CONTINENTAL flood basalt provinces represent important magmatic events, which may contribute significantly to the generation of new crust. In a typical flood basalt province, large volumes of magma are erupted in a short time: for example, at least 106 km3 in 2–3 Myr in the Parana–Etendeka province of southeast Brazil and northern Namibia. In many areas flood basalts are associated with mantle plumes, but details of their origin, such as the site of major-element melting, remain unresolved. Recent authors1–3 have assumed that partial melting took place at the anhydrous peridotite solidus, and thus concluded that during continental extension, more than 95% of the erupted magmas are generated in the sublithospheric upper mantle. In this situation, the distinctive isotope and trace element geochemistry of the basalts is attributed not to this process, but to the addition of low-degree partial melts scavenged from the overlying lithosphere4. Here we present an alternative model in which, in the presence of small amounts of water (∼0.4%), the observed quantities of melt may be generated entirely within the mantle lithosphere. As rifting proceeds, however, the basalts acquire an increasingly 'asthenospheric' chemistry, as melts from the asthenosphere come to dominate those from the lithosphere.