Oceanic islands and seamounts

Abstract

In the oceanic environment the majority of volcanic activity is concentrated at plate margins, especially at ocean ridges or spreading centres where the oceanic crust is generated. However, the interiors of oceanic plates are invariably pockmarked by numerous basaltic submarine volcanoes (sea-mounts and guyots) and emergent oceanic islands that testify to extensive, but often localized intraplate volcanism. Oceanic islands within 30° north and south of the equator may be encircled with reefs, or form atolls if the volcanic pedestal is submerged and capped by reefal limestone. The Hawaiian islands are probably the best known example of intraplate volcanism and were considered by Wilson (1963a, b) to be generated by a local upper mantle thermal perturbance that was the source of anomalous mid-plate or hot-spot volcanism. Many of the characteristic linear chains and island groups related to hot-spot activity are sited on large-scale topographic swells (1000–2000 km in width) that elevate some volcanic domains 1–2 km above the adjacent sea-floor (Monnereau and Cazenave, 1988; White, 1989). The geophysical properties of swells (e.g. geoid, gravity and heat flow anomalies) suggest that they are probably supported by deep convective thermal perturbances derived from below the lithosphere (Crough; 1983; Watts et al., 1985).