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We present the results of a seismic refraction experiment that constrains crustal structure and thickness along 225 km of the Kolbeinsey Ridge and Tjörnes 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 …