Plate tectonics is a unifying theory that explains the structure of the Earth's near surface. The plates (lithospheric plates) are delineated by the earth's major earthquake zones and plate motions are thought to be driven by thermal convection in the upper mantle as well as the gravitational pull of cold dense parts of plates descending into the mantle. Plates move apart at ocean ridges, or divergent plate margins, and towards one another along magmatic (island) arcs or orogenic belts at convergent plate margins. Some important aspects of the plate tectonic theory are:Ocean ridges are approximately linear zones of extensional faulting or rifting, characterised by basaltic volcanism.
Sea floor spreading occurs as new ocean floor is added thus spreading occurs away from the ocean rifts. Continents may be rifted apart to drift passively along as new oceans spread between them. Distribution of heat flow, ages of volcanic islands and fringing reefs, barrier reefs, atolls and volcanic sea mounts, all support the view of slow cooling and subsidence of ocean floor as it moves away from spreading centres. Sea floors have the following features:
Deformation of the continental crust has occurred throughout most of geological time. A typical orogenic episode is preceded by subsidence of marginal troughs in which sediments accumulate; plate convergence then initiates deformation in a belt that extends hundreds of kilometres from the original troughs. The marginal sediments are deformed by folding or faulting; thrust sheets ten to twenty kilometres thick slide over one another, often for distances of tens to hundreds of kilometres. Huge foreign terranes (a group of rocks with similar history) brought in with the subducting plate may accrete to the continental plate. Intrusions of batholiths and metamorphism typically occur with the orogeny. The mountains are raised in a deformed belt and erode after the orogeny ends. Renewed stages of uplift, or block faulting, that again raise the region, account for many of the large-scale topographic features we see today.Plate collisions are the main cause of these orogenic episodes, i.e. orogenesis. Collisions may involve either two continents, or varying combinations of magmatic arcs, continents or micro continents. Orogenic belts appear to be made up of collages of diverse terranes jammed together by successive collisions. Suture zones mark the boundaries of such collisions: they are often characterised by crumpled mafic and ultramafic ocean rocks. The collision of two or more continents forms a supercontinent. Plate margins can exhibit the following features:
Hot mantle plumes rising from the lower mantle seem to cause the initial break up of the crust at triple junctions, e.g. the Indian Ocean Ridge connects with the East African Rift System in a triple junction where three rifts meet.
Aulacogens are sediment filled troughs associated with initial rifting of continents at triple junctions.