The deep structure of continents

Gravity and heat flow observations demonstrate that, on the average, mass and radioactivity per unit area are equal under continents and oceans. A global representation of the anomalies in the heat flow and gravity fields shows many similarities, and horizontal gradients in both fields are correlated with earthquake zones. The regional variations of Rayleigh and Love wave velocities provide evidence additional to that of gravity and heat flow that the mantle under continents differs from that under oceans to a depth of 400 to 700 km. Taken together the observations of the planetary fields imply that vertical segregation has been the dominant feature of the process of continent formation. The significance of deep structure of continents and oceans is explored by a number of numerical experiments. The difference in radioactivity between continents and oceans results in a concentration of thermal stresses at the continent‐ocean boundary. The ensuing fault zones provide a preferred location for the release of strain energy accumulated by dynamic processes such as those associated with changes in the earth's rotation. The decay of radioactivity implies a past rate of heat production in excess of the heat loss. During the first two to three aeons, the earth expanded at an average rate of about 10 km aeon −1 . The resulting fracturing of the outer surface with accompanying upward flow of matter led to the localization of the early continents. Continental growth proceeded by secular differentiation of the subjacent mantle.