For example, paleomagnetic evidence indicates that continents have moved significantly in the relatively recent geologic past. The absence of very old oceanic crust and ridges suggests that convection completely replaces the sea floor every 200 to 300 million years, meaning that the present constituents and configurations of ocean basins are relatively new. Discussion Ideas Suggested by Hess That Are Generally Accepted Today Crustal plates ride passively atop convecting mantle cells. In Hess's model, whole-mantle convection is assumed. This is true, although the global picture is somewhat more complicated.
All oceanic crust is new, geologically speaking.
New crust is created at mid-ocean ridges, and is consumed at the descending limbs of convection cells. Sea floor sediment can be incorporated into continents. Continental crust, due to its low density, cannot be pulled down very far at descending limbs of mantle convection cells. Instead, the leading edge of the continent will be shortened and thickened. Major mountain belts of the world are, of course, formed in this way.