Africa

General considerations

The African continent essentially consists of five ancient Precambrian cratons—Kaapvaal, Zimbabwe, Tanzania, Congo, and West African—that were formed between about 3.6 and 2 billion years ago and that basically have been tectonically stable since that time; these cratons are bounded by younger fold belts formed between 2 billion and 300 million years ago. All these rocks have been extensively folded and metamorphosed (that is, they have been modified in composition and structure by heat and pressure). Precambrian rock outcrops appear on some 57 percent of the continent’s surface, while the rest of the surface consists of largely undeformed younger sediments and volcanic rocks.

The oldest rocks are of Archean age (i.e., about 4 to 2.5 billion years old) and are found in the so-called granite-gneiss-greenstone terrains of the Kaapvaal, Zimbabwe, and Congo cratons. They consist of gray, banded gneisses, various granitoids, and rather well-preserved volcanic rocks that show evidence of submarine extrusion (i.e., emission of rock material in molten form) and formation under high temperatures. The rock type komatiite is particularly diagnostic of these volcanic sequences and is almost exclusively restricted to the Archean Eon. The cratons were tectonically stabilized by voluminous ... (200 of 36,103 words)