What is "plate tectonics"

plate tectonics \plate tectonics\, n. (Geol.) A geological theory which holds that the crust of the earth (the lithosphere) is divided into a small number of large separate plates which float and move slowly around on the more plastic asthenosphere, breaking apart and moving away from each other at points where magma upwells from below, and, driven by such upwellings and other currents on the athenosphere, sliding past each other, colliding with each other, and in some cases being submerged (subducted) one below the other. This theory is now widely accepted, and explains many geological phenomena such as the clustered locations of earthquakes, mountain building, volcanism, and the similarities observed between the geology of continents, such as South America and Africa which are now far apart, but, according to the theory, were once joined together. The motions of such tectonic plates are very slow, typically only several centimeters per year, but over tens and hundreds of millions of years, cause very large changes in the relative positions of the continents. The consequence of such movement of plates is called continental drift.

Tectonics \Tec*ton"ics\, n.

1. The science, or the art, by which implements, vessels, dwellings, or other edifices, are constructed, both agreeably to the end for which they are designed, and in conformity with artistic sentiments and ideas.

2. (Geol. & Phys. Geog.) the branch of geology concerned with the rock structures and external forms resulting from the deformation of the earth's crust; also, similar studies of other planets. Also called structural geology.

   plate tectonics a geological theory which considers the earth's crust as divided into a number of large relatively rigid plates, which move relatively independently on the more plastic asthenosphere under the influence of magmatic upwellings, so as to drift apart, slide past, or collide with each other, causing the formation, breakup, or merging of continents, and causing volcanism, the building of mountain ranges, and the subduction of one plate beneath another. In recent decades a large body of data have accumulated to support the theory and provide some details of the mechanisms at work. One set of supporting observations consists of data showing that the continents have slowly moved relative to each other over long periods of time, a phenomenon called continental drift. Africa and South America, for example, have apparently moved apart from a connected configuration at about 2 to 3 cm per year over tens of millions of years.

n. (context geology English) The large-scale movement of tectonic plates that contributes to continental drift.

n. the branch of geology studying the folding and faulting of the earth's crust [syn: tectonics, plate tectonic theory]

Plate tectonics (from the Late Latin tectonicus, from the "pertaining to building") is a scientific theory describing the large-scale motion of Earth's lithosphere. The theoretical model builds on the concept of continental drift developed during the first few decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.

The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken up into tectonic plates. The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates. Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The relative movement of the plates typically ranges from zero to 100 mm annually.

Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories (that still have some supporters) propose gradual shrinking (contraction) or gradual expansion of the globe.

Tectonic plates are able to move because the Earth's lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topography and density of the crust, which result in differences in gravitational forces) and drag, with downward suction, at the subduction zones. Another explanation lies in the different forces generated by tidal forces of the Sun and Moon. The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.