In part 1 (Rosendahl et al., 1976) of this study it was found that a wedge-shaped low-velocity zone underlies the axial block of the East Pacific Rise. This low-velocity zone, which occurs within the oceanic crust, is thought to represent a magma reservoir containing approximately 30% melt by volume. One consequence of this interpretation is that isostatic uplift alone can produce the axial block morphology observed along much of the East Pacific Rise crest. Based upon mantle intercept time anomalies, it appears that the axial block of the Reykjanes Ridge also could be a morphologic expression of an underlying magma reservoir. Because the Mid-Atlantic Ridge axial valley is about an order of magnitude larger than the probable width of any underlying crustal zone of partial melt, the morphology there could not be generated by volumetric changes (e.g., via magma withdrawal) in a shallow magma chamber. Another consequence of the magma reservoir interpretation is that it leads to new inferences on the constitution of ocean crust. In particular, it is suggested that crust generated at spreading systems with axial block morphology may show both chemical and stratigraphic differences from crust generated at axial valley spreading systems.