Early Tertiary lithospheric breakup between Eurasia and Greenland was accompanied by a transient (~3 m.y.) igneous event emplacing both the onshore flood basalts of the North Atlantic Volcanic Province (NAVP) and huge extrusive complexes along the continent-ocean transition on the rifted continental margins. Seismic data show that volcanic margins extend >2600 km along the early Eocene plate boundary, in places underlain by high-velocity (7.2--7.7 km/s) lower crustal bodies. Quantitative calculations of NAVP dimensions, considered minimum estimates, reveal an areal extent of 1.3¿106 km2 and a volume of flood basalts of 1.8¿106 km3, yielding a mean eruption rate of 0.6 km3/yr or 2.4 km3/yr if two-thirds of the basalts were emplaced within 0.5 m.y. The total crustal volume is 6.6¿106 km3, resulting in a mean crustal accretion rate of 2.2 km3/yr. Thus NAVP ranks among the world's larger igneous provinces if the volcanic margins are considered. The velocity structure of the expanded crust seaward of the continent-ocean boundary differs from standard oceanic and continental crustal models. Based on seismic velocities this ''volcanic margin'' crust can be divided into three units of which the upper unit corresponds to basaltic extrusives. The regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust, indicating the high-velocity lower crust which extends some distance landward of the continent-ocean boundary, was emplaced during and subsequent to breakup. The volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling, probably reflecting the interaction of a mantle plume and a lithosphere already extending.