Young rhyolites and associated lavas and magmatic enclaves from the KatmaiEEENovarupta volcanic system (Alaskan Peninsula), and the Crater Lake and Medicine Lake volcanic system (Cascades) were analyzed for U and Th isotope abundances, as well as major and trace element concentrations, to investigate the timescales of the processes that lead to rhyolite generation in continental arcs. Basalts and basaltic andesites typically migrate from the mantle to the surface within several thousand years. Variations in (230 Th)/(232 Th) and (238 U)/(232 Th) ratios with SiO 2 concentrations in intermediate lavas appear to result from crystal fractionation combined with assimilation of recently crystallized magmas. These data also suggest that $10 4 EEE10 5 years of mafic magmatism are required at a volcanic center to generate silicic andesites and dacites. Rhyolite genesis involves varying proportions of crystal fractionation of intermediate magmas and assimilation of crust. The nearequilibrium (238 U)/(230 Th) ratios for all of the rhyolites suggest an average time since U was last fractionated from Th for the constituents making up these rhyolites of 410 5 years. Therefore, the residence times of continental magmas and their entrained crystals appear to increase by a minimum of 2EEE3 orders of magnitude with increasing SiO 2 concentrations from basalt to rhyolite.