CPT Q. 063: Does accumulating basalt from a subducting plate buoy up the adjacent continent edge?

Q. 63. CPT posits the flooding of the continents was caused in part by the downward drag of the continents by descending plates. If so, would not the rapid addition of so much volume of new basaltic crust (from the subducting plate) immediately beneath the continents add an upward component of force during subduction?

Response: First of all, the roughly 7 km thick layer of basalt and gabbro, which is the top crustal part of an oceanic plate, converts to a much denser type of rock called eclogite at a depth of only about 50 km. Eclogite is intrinsically denser than normal mantle rock at mantle depths, so it has a strong tendency itself to sink and also to help the slab to which it is attached to sink. So this basalt/gabbro crust generally does not pile up or accumulate because of positive buoyancy beneath a continent. Instead it tends to sink to the bottom of the mantle. On the other hand, when the subduction is very rapid, there is evidence that a significant fraction of the sediment that is on top of the ocean plate can and does get subducted. It tends to melt easily and to generate magma with a more or less granitic composition which commonly forms plutons in the overlying continental plate. So this sediment does act to thicken the edge of continent when it is subducted beneath it. Volumes appear to have been large enough during the Flood to generate several of the major mountain ranges on earth today. However, while the rapid subduction is occurring, there are downward dynamic forces acting on the edge of the continental plate that more than compensate for that growing thickness of low density rock to keep that strip of continent below sea level. On the other hand, when the rapid subduction ceases, this zone of thickened crust, because of its positive buoyancy, rises rapidly and in most cases forms a prominent belt of mountains.