Abstract. Results from the Reykjanes-Iceland Seismic Experiment (RISE)show that the thickness of zero-age crust decreases from 21 km in southwest Iceland to 11 km at 62ø40•N on the Reykjanes Ridge. This implies a decrease in mantle potential temperature of •-130 øC, with increasing distance from the center of the Iceland mantle plume, along this 250 km transect of the plate boundary. The crust thins off-axis at 63øN, from 12.7 km thick at 0 Ma to 9.8 km at 5 Ma, most likely due to a •40øC change in asthenospheric mantle temperature between these times. This provides evidence for the passage of a pulse of hotter asthenospheric mantle material beneath the present spreading center. A reflective body, the top of which lies at 9-11 km depth, is identified in the lower crust just west of the tip of the Reykjanes Peninsula. Synthetic seisrnogram modeling of the wide-angle reflections from this body suggests that it corresponds to a zone of high-velocity (>_7.5 krn s -x), high-magnesium rocks in the lower crust. The P to S wave velocity ratio beneath the peninsula is 1.78, implying that crustal temperatures are below the solidus. Gravity modeling shows the RISE models to be consistent with the observed gravity field. Mantle densities are lower beneath the ridge axis than beneath older crust, consistent with lithospheric cooling with age. Iceland and the Reykjanes RidgeIceland has been created where the slow spreading Mid-Atlantic Ridge passes directly over a mantle plume.McKenzie and Bickle [1988] showed that for decompression melting of the mantle at oceanic spreading centers the melt volume produced relies sensitively on the mantle temperature. Thus, assuming passive upwelling, the presence of the spreading center allows us to estimate the mantle temperature at a range of distances from the plume by measuring the igneous crustal thickness.•