6. Question: How can plates rift apart, and should they do so, how could they move over the underlying rock?
Response: While plates in general display considerable strength, there is impressive observational evidence that they can and do rift, or split apart, under the right circumstances. A good example is the rifting of the Arabian block from Africa with the formation of the Red Sea and the Gulf of Aden. Just what the forces were that initiated the rifting may not be so clear, but the reality that a once coherent plate has split into two pieces and that the two pieces are presently moving away from each other is documented by many lines of evidence, including GPS measurements. Another example of continental rifting is the separation of Baja California from Mexico and the subsequent migration of this block several hundred kilometers to the northwest. The cause in this case seems to be clearer, given that according to present GPS measurements the motion of this block is essentially identical to that of the Pacific Plate. The implication is that some of the western portion of the North American Plate has overridden the ridge that earlier formed the southeastern boundary of the Pacific Plate and that the forces associated with the divergence at this ridge were sufficient to cause the rifting away of this sliver of North American Plate. Careful numerical simulation indicates that lithospheric plates can also fail in compression. The most common circumstance is when a slab of oceanic lithosphere becomes sufficiently thick through cooling, it begins to founder under its own weight, initially producing a broad depression above it and then fracturing and sinking into the weaker and less dense mantle beneath it.
In regard to the issue of a lithospheric plate moving relative to the asthenospheric layer beneath it, I pointed out earlier that the case is compelling that this layer is on the order of a thousand to ten thousand times weaker than normal lithosphere and that the drag forces exerted by the asthenosphere on the base of the lithosphere tend to be extremely small.