rocks older than 600 to 700 Ma. Correlation must, therefore, be based on physical field criteria and isotopic dating.

Prephanerozoic time may be divided into the oldest rocks, the Archean, followed by the Proterozoic.

The Archean, from 4600 to 2500 Ma, was dominated during its early part by a massive heat flux which was so great that little permanent crust could survive; an oxygen rich atmosphere had not yet developed. It has two main assemblages of rocks:

By Proterozoic times, from 2500 Ma to 545 Ma, heat generation had apparently declined sufficiently to allow much larger masses of continental crust to survive. Importantly, clearly recognisable cratons bordered by well-defined orogenic belts suggested that plate tectonic processes may have been operating much as today.

Early Proterozoic sediments differed from the Archean ones in that they were texturally and compositionally more mature. They included terrigenous (land derived) clastic material, with well sorted and well rounded quartz grains, and non-terrigenous chemical carbonate and evaporate strata. Stromatolites formed by algae or bacteria occurred quite widely in carbonate rocks all of which were deposited in broad shallow seas on stable cratonic areas.

By late Proterozoic times, important large-scale rifting accompanied by eruption of widespread flood basalts was occurring. Climates, as far as they can be deduced, seem not to have been too dramatically different from Phanerozoic ones except that more carbon dioxide may have caused warmer global temperatures, due to the greenhouse effect. There is clear evidence of extremes of both glaciation and aridity. The more the later Prephanerozoic sedimentary record is studied, the more it seems to resemble that of Phanerozoic times, but with the absence of animal remains.