Richter and Parsons (1975) in their two-scale model of mantle convection predict that if the depth of the convection layer is about 600 km, then for a plate moving at 10 cm yr-1, longitudinal convective rolls will be produced in about 50 m.y., and the strike of these rolls indicates the direction of motion of the plate relative to the upper mantle. In the case of the Pacific plate there should then exist a series of longitudinal convective rolls probably striking WNW. These predictive features of two-scale mantle convection are tested by examining a new global free air gravity model which is complete to the thirtieth degree and order. To isolate only those anomalies with wavelengths of probable interest, the low degree and order field (degree of the field n?12) is subtracted from a field complete to the twenty-second degree and order. The resulting free air gravity map shows a series of linear positive and negative anomalies spanning the Pacific Ocean. Anomalies of this type are found only in the Pacific Ocean area. These anomalies cross the east Pacific rise and strike parallel to the hawaiian seamounts. Their transverse wavelength is about 2000 km. Visual correlations between residual depth anomalies and free air gravity in the central Pacific and along the east Pacific rise are fair. We suggest that the long linear pattern of free air gravity anomalies may indicate the presence of longitudinal convective rolls beneath the Pacific plates. In order for these rolls to have been developed in the course of 43 m.y., the age of the bend in the Hawaiian-Emperor seamount chain, for an absolute plate velocity of about 10 cm yr-1 the convective depth must be about 600 km or less. These results tend to support the predictions of the two-scale model of Richter and Parsons. |