The Precambrian covers approximately 4 billion years (and 87%) of Earth history.
The Precambrian is divided into 2 eons:
The Precambrian is not well known or completely understood. Why?
Most information on the Precambrian is from cratons - large portions of continents which have not been deformed since Precambrian or Early Paleozoic time.
The most extensive exposures of Precambrian rocks are in geologically stable regions of continents called shields.
Example = Canadian shield in North America. Mostly igneous and metamorphic rocks; few sedimentary rocks. Overlying sedimentary rocks were scraped off by glaciers during last Ice Age.
Stable regions of the craton where shields are covered by sedimentary rocks are called platforms.
Precambrian rocks are often called basement rocks because they lie beneath a covering of fossil-bearing sedimentary strata.
Various Precambrian provinces can be delineated within the North American continent, based on radiometric ages of rocks, style of folding, and differences in trends of faults and folds.
By about 4 b.y. ago, the Earth had probably cooled sufficiently for plate formation.
Once plate tectonics was in progress, it generated crustal rock that could be partially melted in subduction zones and added to the continental crust.
Continents also increased in size by addition of microcontinents along subduction zones.
Greater heat in Archean would have caused faster convection in mantle, more extensive volcanism, more midoceanic ridges, more hot spots, etc.
Growth of volcanic arcs next to subduction zones led to formation of greenstone belts.
The major types of Archean rocks on the cratons are:
Pillow basalts (look for large rounded structures). Also note glacial striations.
Greenstones are mostly found in trough-like or synclinal belts.
There is a specific sequence of rock types in greenstone belts. These include:
Granulites are present between greenstone belts.
Generalized cross-section through two greenstone belts. Note sequence of rock types and relationships between granulites and the greenstones.
By 2.8 billion years ago, Earth had cooled sufficiently for glaciation to occur. Earth's earliest glaciation is recorded in 2.8 billion year-old sedimentary rocks in South Africa.
October 29, 2005