The unifying concept that has drawn continental drift, sea-floor spreading, seismic activity, crustal structures, and volcanic activity (see volcanicity) into a coherent model of how the outer part of the Earth evolves. The theory proposes a model of the Earth’s upper layers in which the colder, brittle, surface rocks form a shell (the lithosphere) overlying a much less rigid asthenosphere. The shell comprises several discrete, rigid units (tectonic plates) each of which has a separate motion relative to the other plates. The plate margins are most readily defined by present-day seismicity, which is a consequence of the differential motions of the individual plates. The model is a combination of continental drift and sea-floor spreading. New lithospheric plates are constantly forming and separating, and so being enlarged, at constructive margins (ridges), while the global circumference is conserved by the subduction and recycling of material into the mantle at destructive margins (trenches). This recycling results in andesitic volcanism and the creation of new continental crust, which has a lower density than the oceanic crust and is more difficult to subduct. Many features of the Earth’s history are explicable within this model which has served as a unifying hypothesis for most of the Earth sciences. Previous mountain systems are now recognized as the sites of earlier subduction, often ending with continental crustal collision: the movement of plates has been used with varying success in interpreting orogenic belts as far back as the early Proterozoic. Plate motions are driven by mantle convection and are likely to have occurred throughout Earth history, although the resultant surface features are likely to have changed with time. See ridge-push; slab-pull.