Magmagenesis

The lithosphere, the outer 100-200 km of the Earth, is divided into seven major plates each bounded by well-defined earthquake zones. Plate tectonics, the mechanism by which these plates are formed, move and are destroyed, provides a convincing explanation of how the oceanic lithosphere, forming over 70% of the Earth's surface, has been produced beneath oceanic ridges during the last 200 million years of geological time. But, as the total surface area of the Earth has not changed appreciably in this time, some of the lithosphere must have been destroyed to make way for the new. It is the ocean floor itself that has a built-in self-destruct mechanism and disappears back into the Earth, only to be partly recycled, as the magmatic products that form island arcs and embellish continental margins. The major magmatic processes at plate margins, and the apparent anomaly of within-plate magmatism, are described before the evolution and interrelation of magmagenetic sites and the role of magmagenetic processes in crustal evolution are briefly discussed.