ABSTRACT
Magmatism is a series of processes including the melting of the mantle or crust and the solidification of magma as magmatic rock. The model of the Earth as a series of concentric layers is the culmination of a long period of observation. The coupling of mechanical phenomena and the generation of rocks helps people understand the diversity of magmatic rocks. The history of plate tectonics began long ago. As early as 1620 the philosopher Bacon noted the similarity between the shapes of the African and South American coasts. Magnetic surveys show interesting linear anomalies on the oceanic crust on both sides of the Mid-Atlantic Ridge. The striped pattern of magnetic bands signifies the inversion of the magnetic field. Positive anomalies indicate that the magnetic field at some time in the past was the same as the present-day field. The ridge passes between Australia and Antarctica and reappears in the Indian Ocean.
