We present the results of a seismic refraction experiment that constrains crustal structure and thickness along 225 km of the Kolbeinsey Ridge and Tj¿rnes Fracture Zone and thus quantifies the influence of the Iceland hot spot on melt flux at the spreading center north of Iceland. North of the Iceland shelf, crustal thickness is relatively constant over 75 km, 9.4 ¿ 0.2 km. Along the southern portion of the Kolbeinsey Ridge, on the Iceland shelf, crustal thickness increases from 9.5 ¿ 0.1 km to 12.1 ¿ 0.4 km over 90 km. Gravity inversion indicates a residual crustal gravity anomaly that decreases by about 30--40 mGal toward Iceland. We infer that the variations in crustal thickness and gravity are accompanied by mantle temperature changes of 40¿ to 50¿C. At similar distances from the Iceland hot spot, crustal thickness along the Kolbeinsey Ridge is 2--2.5 km less than at the Reykjanes Ridge, consistent with the asymmetry in plume-ridge interaction that has been inferred from the axial depth and geochemistry of these ridges. Average lower crustal velocities are also higher along the Kolbeinsey Ridge, consistent with a lower degree of active upwelling than along the Reykjanes Ridge. Topography and crustal thickness patterns at the spreading centers around Iceland are consistent with isostatic support for normal crustal and mantle densities. However, we infer that the lower crust beneath central Iceland is considerably denser than that beneath the adjacent ridges. Crustal thickness and geochemical patterns suggest that deep melting is spatially limited and asymmetric about Iceland while shallow melting is enhanced over a broad region. This asymmetry may be due to a mantle plume that is tilted from south to north in the upper mantle and preferentially melts deeper enriched material beneath the Reykjanes Ridge.