| 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. [ PJC ] |
| Tectonics | 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. [ 1913 Webster ] 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. [ PJC ]
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. [ PJC ]
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