Heat flow anomalies on the oceanic ridges and the large free air gravity anomalies observed from the earth’s surface and from satellites are often believed to be surface expressions of high temperatures and flow within the mantle. A simple model for the temperature within a spreading sea floor can, however, reproduce the shape and magnitude of the observed anomalies. Thus, it is not necessary for the upper mantle to be hotter beneath ridges than it is elsewhere. A similar model may be used to relate the free air gravity anomaly to the stress in the lithosphere. The results show that long-wavelength harmonics of the external gravity field cannot be supported by the strength of the lithosphere. Most free air anomalies observed on the surface can be maintained in this way, except possibly the largest of those over the trenches.