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476 SEAMOUNTS magma body (10–100 km3) at a depth of km below the surface Small melt bodies that have been detected seismically in the uppermost mantle near the EPR (East Pacific Rise) may be evidence that even very small near-ridge seamounts have ephemeral magma chambers Distribution Global Distribution and Spatial Arrangement The earliest studies of seamount distribution and abundance in the ocean basins relied on estimates of height and volume obtained from single-beam echosounder crossings of seamounts Since single-beam data provide only a cross-sectional view of the topography, with no guarantee of crossing the summit, approximations were required to estimate true seamount height and shape Nevertheless, these early studies provided valuable indications of the distribution of seamounts in the ocean basins Modern multi-beam echosounders have revealed the complexity and variety of seamount shapes and distributions, but still only cover regional areas (a few hundreds of square kilometres) The development of satellite altimetry has allowed the complete mapping of the larger seamounts in the ocean basins (Figure 1) Studies of seamounts suggest that satellite altimetry reliably detects seamounts that are at least km high The global distribution of seamounts is best approximated by power-law models Power-law models predict that there are in the order of 105 seamounts over km high in the ocean basins and in the order of 107 seamounts including seamounts that are much less than km high Over 50% of all seamounts are found in the Pacific Ocean Statistical studies of seamount distribution find very clear differences in the distributions of seamounts in different oceans and in different tectonic settings (Table 1) Seamounts are distributed in several spatial patterns The most widely known pattern is the seamount chain (Figure 2) In seamount chains, several very closely spaced individual volcanoes occur in a line The Hawaiian–Emperor Seamounts are a classic example of a seamount chain In this chain, as in many others, the bases of individual seamounts nearly touch or may completely overlap, but each volcano creates an individual peak When seamounts are even more closely spaced, so that individual seamounts along the chain are difficult to recognize, the arrangement is called a volcanic or aseismic ridge Aseismic ridges often occur along the track of a hotspot, for example the Ninety Figure Global distribution of seamounts (indicated by plus symbols scaled according to the size of the seamount edifice) detected by Geosat/ERS altimetry Approximately 14 675 seamounts ranging in size from approximately km to km were identified Reproduced by permission of American Geophysical Union from Wessel P (2001) Global distribution of seamounts inferred from gridded Geosat/ERS altimetry Journal of Geophysical Research 106: 19 431 19 441 Copyright 2001 American Geophysical Union
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