<P> The outer layers of the Earth are divided into the lithosphere and asthenosphere . The division is based on differences in mechanical properties and in the method for the transfer of heat . The lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily . In terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient . This division should not be confused with the chemical subdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure . </P> <P> The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid - like (visco - elastic solid) asthenosphere . Plate motions range up to a typical 10--40 mm / year (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm / year (Nazca Plate; about as fast as hair grows). The driving mechanism behind this movement is described below . </P> <P> Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). Average oceanic lithosphere is typically 100 km (62 mi) thick; its thickness is a function of its age: as time passes, it conductively cools and subjacent cooling mantle is added to its base . Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed . For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km (4 mi) thick at mid-ocean ridges to greater than 100 km (62 mi) at subduction zones; for shorter or longer distances, the subduction zone (and therefore also the mean) thickness becomes smaller or larger, respectively . Continental lithosphere is typically about 200 km thick, though this varies considerably between basins, mountain ranges, and stable cratonic interiors of continents . </P> <P> The location where two plates meet is called a plate boundary . Plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and oceanic trenches . The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being the most active and widely known today . These boundaries are discussed in further detail below . Some volcanoes occur in the interiors of plates, and these have been variously attributed to internal plate deformation and to mantle plumes . </P>

Motion of the plates of earths crust is thought to be due to
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