SEAMOUNTS 475 SATURN See SOLAR SYSTEM: Jupiter, Saturn and Their Moons SEAMOUNTS S M White, University of South Carolina, Columbia, SC, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction Maps of the seafloor reveal a surface that is dotted with a vast number of cones and truncated cones, arranged in lines, grouped in clusters, or isolated These are known as seamounts, the name given to any steep-sloped more-or-less conical feature on the seafloor The overwhelming majority of seamounts are volcanoes In fact, seamounts are the most common volcanic landform on Earth, but one of the least studied, owing to their wide dispersal and relative inaccessibility As a consequence, much of the information presented here is derived from a small number of well-studied seamounts and applied to the general population of seamounts Seamounts are officially defined as volcanic cones on the seafloor with at least km of relief This distinction arose during the days of seafloor mapping with wide-beam echosounders when only these relatively large features could be unambiguously identified Technological advances in remote sensing have improved our ability to locate and image seamounts of increasingly smaller size Modern shipboard multibeam echosounders provide high-resolution maps showing the morphology and distribution of small (>50 m high) seamounts Deep-towed sonars reveal tiny (2 km high) seamounts There is little scientific justification for a km height cut-off for seamounts All volcanoes start growing from the seafloor, regardless of their current size For the purpose of this work, seamounts are defined broadly to include submarine volcanic edifices of any size Seamounts, ocean islands, and guyots form a natural continuum in the process of submarine volcanic construction from submerged, through emergent, to erosional This article focuses on the processes that build and erode seamounts and their distribution in the oceans The quasi-conical shape and volcanic origin of seamounts distinguish them from other relief-forming features on the seafloor, such as abyssal hills and carbonate reefs, which are not dealt with here One exception is the process of serpentine mud volcanism, which builds seamounts in convergentmargin settings Geochemical and Geophysical Characteristics Nearly all seamounts, aside from some in island arcs, are composed of basalt Geophysical studies also suggest that seamounts have densities, magnetizations, and seismic velocities that are consistent with porous basalt Most seamounts consist of a base layer of alkali basalt that grades into voluminous edifice-building tholeiitic basalt, which is capped by alkali basalt Occasionally differentiation products of alkali basalt (hawaiite, mugearite, benmoreite, and trachyte) are observed in the latter stages of growth However, not all seamounts exhibit all these growth stages, and small seamounts in particular may cease activity before reaching the main tholeiite stage Smaller seamounts located near mid-ocean ridges have a composition that strongly resembles that of the lava erupted at the mid-ocean ridge but have a tendency to be both more primitive and more depleted than ridge-axis basalt Seismic, geodetic, and gravity studies have identified magma chambers beneath several ocean islands Geophysical studies of the Kilauea volcano on Hawaii provide perhaps the clearest picture of a magma system anywhere These studies define a partly molten conduit that rises to about km beneath the summit caldera, with large blade-like dikes running laterally down the rift zones Large positive gravity anomalies centred beneath many of the islands of French Polynesia suggest large (100 km3) frozen magma chambers at shallow levels (2–4 km) At Krafla on Iceland, attenuation of seismic shear waves suggests a large