A detailed analysis of high-resolution side-scan sonar data shows that the volcanic edifices built within the axial zones of the slow spreading (20 mm/yr) Reykjanes Ridge and the slow spreading (25 mm/yr) Mid-Atlantic Ridge are remarkably similar. In fact, individual seamounts, hummocks (rounded mounds), and hummocky ridges at the Reykjanes Ridge have nearly identical morphological counterparts at the Mid-Atlantic Ridge (MAR), arguing that commonly eruptive conditions are similar at the two ridges. The construction of these volcanic edifices, typical of slow spreading ridges, occurs at the Reykjanes Ridge despite its proximity to the Iceland hot spot and the influence of the hot spot on other characteristics of the ridge axis, such as its overall shape (an axial high north of about 59 ¿N), its shallow axial depths, and its thicker crustal section. The volume of magma that goes into constructing volcanic edifices at each of 12 segments of the MAR and four regions of the Reykjanes Ridge is calculated using multibeam bathymetry data. Our results indicate that edifice volume per unit seafloor area, equivalent to a uniform thickness over the area of interest, is larger at the MAR than at the Reykjanes Ridge, on average.

Taking into account spreading rate, we then estimate B, the average building rate per unit length along the axis, which is also larger at the MAR. We conclude that the same volcanic edifices build the crust at the Reykjanes Ridge and MAR, but that their emplacement is more frequent at the MAR. To explain our results and reconcile them with those of Magde and Smith (1995) who documented that seamounts are taller and more abundant at the Reykjanes Ridge than at the MAR, we consider the controls on diking and eruption within the axial zones of the two ridges. We suggest that the combination of slower spreading rate and the oblique geometry of spreading at the Reykjanes Ridge leads to fewer eruptions which are more widely dispersed across the axial zone than at the MAR. If this hypothesis is correct, the proportion of seamounts modified by subsequent eruptions will be less. This might also explain the taller heights of the seamounts if there is less overlap and infilling between edifices. ¿ American Geophysical Union 1995