- © 2006 by the Mineralogical Society of America
Tracing the origin of the oceans and the division of the crust into distinct oceanic and continental realms relies on incomplete information from tiny vestiges of surviving oldest crust (>3.6 billions years old). Billions of years of tectonism, melting and erosion have obliterated the rest of that crust. Oceans and continental crust already existed almost four billion years ago because water-laid sedimentary rocks of this age have been found and because tonalites dominate in gneissic sequences dating from this period. Tonalites are igneous rocks produced by partial melting of hydrated basaltic crust at convergent plate boundaries. Collisional orogenic systems produced granites by partial melting of tonalite crust 3.7-3.6 billion years ago. Thus the oldest rocks can be understood in terms of a plate tectonic regime. The chemistry of even older detrital zircons may argue for continental crust and oceans back to 4.4 and 4.2 billion years ago, respectively. Maybe only within the first 200 million years was Earth's surface hot, dry and predominantly shaped by impacts.