<Dd> h ∼ 2 κ t (\ displaystyle \, h \, \ sim \, 2 \, (\ sqrt (\ kappa t)) \,) </Dd> <P> Here, h (\ displaystyle h) is the thickness of the oceanic mantle lithosphere, κ (\ displaystyle \ kappa) is the thermal diffusivity (approximately 10 m / s) for silicate rocks, and t (\ displaystyle t) is the age of the given part of the lithosphere . The age is often equal to L / V, where L is the distance from the spreading centre of mid-oceanic ridge, and V is velocity of the lithospheric plate . </P> <P> Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years but after this becomes increasingly denser than asthenosphere . This is because the chemically differentiated oceanic crust is lighter than asthenosphere, but thermal contraction of the mantle lithosphere makes it more dense than the asthenosphere . The gravitational instability of mature oceanic lithosphere has the effect that at subduction zones, oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental . New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones . As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old . The oldest parts of continental lithosphere underlie cratons, and the mantle lithosphere there is thicker and less dense than typical; the relatively low density of such mantle "roots of cratons" helps to stabilize these regions . </P> <P> Geophysical studies in the early 21st century posit that large pieces of the lithosphere have been subducted into the mantle as deep as 2900 km to near the core - mantle boundary, while others "float" in the upper mantle, while some stick down into the mantle as far as 400 km but remain "attached" to the continental plate above, similar to the extent of the "tectosphere" proposed by Jordan in 1988 . </P>

Is the oceanic crust part of the lithosphere