Introduction to Tectonics

The Earth is composed of layers of different composition and physical properties, principally the solid central core, the fluid peripheral core, the viscous mantle, and the solid lithosphere. The lithosphere is comprised of the upper mantle and the crust, the outer shell of the Earth. There are two types of lithosphere, according to the crust resting on the solid mantle lithosphere (lithospheric mantle): the oceanic lithosphere has a 5 to 8km thick oceanic crust (with a basaltic composition) and the continental lithosphere has a 30km to 40km thick granitic-dioritic crust. The lithosphere is fragmented into pieces of variable shape and size, the plates. The edges of the plates are called plate boundaries. The Earth has 7 major plates (Africa, Antarctica, Australia, Eurasia, North America, South America and Pacifica) and several minor ones (Adria, Arabia, Caribbean, Nazca, Philippines and others). Most of the plates are composed of continental and oceanic lithosphere. These plates move independently relative to one another, with a restricted independence from the 7 large plates, however. The relative, horizontal movements are ideally described as rigid body motions that produce space and friction problems at the contacts between adjacent plates. Plate boundaries are not fixed; they also move and change shape. The global mosaic of plates periodically reorganizes itself and new plate boundaries form while others close up. Plate tectonics, the study of such relative motions and their consequences, allows relating surface, geological and geophysical structures with quantified movements attributed to deep processes of the Earth’s heat engine: The interior is hot, space is cold; the second law of thermodynamics states that this gradient will drive spontaneous convection processes in pursuit of equilibrium. The motion of lithospheric plates is a considerable consequence of thermally-driven mass movements on the Earth.