Many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle. Flood basalts, radial dike swarms, and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere. The buoyant plume head material ponds at the base of the lithosphere and then flows laterally. Relief on the base of the lithosphere acts as an upside-down drainage pattern with enclosed catchments for this flow. That is, plume material preferentially collects beneath regions where the lithosphere is locally thin, such as thermally subsiding sedimentary basins. An example is uplift associated with the Iceland starting plume which inverted the Mesozoic Irish Sea basin. Additional relief at the base of the lithosphere is associated with rifting and seafloor spreading. In the case of the northern North Sea, material from the Iceland starting plume ponded beneath a basin with locally thin lithosphere. Rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading. Rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head. An example is the volcanic passive margin off Nova Scotia which is 2000 km from the center of the Bahama starting plume head. Numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 Pa s.¿ 1997 American Geophysical Union