The submarine volcanoes that form the pedestals on which basaltic islands built of subaerial lava flows stand must be composed largely of pyroclastic materials, for basaltic lava emitted under water—at any rate under shallow water—generally explodes, producing tuff and scoria. The steepness of their submarine slopes confirms this, for it indicates accumulation of fragmentary materials.
Under very deep water hydrostatic pressure prevents vesiculation, so that lava flows are poured out quietly on the sea-floor. A more or less cylindrical core built of such flows probably, therefore, underlies oceanic basaltic islands at a considerable depth; the non-vesiculated basalt of this core will be sufficiently dense to account for the distribution of the Bouguer anomalies of gravity on the islands and for seismic-wave velocities under them. At a depth of about 2000 m, however, the dense core will be buried and surrounded by pyroclastic material, typical of the pedestal.
On the island of St Helena a scoriaceous formation underlying the subaerial basalt flows is exposed as a result of erosion following an anticlinal upheaval; it seems to have originated as the submarine pedestal of the island. Certain uniformly upheaved volcanic islands—Mangaia in the Cook Group, Nuku Hiva and Hiva Oa in the Marquesas, and probably Moorea, Huaheine, and Borabora in the western Society Islands—seem to be largely composed of pedestal material, and some of them are capped by remnants of the much more resistant subaerial lava flows of the original islands, which may form conspicuous buttes.