Rabaul Caldera has been the source of many voluminous explosive eruptions in the past few hundred thousand years. Reliable dates are available for two such eruptions within the past 3,500 years. A caldera-modifying eruption about 3,500 yr b .P. produced primarily rhyolite but may have been triggered by an injection of basalt (Walker and others, 1981). A large eruption of dacitic magma roughly 1,400 yr B.P. may have resulted in further caldera collapse, perhaps of an inner caldera inferred from bathymetry and from recent seismicity. Postcaldera volcanics at Tavurvur, Sulphur Creek, and Rabalanakaia range from basalt to dacite; those a t Vulcan and Davapia Rocks are dacite. The vent for the 3,500 yr B.P. eruption may have been in the north or northeast part of the present caldera (J. Mori, 198 8); the vent for the 1,400 yr B.P. eruption may have been near the center of the caldera (Walker and others, 1981). Both the 3,500 and 1,400 yr B.P. eruptions " began with small scale explosions... produced small-volume pyroclastic deposits... (and then) activity built up rather quickly to a climax" (Walker and others, 1981). "Rather quickly" in this context means only that there is no geologic evidence for a time gap between the small-scale and larger scale deposits, although Walker speculates that the gaps may have been as short as a few hours. Consideration of repose periods between large explosive eruptions at Rabaul suggests that another might occur in the geologically near future (McKee and others, 1985).