A study of five major basaltic provinces, including oceanic plateaus, volcanic passive margins, and continental flood basalts, shows that they are voluminous constructions of extrusive igneous rock underlain by intrusive rock. Crustal thickness ranges from 20 to 40 km, and lower crust is characterized by high (7.0-7.6 km/s) seismic velocities. Volumes and emplacement rates derived for two oceanic plateaus, the Ontong Java and Kerguelen-Broken Ridge, reveal short-lived pulses of increased global crustal production and suggest an origin involving the lower mantle. The Ontong Java rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid-ocean ridge system. Despite the importance off large igneous provinces in studies of mantle dynamics and the global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.