Seismic data from the B96 array in northern Iceland are used to constrain the compressional and shear velocity structure of the crust and uppermost mantle in northern Iceland, at the western flank of the northern volcanic zone (NVZ) (the mid-Atlantic plate boundary in northern Iceland). Travel times from P and S waves from shots and microearthquakes north and south of the array are used. Two dome structures, with velocities 6% faster than average, are detected at depths down to 7 km. They may be the fossil roots of extinct central volcanoes. P, S, PmP, and SmS wave travel times indicate that the compressional-to-shear-wave velocity ratio in the crust is 1.75 to 1.76, with no significant variation detected between the mid and lower crust. Near-solidus lower crustal temperatures, previously predicted on the basis of high surface heat flow, are ruled out. The crust is 25 to 31 km thick, with the southward thickening occurring in an abrupt step. The relatively high Pn and Sn wave apparent velocities of 8.00 ± 0.1 km/sec and 4.31 ± 0.04 km/sec, respectively, from an earthquake in southern Iceland, are consistent with a mantle lid, that is, a layer of subsolidus mantle separating the Moho from a deeper, partial melt zone. Travel times from P waves crossing the NVZ from a fan shot in eastern Iceland cross the previously identified Krafla high-velocity dome at a depth of 8 to 10 km. They provide new support for the existence of this mid-crustal feature and indicate that its base extends 40 to 50 km along the strike of the NVZ. The neighboring Þeistareykir central volcano has no dome at 8 to 10 km depth, suggesting that it has had a more subsidiary role in the formation of the lower crust.