Igneous rock was recovered at three sites on Hess Rise, north-crystal Pacific Ocean, during Deep Sea Drilling Project leg 62: tholeiitis basalt at site 464 (northern Hess Rise), trachyte at site 465, and rounded clasts of alkalic basalt at site 466 (both on southern Hess Rise). The depths at which the igneous rocks from sites 464 and 465 were recovered correspond to strong acoustic reflectors, whereas the clasts from site 466 were recovered from within calcareous sediment. Although all the rocks are moderately to highly altered, petrographic and chemical studies, especially rare earth element determinations, provide data on the nature of rocks. Data from thee studies, combined with geophysical models of the evolution of the Pacific plate during the Mesozoic, allow an interpretation of the formation of Hess Rise involving building of a volcanic platform by eruption of tholet tholeiitic mid-oceanic ridge basalt caused by the breakup of a R-R-R-type triple junction during the middle Cretaceous. This period of platform building was accompanied by edifice building along the south edge of Hess Rise involving alkalic basalt and its differentation products associated wth a leaky transform fault. This interpretation agrees with proposed models for the origin of the tholeiitic parts of other oceanic plateaus but requires a different mechanism-namely, volcanism associated with leaky transform fault--so account for the alkalic rocks.