According to the theory of isostasy, Earth's continents should float in equilibrium upon the denser underlying mantle, much in the same way that icebergs float on the ocean surface. However, this understanding may not accurately portray dynamic continent-mantle interaction. A new study suggests that continents may reside below the level predicted by isostatic theory—nearly 2–3 km below in some areas.
In the February issue of Geophysical Research Letters, A. M. Forte, A. M. Dziewonski, and R. L. Woodward of Harvard University and W. R. Peltier of the University of Toronto present evidence that some continents are actively pulled downward by the global-scale flow in the mantle. This flow is also responsible for the so-called “drift” of the Earth's tectonic plates. According to Forte, “our models demonstrate a clear pattern of continental depressions, which are forced by downwelling flow due to colder mantle material extending to great depths. We believe that such forces also operated in the geologic past and were partially responsible for the creation of deep continental basins that are now filled with sediments.”