1. Formation of an embryonic crystallite from a melt which is followed by the growth of a nucleus to crystal dimensions. See also microlite.

2. Theory according to which geosynclines developed on the edges of cratons; the ensuing orogenic belt then became part of the craton and the products of subsequent erosion filled a new geosynclinal trough which developed on the edge of the enlarged craton. The theory of plate tectonics, with the emphasis on horizontal movement, modified nucleation in many ways, not least by including oceanic sediment, scraped off the subducting plate (see subduction), into the new craton. See accretion.