The geophysical data for the Atlantic Ocean north of 30°N, and for the Labrador, Greenland, and Norwegian Seas, are sparse in comparison with those from the midlatitudes, but new information is being accumulated rapidly due to the stimulus of new ideas about ocean-floor spreading. This summary is based on, published data and such unpublished information as investigators have released.

The North Atlantic and the northern marginal seas are dominated by the Mid-Atlantic Ridge and its extension toward the Arctic Ocean. As elsewhere, the axial zone of the ridge is also an earthquake epicentral belt and is associated with high heat flow. Where sufficient magnetic observations have been made, magnetic lineations have been found to parallel the ridge axis.

In deep water north of 50°N, almost irrespective of geographic location, seismic-refraction results more closely resemble those from the Mid-Atlantic Ridge than those from ocean basins. In only one instance has a velocity as high as 8 km/sec been reported. Velocity below the deepest interface is commonly 7.4-7.7 kim/sec. In contrast to the zero or negative free-air gravity anomalies elsewhere in the Atlantic, a broad zone with free-air anomalies greater than +20 mgal spreads north of 25-30°N along the ridge, possibly extending over Iceland, the Denmark Strait, and the Iceland-Faeroe Ridge.

South of Iceland the sediment cover in the North Atlantic is greater west of the ridge than it is on the east, and is thin or absent over the axial zone.

The ocean-floor spreading rate inferred from magnetic data is lower than elsewhere, being of the order 1 cm/yr. There is some indication of a magnetic pattern similar to that observed over midocean ridges paralleling the buried ridge structure in the Labrador Sea and Davis Strait. Anomalies are most apparent away from the center of the Mid-Atlantic Ridge, and are small or absent near the axis. This situation suggests that, although spreading may have occurred there, the region is now quiescent.

In summary, the region north of 50°N is characterized (1) by the almost complete absence of sub-moho velocities of 7.9-8.5 km/sec; (2) by a broad zone of positive free-air anomalies; (3) by widespread volcanism; (4) by magnetic anomalies in the ridge zone indicating a low rate of spreading that was rejuvenated recently; and 5) by topography strongly reflecting the presence of nearby landmasses and the ridge structure.