Plate tectonics is the theory that Earth crust is compromised of several large plates that are constantly in motion, sometimes creating mountains, earthquakes or volcanos.
In the 19th century, Alfred Wegener, a German geologist, recognized the geometric similarity of the coastlines of Africa and South America. He proposed the concept of continental drift -that continents, now far removed from one another, may have previously been joined. However, it is only since the development of modern geophysics that a feasible explanation has been provided for movements in the Earth's crust
In the 1960s, geophysicists noticed that major earthquakes and volcanoes are restricted to narrow zones on the Earth's surface. They concluded, therefore, that portions of the crust behave as rigid plates which deform at their edges. The term lithosphere is used to describe the crust and uppermost mantle that is mechanically rigid.
Further geophysical investigations into the Earth's interior showed that below the lithosphere there is a zone of ductile deformation known as the asthenosphere. It permits both lateral movements of the plates, and vertical crustal movements to compensate changes in crustal thickness dutring plate motions.
Plate tectonic theory therefore describes plates of lithosphere about 100 km thick overlying a layer of relatively low strength (the asthenosphere). The movement of these plates is driven by the continuous cooling of the Earth, which produces large scale convection cells in the mantle.
In the oceans, oceanic lithosphere is created at divergent margins, and consumed at convergent margins. Along a third type of margin, oceanic plates slide past each other, forming conservative margins.
Due to the structural and compositional heterogeneity of continental lithosphere, plate tectonics in the continental regions is considerably more complicated than in the oceans. Rather than occurring in narrow zones, deformation of the continents takes place over large regions. For example, the Himalayan fold belt, the result of continental collision between the Indian subcontinent and Eurasia, is over 2000 km wide.
In the following sections, the typical structures formed at divergent, convergent, and conservative margins are described.
Divergent Plate Margins
Divergent plate margins in oceans generate oceanic crust by allowing molten mantle material to solidify at the surface. Oceanic crust covers about 70% of the Earth's surface and has a simple structure. Ocean ridges form over the rising asthenosphere because this part of the crust is hotter, and therefore less dense, than the surrounding regions. Mantle material rises under the ridge, "filling the gap" as the two plates move away from each other. As the newly formed crustal material moves away from the spreading center, it cools, and so its elevation decreases.
Divergence in a continental plate causes thinning of the lithosphere by extension to form a sedimentary basin. As divergence continues, the two parts of the continental plate move apart, oceanic crust is formed at a new mid-ocean ridge and an ocean is born with continental or passive margins on either side.
Convergent Plate Margins
There are three types of convergent plate margins: ocean-ocean convergence, ocean-continent convergence, and continent-continent convergence.
Convergence involving oceanic lithosphere results in consumption of lithospheric material at a subduction zone as it sinks into the underlying mantle. This material then heats up and melts to form a source of magma for volcanoes at the surface. A line of volcanoes, called a volcanic arc (or island arc if it is partly immersed), normally forms above a subduction zone.
Continental lithosphere is too buoyant to be consumed at convergent margins, so the lithosphere becomes very thick to form mountain belts. The high pressures and temperatures associated with mountain formation, or orogenesis, cause metamorphism of the existing rocks. Most mountain belts have a thickened core of high pressure metamorphic rocks and some igneous material.
Conservative Plate Margins
The features formed at conservative margins can be considered as a combination of those from divergent and convergent margins, depending on the overall direction of plate movement and the shape of the plate boundary. If the plates are moving obliquely away from one another, the margin is said to be transtensional and small extensional basins are formed. Conversely, if the relative motion of the plates is obliquely toward each other, small mountains and compressional basins form and the margin is termed transpressional. Inevitably, real conservative margins do not have a straight line boundary, and both extensional and compressional features exist. Tectonic activity at all of these margins causes the formation of sedimentary basins. Each margin is typified by certain kinds of basin. In the following section, the mechanisms are described which lead to the formation of sedimentary basins.