A northwest trending group of seamounts located just west of the East Pacific Rise (EPR) axis has been mapped using high-resolution Sea Marc I side-scan sonar and Sea Beam multibeam bathymetry. These seamounts have 9--13 km basal diameters, and the shapes of the volcanos change from conical domes near the EPR axis to truncated cones and larger, elliptical truncated cones farther away from the rise axis. The detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions (craters to large complex calderas) and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow. Our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse. The creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera.

The concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau. The steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera. The steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain. As a seamount evolves, the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits. The final phase of volcanism may be a caldera-filling episode that builds a rough conical mount over the summit plateau.