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 ]