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.
Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust.
Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading.
Mechanically, 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.
The outer layers of the Earth are divided into the lithosphere and asthenosphere.
This is based on differences in mechanical properties and in the method for the transfer of heat.
Earlier theories, since disproven, proposed gradual shrinking (contraction) or gradual expansion of the globe.
Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere.
The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates.