For the lowest frequency oscillations, it is important to consider the effect of the earth's gravity and rotation on the restoring force of particles from tbeir equilibrium position. A best fitting average earth model has been determined. Si rich, chemically different from mantle Bouyant materal that cannot be subducted far into the mantle. Continental crust 20-60 km thick. Oceanic crust 5-10 km thick.
Oceanic crust is basalt.
Best developed beneath tectonically active regions. Weak or absent beneath old continental cratons. Likely zone decreased viscosity, plate decoupling and zone of strong shear deformation. Lower mantle near the core-mantle boundary. Global average models have P and S velocity gradients reduced.