A simple model is developed which explains the occurrence of volcanic continental margins and flood basalts as a consequence of their association with nearby plumes that were active at the time of rifting. In the model, asthenosphere temperatures are increased by 100-150 C over large regions of the earth by heat advected upward in mantle plumes. The amount of partial melt generated by the asthenosphere as it wells up beneath rifts in these hot areas is calculated. Observational constraints from all known examples of volcanic continental margins are reviewed and the model is used to explain these observations.