The Evolution of Continental Crust
Department of Geology and Geography
Denison University, Granville, Ohio 43023
The early history of Earth is difficult to interpret due to the lack of material preserved from the first 500 Ma of its formation. Early Earth was extremely hot due to impacting of large bodies and an increase of heat as a result of radioactive decay. Widespread melting and the formation of a magma ocean was a result of the excessive heat. A homogeneous transient crust of komatiite was formed by crystallization of the magma ocean. Heat generaged by the breakdown of radioactive elements from the interior of the Earth was partially removed by convection initiating an early form of plate tectonics. Tonalite, trondhjemite, and granodiorite (TTG) were formed by the subduction of komatiite and large amounts of water, resulting in the first moderately buoyant material. Crust formation at oceanic ridges varied in composition from komatiite to basalt due to decreased melting of the mantle with decreasing temperatures. Subduction of hydrous basalt under an area of TTG accumulation resulted in the production of granite by partial melting of the subducted material and contamination of the magma by surrounding TTG. Increased production of granite and accumulation of the buoyant crust through accretion resulted in the formation of the present continental crust.
The Significance of Ophiolites
Ophiolites represent fragmented slabs of oceanic crust that have escaped recycling by subduction. They are composed of a sequence of mafic and ultramafic rocks formed at oceanic spreading centers. For the preservation of ophiolites it is necessary that they be placed on continental lithosphere which is too buoyant to be subducted. Ophiolite emplacement is believed to occur by obduction, which consists of a fragment of oceanic crust being thrust over continental material instead of being subducted. Ophiolites were first discovered in the geologic rock record at 2.0 Ga and reasons concerning the lack of older ophiolites are disputed. It is a possibility that greenstone belts represent preserved Archean oceanic crust, but that model is not widely accepted. Ophiolites provide the best means of gathering information about processes which occurred early in geologic history and can provide a possible model for the characteristics of early oceanic crust.