Stratigraphic analyses of cores recovered by the Deep Sea Drilling Project from the western Pacific Ocean confirm that Cretaceous volcanic activity in that region was common, and that two separate major episodes of activity can be distinguished. The older episode, Aptian to Cenomanian in age, resulted in the formation of most of the oceanic plateaus in the western Pacific. In the younger, Santonian to Maastrichtian episode, several island and seamount chains formed, especially those in the west-central Pacific that trend north-northwest. These volcanic events, among the most extensive in the marine geologic record, covered an area of at least 30 × 106 km2. Possibly, the older episode was responsible for a Cretaceous sea-level high and concomitant epi-continental transgression. The younger episode of volcanism certainly covered an extensive area, but its effect upon sea level may not have been as great as that of the older event.