A topographic dome, radial drainage pattern, and regional negative gravity anomaly, all centered on Lake Superior, are vestiges of a 1100 Ma mantle plume that formed the Midcontinent Rift System of central North America. The topographic and gravimetric relations suggest that the dome is maintained isostatically by fundamental changes imprinted on the lithosphere, by the plume, including magmatic underplating of the crust and depletion of the upper mantle. These changes occurred at distances of up to 500 km or more from the center of the plume, well beyond the margins of the rift. ¿ American Geophysical Union 1992