HOT-SPOTS, MAGMA UNDERPLATING, AND MODIFICATION OF CONTINENTAL-CRUST

Most common mantle-derived magmas have densities at Moho pressures that exceed the density of average continental crust. Thus, unless the crust is tightly coupled with the mantle, many such magmas should penetrate at or near the Moho interface. The scale of such a phenomenon can be impressive; recent hot spots may have perturbed more than 5% of the planet's surface. The details of the process will depend on the ease of melting of the base of continents. Underplating can explain continental construction from below; Moho structures, or lack thereof; delamination of the continental basement; partial subduction and density cleaning of crust; the formation' of massive anorthosite complexes; and major changes of elevation of continental blocks. In the thermally turbulent Archean, the very dense and hot komatiite magmas must have had even more profound influences. Modem studies of the subduction process show that the mantle must be heterogeneous and that volatiles can be carried to great depths in significant volumes.