CPT Q. 067: How can horizontal forces move a large tectonic plate without rupturing it?

Q. 67. Although ‘buoyancy anomalies in the mantle’ and ‘gravitational potential energy’ are envisioned as the vertically-downward forces need for subduction, how would lateral force continue to be transmitted by ‘trench pull’ and ‘ridge push’ throughout the entire volume of a tectonic plate without completely rupturing or subdividing such a huge crustal slab until it could be dragged, so to speak, “in toto beneath the Moho”?

Response: Most people fail to appreciate just how weak the asthenosphere, which lies just beneath the lithospheric plates, really is. Various lines of evidence indicate that the asthenosphere is at least a thousand times weaker and more likely ten thousand times weaker in most places than the overlying lithosphere. This means that the asthenosphere exerts negligible forces on the plates. It also means that the forces responsible for plate motions are the forces acting at their edges, for example, the slab pull and the ridge push forces you mention. Indeed, it seems astonishing that on today’s earth, GPS measurements indicate the Pacific Plate, which is some 14,000 km wide but only some 80 km thick, is moving toward the west northwest essentially as a rigid block with no discernible internal deformation. Mechanically, this is possible only if the tractions exerted by the underlying lithosphere are negligible.

Some clarification is in order here regarding the meaning of the term ‘Moho’. The Moho is the boundary between crust and mantle, distinguished seismically by an observable change in seismic velocities due to a change in chemical composition and mineralogy. In ocean lithosphere, this boundary typically occurs at a depth of 6-7 km, while the thickness of an oceanic plate varies between near zero at the axis of a spreading ridge to about 80 km for lithosphere 2000 km or more from the ridge. In the case of continental lithosphere, the depth to the Moho is on average about 35 km, while the thickness of the continental lithosphere, or plate, can be as much as 250 km. Subduction occurs then, not beneath the Moho which is merely a seismological boundary inside a lithospheric plate, but rather into the asthenosphere beneath an adjacent lithospheric plate.