Geochemical, K-Ar age, and paleomagnetic data are presented for volcanic rocks from Truk Lagoon (7°20′N, 151° 15′E). Petrographic observations and 52 analyses for major and trace elements reveal 2 shield-building magma types—a mildly alkalic Truk Main Lava Series (TMLS) and a less alkalic Truk Transitional Lava Series (TTLS); and a post-erosional, highly silica-undersaturated, and incompatible-trace-element–enriched Truk Nephelinite Series (TNS). The TMLS and TTLS shield-building lavas give a mean K-Ar age of 10.9 m.y.; post-erosional TNS lavas are dated at 4.8 m.y. B.P. Paleomagnetic results for 15 stably magnetized sites give a paleopole of declination and inclination of 359.3°E and 8.8°30′, respectively, which suggests that Truk was formed 4.4° north of the equator. These data are consistent with a hot-spot origin for the volcanics of Truk Atoll.