Space-Time Distributions of Seamounts on the Ocean Floor: Constraints on Volumes and Patterns of Asthenospheric Melting

The seafloor features tens of thousands of seamounts of various sizes, most of which are the result of intraplate volcanism occurring on the seafloor. To understand the tectonic and asthenospheric processes that produce this volcanism, we used a recent global database of seamount locations and sizes obtained from satellite altimetry [Kim & Wessel, 2011] to estimate the volume of seamounts as a function of seafloor age. Overall, the volume of large seamounts (heights > 1 km) approximately equates to an ~12 m thick layer spread across the seafloor. Small seamounts (heights < 1 km) are more difficult to detect, especially in deep water, but volume estimates from young (shallow) seafloor indicate that their volume can be estimated as an ~11 m thick layer covering the seafloor. If these seamounts were formed by tapping partial melt at the top of the asthenosphere, they require an average melt fraction of at least ~0.1% over a 20 km layer in their source region. If they source from thinner layers or more restricted domains, such as embedded heterogeneities, larger melt fractions are required. As most seamounts were formed on young seafloor, observed variations in total seamount volume as a function of seafloor age likely indicate variations in seamount volcanic flux over time. Indeed, we observe such variability, with seamount volume correlated to paleo-spreading rate. This correlation indicates that a large fraction of seafloor volcanism was produced on the flanks of fast-spreading ridges (e.g., the East Pacific Rise). Such a relationship may indicate a role for shear heating of the asthenosphere or shear-driven upwelling, in which horizontal shearing of the asthenosphere is diverted vertically by viscosity heterogeneity in the asthenosphere.