As the primordial Earth began to cool, almost 4.6 billion years ago, the magma across its surface was subjected to the planet's gravity and lighter elements and minerals moved upward, forming a layer of inorganic material that would eventually coalesce to cover 30% of the globe. The other 70% would cool to form basalt, a more dense, fine grained rock, rich in silica, ferro-magnesium minerals and, in some regions, olivine. The lighter elements formed the continental crust while basalt formed the oceanic crust; as heat currents evolved in the underlying mantle, the crust broke into tectonic plates, some of which were comprised of both continental and oceanic crustal segments.
Throughout the history of our planet and continuing today, basalt forms from magma along the mid-oceanic ridges and moves outward as oceanic crust on the diverging plates. Should the oceanic plates move over a hotspot, produced by a mantle plume, basalt volcanic islands are produced; the Hawaiian Chain provides the classic example of hotspot basalt islands while Iceland has formed from basalt volcanism above the mid Atlantic ridge. At subduction zones, where an oceanic plate is forced down toward the mantle by an overriding continental or oceanic plate, the basalt melts and returns to the surface via subduction volcanoes, mixing with andesite if the upper plate is continental crust. Basalt also rises to the surface at rift zones, where continental crust is pulling apart; this can occur in the form of volcanoes or surface flows. Among the latter were the massive flows of the Siberian Traps (late in the Permian Period), the Deccan Traps of India (late in the Cretaceous Period) and the Columbia River Plateau of the Pacific Northwest (late in the Tertiary Period). Smaller areas of basalt volcanism continue today in the Rio Grande and East African Rift Valleys, among other locations; such rifting activity may abort or may eventually produce a new seaway and thence a new ocean (as the rift zone converts to a mid oceanic ridge).
Having played a major role in the formation of Earth's crust and in the ongoing process of continental drift, basalt is truly a rock of the ages. Needless to say, it is the most abundant and widespread rock on the surface of our planet (though most remains hidden beneath the ocean waters).
