Seamount morphology in the Bowie and Cobb hot spot trails, Gulf of Alaska

Abstract

Full‐coverage multibeam bathymetric mapping of twelve seamounts in the Gulf of Alaska reveals that they are characterized by flat‐topped summits (rarely with summit craters) and by terraced, or step‐bench, flanks. These summit plateaus contain relict volcanic features (e.g., flow levees, late‐stage cones, and collapse craters) and as such must have been constructed by volcanic processes such as lava ponding above a central vent, rather than by erosion above sea level. The terraced flanks are composed of a sequence of stacked lava deltas and cones, probably tube‐fed from a central lava pond, a morphology which is suggestive of long‐lived, stable central lava sources and low to moderate eruption rates, indicative of significant time spent above a hot spot outlet. Most of these seamounts have summit plateaus surrounded, and cut into, by amphitheater headwall scarps, and flanks that are scarred by debris chutes, but lack visible debris accumulations at their base. We interpret the lack of blocky debris fields as evidence that the slope failures are mainly small‐scale debris flows, rather than large‐scale flank collapses. However, we cannot rule out the possibility that large flank‐collapse blocks from early in the histories of these seamounts are now hidden beneath the thick glacio‐fluvial fan deposits that cover the Gulf of Alaska seafloor. These slope failure features become smoother and longer and increase in size and abundance with increasing age of a seamount, suggesting that slope failure processes continue long after volcanic activity ceases.