11.   Continental lithosphere stands higher than oceanic lithosphere because continental crust is both of greater thickness and lower density than oceanic crust.

12.   This kind of behaviour, known as flexural isostasy, is especially important for oceanic lithosphere and for thick continental lithosphere with a high rigidity.

13.   Oceanic lithosphere has a mean density rather close to that of the immediately underlying asthenosphere.

14.   However, as oceanic lithosphere ages, cools and thickens it becomes more dense than the asthenosphere and rests upon it in an unstable state.

15.   Alternatively, it seems possible that old, cold and thick oceanic lithosphere can attain a density sufficient for it to subside spontaneously into the asthenosphere.

16.   By contrast continental lithosphere has a significantly lower mean density than oceanic lithosphere and its buoyancy with respect to the asthenosphere prevents all but limited subduction.

17.   Other evidence supports the idea of a lack of major deformation within plates, at least over plates composed of oceanic lithosphere.

18.   Convergent boundaries between oceanic lithosphere are marked by an oceanic trench, a volcanic island arc and a Wadati-Benioff zone.

19.   Cool, thick, old oceanic lithosphere is gravitationally unstable as it is generally denser than the asthenosphere over which it lies.

20.   The subduction of oceanic lithosphere is a steady-state process in that it can continue until a continent or island arc arrives at the subduction zone.