Irregularly Shaped Seamounts Near the East Pacific Rise: Implications for Seamount Origin and Rise Axis Processes

Abstract : We present new Seabeam and SeaMARC I data for small (<1 km high) volcanoes near the East Pacific Rise (EPR) that exhibit irregular (non-circular) plan-form shapes and whose surfaces are commonly cut by young faults and fissures. The structural control, petrology and close proximity of these irregular seamounts to the EPR strongly suggests a close genetic link between EPR processes and seamount formation and growth. We infer that their irregular shapes are due to early growth from eruptive conduits (fractures) that consist of intersecting sets of EPR parallel, EPR-perpendicular and oblique faults. Since many small volcanoes near the EPR have irregular shapes, we suggest that larger circular volcanoes may evolve from small irregular ones. This could occur if volcano shape is dominated by the geometry of their conduits in the early stages of growth (in turn controlled by local tectonic stress) but later their shapes are dominated by caldera collapse and gravitational stress within the edifice.