Q. 84: How catastrophic was the effect of runaway slabs contacting the mantle-core boundary?
CPT Q. #84 – 101 Q&A on Catastrophic Plate Tectonics
Question: 84. In your opinion, how catastrophic was the effect of the runaway slabs contacting the mantle-core boundary? Was the deceleration seen near the surface significant enough to cause continental compression which some creationists (e.g., Walt Brown) believe was very significant? Walt Brown suggests that a rapid deceleration of the continents caused such severe compression that the plates thickened significantly. Do you think that such a compression event could have been caused by slab contact with the mantle-core boundary, or was there sufficient “mantle/slab buffering” to reduce surface deceleration rates? If not, I assume that you believe most continental compression was purely the result of subduction effects.
Response: In my answer to question 71 above, I pointed out the specific kinetic energy of a slab moving at 45 mph or 20 m/s is only 200 J/kg. That amount of energy will raise rock only 20 meters in the earth’s gravity field. I showed that the amount of energy required to double plate thickness is at least 450 times greater than what Brown is proposing. Therefore, as far as I can determine, Brown has neither an energy source nor a physical mechanism to increase crustal thickness of the continents by the factor of two his hypothesis requires. Besides these difficulties, there is the geometry problem of not having enough crustal mass between the present continent and the adjacent ridge to achieve such a doubling of thickness, even if an energy source and mechanism were available.
In my runaway calculations I rarely see velocities higher than 10 m/s, and these tend to be in the middle of the mantle. As runaway material approaches either the top or bottom boundary, it tends to decelerate smoothly and have what can be described as a ‘soft landing’. More importantly, the material is so weak that decelerations which occur at one boundary have essentially no effect at the other.
Finally, let me say that the evidence seems to be strong that the continental cratons did not experience any thickening of their crystalline basements during the Flood. Most of the continental shield areas actually appear to have experienced moderate to severe beveling of their top surface by erosion during the Flood. Large portions of the Canadian Shield, for example, display Precambrian mountain roots exposed at the surface, suggesting a kilometer or more of crystalline rock has been removed by erosion since those Precambrian mountains were present. Nowhere in the continental cratons is there any systematic evidence for the sort of crustal compression and shortening that Brown postulates.
The two primary places where the thickness of the continental crust was increased during the Flood were, first, just inboard of subduction zones along continental margins such as around the Pacific perimeter, as is clearly evident on the western coasts of South and Central America, and, second, in continent-continent collision zones, most notably along the Himalaya-Alpine belt. In both cases subduction is what brings about the increase in the thickness of the crustal layer. In the first case, much of the thickening is due to emplacement of magmatic material from below. In the second, the crustal portions of two continental plates are deformed into one another by the subduction of the mantle lithosphere of one or both of the plates. In neither case does the inertia of the plates play any significant role. The accelerations, even in the case of CPT, are far too small to matter. What drives everything is gravity. Gravity drives the subduction, and it also drives the subsequent isostatic adjustment.