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Definition of a Seamount

Traditionally, geologists have defined seamounts as undersea mountains whose summits rise more than 1,000 meters above the surrounding sea floor and that, to first order, exhibit a conical shape with a circular, elliptical or more elongate base. With few exceptions, seamounts are volcanic in origin. Initially, a seamount was defined as a “large isolated elevation characteristically of conical form” [Murray, 1941], where “large” would only later be quantified to mean a relief of 1000 m or more [Menard, 1964]. When the Davidson Seamount was named in 1938, the U.S. Board on Geographic Names stated, "The generic term 'seamount' is here used for the first time, and is applied to submarine elevations of mountain form whose character and depth are such that the existing terms bank, shoal, pinnacle, etc., are not appropriate”. As our understanding of the geologic processes that form seamounts and their distribution has improved, the strict 1000-m-relief limitation has been relaxed in practice, as the geological literature now routinely apply the term “seamount” to much smaller structures (down to a few tens of meters). Studies of seamount populations reveal that their size-frequency distributions are continuous with no obvious break. Thus, seamounts do not have a clear lower-size limit, making a ny size-based criteria for defining them arbitrary. Consequently, the term “seamount” has been applied more generally to topographic “hill” elevations regardless of size and relief [e.g., Epp and Smoot, 1989; Rogers, 1994] (see Summary Section of Chapters 1-3).

For the purpose of this book (which is biological rather than geological) we adopt a more functional approach and define as a seamount any topographically distinct seafloor feature that is at least 100 meters high but which does not break the sea surface. We will, however, exclude large banks and shoals (e.g. Georges Bank, Porcupine Bank) as well as topographic features on continental shelves since these have been dealt with elsewhere in the literature and, in any case, differ from true seamounts in terms of size (in the case of large banks and shoals) and proximity to other shallow topography (in the case of topographic features on the continental shelf).

We will also classify individual seamounts on the basis of summit depth. However, rather than arbitrarily picking absolute depth limits, we have chosen to use functional criteria that are important in regulating biological productivity (see Summary Section). Consequently, we define shallow seamounts as those that penetrate the euphotic zone, intermediate seamounts as those that are shallower than the daytime depth of the deep scattering layer (but which do not reach the euphotic zone), and deep seamounts as those with summits below the deep scattering layer. Oceanic islands, many of which have the same origins as seamounts, share many common features and ecological effects on their submerged slopes.

Finally, we will classify seamounts as being large or small, depending on whether the heights exceed 1500 meter (regardless of depth). This height separation is useful in isolating large seamounts, whose global distribution is well resolved by satellite altimetry, from small seamounts whose distribution must be inferred from local, acoustic mapping and therefore remain poorly sampled.

References cited
Epp, D. and N. C. Smoot (1989) Distribution of seamounts in the North Atlantic. Nature 337: 254–257.
Menard, H. W. (1964) Marine geology of the Pacific. New York, McGraw-Hill.
Murray, H. W. (1941) Submarine mountains in the Gulf of Alaska." Bull. Geol. Soc. Am. 52: 333–362.
Rogers, A. D. (1994) The biology of seamounts. Adv. Mar. Biol. 30: 305–350.