3. Ocean Basin and Continental Evolution

Earth’s surface can be readily divided into oceanic and continental areas. Each is characterized by particular rock types and compositions, which can be approximated by the terms mafic (basaltic) for ocean crust and felsic (granitic) for continental crust. Oceanic crust is thin (ca. 10 km) and relatively dense while continental crust is rather thicker (ca. 40 km) and somewhat less dense. Both types “float” on the solid, higher density mantle of ultramafic rock. The thinner ocean crust sits at lower elevations than the continents, providing the basins which have filled with water, that is, the oceans. At the earliest stages of Earth’s history there was little or no continental crust, but a global ocean crust, thinly covered by the primitive ocean. This situation would have existed even before the bulk of volatile degassing, implying that the impact of late accreting meteoritic material would have largely been into the global ocean. This is an important factor in considering the chemical processes that would have occurred during degassing of most of the early Earth’s volatiles. It is likely that the earliest prebiotic molecules were formed at the same time as much of the primordial degassing. Production of continental crust by geochemical (igneous) processing of oceanic and some mantle material has taken place over most of Earth’s history. The relative stability of continental crust results in its preservation and accumulation over time, leading to the present situation in which continental masses cover about 40% of Earth’s surface.