We report results of a seismic, gravity, and magnetic survey of the Reykjanes Ridge spreading center at 61¿--62 ¿N, about 600 km from the center of the Iceland mantle plume. Anomalously shallow water on the ridge crest enabled us to record seismic refractions on a 2.4 km hydrophone streamer. The velocity within layer 2A is 2.4¿0.3 km s-1, and its mean thickness is 400¿100 m. The velocity at the base of layer 2A is 3.3¿0.3 km s-1 on the ridge axis, increasing with crustal age to ~4.0 km s-1 at 1.5 Ma and ~4.5 km s-1 at 5 Ma. Assuming that seismic layer 2A on the ridge axis is also the extrusive layer, i.e., the magnetic source layer, we have successfully modeled the variations in amplitude of the magnetic field. The best magnetic model includes enhanced magnetization within layer 2A at the sites of recent volcanic activity as independently recognized in side-scan sonar data. We also present a full crustal seismic model, based on wide-angle seismic recordings on digital ocean bottom hydrophones and disposable sonobuoys. The seismic model is complemented by gravity modeling, which further suggests that the ridge crest is in isostatic equilibrium. The zero age crust is 10.0 km thick, while crust of age 5 Ma is 7.8 km thick. These crustal thicknesses are greater than those of normal oceanic crust, which we attribute to the presence of anomalously hot asthenospheric mantle beneath the spreading center. We suggest that the variation in thickness between 0 Ma and 5 Ma crust is caused by temporal variation in the plume-fed asthenospheric temperature beneath the Reykjanes Ridge. ¿ 1998 American Geophysical Union