Precambrian Geology

Precambrian time is divided into the Archean Eon and Proterozoic Eon. After the Precambrian, geologic time intervals are commonly subdivided on the basis of the fossil record. The paucity of Precambrian fossils, however, precludes the creation of small-scale subdivisions in this time period. Instead, relative chronologies of events have been produced for different regions based on such field relationships as unconformities and crosscutting dikes. These field relationships, combined with the isotopic age determinations of specific rocks, allow for some correlation between neighbouring regions. The International Commission on Stratigraphy (ISC) and International Union of Geological Sciences (IUGS) divide the Archean Eon into the Eoarchean, Paleoarchean, Mesoarchean, and Neoarchean eras. Likewise, they divide the Proterozoic Eon into the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic eras. These definitions are based on isotopic age determinations. The Archean and Proterozoic eons within Precambrian time are very different and must be considered separately. The Archean-Proterozoic boundary constitutes a major turning point in Earth history. Before that time the crust of the Earth was in the process of growing, and so there were no large, stable continents. Afterward, when such continents had emerged, orogenic belts were able to form on the margins of and between continental blocks.

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