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The Collaborative International Dictionary of English v.0.48 (gcide)

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.
[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]