Seamounts are traditionally defined as undersea mountains whose summits rise more than 1000 m above the sea floor; however, modern studies describe seamounts down to several tens of meters in height. They generally exhibit a conical shape with a circular, elliptical, or more elongated base. Seamounts are some of the most ubiquitous landforms on Earth and are present in uneven proportions in all ocean basins. Being volcanic in nature, seamounts are mostly found on oceanic crust and to a much lesser extent on extended continental crust. They are generated near mid-ocean spreading ridges, in plate interiors over upwelling plumes (hotspots), and in island-arc convergent settings. Oceanic islands form a small subset of large seamounts that have breached sea level.
Most seamounts and islands are constructional aggregates of basalt, reflecting their volcanic origin. Seamounts are typically formed in one of three distinct tectonic settings, each imparting unique tectonic characteristics to its offspring.
The majority of larger seamounts found in the ocean basins were formed in an intraplate setting. Because of their frequent alignment into linear, subparallel chains that correlate with the direction of past plate motions, the consensus origin of such seamounts is given by the hotspot hypothesis, which states that these seamounts formed above more or less stationary mantle plumes (or hotspots) in the Earth's mantle. As the plates move, the seamounts thus formed are carried away from the source of magma and cease volcanic activity, building a line of extinct volcanoes that exhibits a monotonic age progression reflecting the plate motion history. Numerous hotspots have been proposed for sites of unusual volcanic activity, yet conclusive imaging of mantle plumes using seismic tomography remains elusive. Although the simple age progressions predicted by the hotspot hypothesis have been confirmed for several seamount chains, others show complex age patterns, which cast doubt on the hotspot theory as the only explanation.
Seamounts formed by hotspot volcanism may grow quite large (Fig. 1A). In particular, hotspot seamounts formed on old (and hence thicker and stronger) oceanic lithosphere can in some cases reach almost i0 km (measured from their base), making Mauna Kea (one of five volcanoes that form the Big Island of Hawaii) the tallest mountain on Earth. Seamounts formed on oceanic crust must reach at least 2.5 km in height just to match the typical mid-ocean ridge depth; however, most larger seamounts were formed in even deeper water; hence, only truly large seamounts will become islands or have a shallow-water presence. Because large seamounts often penetrate the euphotic zone, they have been the main focus of ecological studies, despite being a small subset of all seamounts globally.
Most seamounts are small and were formed near a divergent plate boundary. Here, excess amounts of magma percolate through the thin, fractured crust to form small, sub-circular seamounts—often just a few tens to hundreds of meters tall. Occasionally, larger seamounts can be formed (Fig. 1B). It is likely that most small seamounts formed in this near-ridge environment as the thickness of the lithosphere rapidly increases away from ridges, making the ascent of small amounts of magma from an increasingly deeper source less likely. Consequently, sea- mount production rates decrease with increasing crustal age and lithospheric thickness, being highest close to the ridge axis. At fast-spreading ridges (e.g., the East Pacific Rise), small seamounts form on the flanks of the ridge where the crust is just 0.2—0.3 million years old, and their abundance correlates with spreading rate. At slow-spreading ridges (e.g., the Mid-Atlantic Ridge), small seamounts are produced almost exclusively within the median valley. Many new seamounts undergo extensive tectonic deformation by normal faulting, which reduces their original heights considerably. Because of increased sediment coverage on older sea floor, the smallest and most numerous seamounts, with heights less than i00 m, are likely to be buried after a few tens of millions of years.
Island arc seamounts form at subduction zones where one oceanic plate is being forced to subduct beneath the other. As plates descend into the mantle, the higher pressure and friction and the increasing temperatures and water content eventually cause decompressional melting that produces an ascending basaltic melt of a different magmatic composition than the basalt available at spreading centers (Fig. 1C). The magma may be more volatile, thus increasing the chance of explosive eruptions. The distribution of island arc seamounts and islands reflects the trend of the convergent plate boundaries, and the overall plate tectonic geometry places strong constraints on the evolution of such seamounts. These island arc seamounts are found in the relatively narrow collision zones between the converging tectonic plates, thus occupying a small area of the total sea floor. Like hotspot-produced seamounts, island arc seamounts can reach considerable height and often form islands. Unlike hotspot-produced seamounts, the volcanic activity along an active arc is essentially simultaneous, geologically speaking, with older seamounts constantly being overprinted by younger ones.
Seamounts are born kilometers below the sea surface. Following a pathway of preexisting cracks or weaknesses, buoyant magma finds its way to the ocean floor. Here, emerging seamounts may be exposed to overburden pressures of 25–50 MPa. Consequently, volcanic gases within the magma cannot expand, and extrusive flows are effusive. The cooling effect of seawater affects the shape of the volcano, allowing construction of steeper flanks (greater than 10°) than would generally be possible once the volcano builds up above sea level (less than 10°). At first, the seamount is fed from a central vent, yielding an almost circular feature, and some develop summit craters. Many seamounts do not develop beyond this stage. However, if adequate magma supply is available, and the seamount is allowed to grow taller, then gravitational stresses in the flanks of the seamount, possibly enhanced by flexural stresses transmitted from the increasingly deformed subsurface, will favor the development of rift zones. These break the circular symmetry and promote construction of long ridges from fissure eruptions. As the summit of the seamount approaches sea level, water pressure can no longer keep gases locked up in the magma, and explosive eruptions become common. The extrusive products tend to be finer-grained, more vesicular, and structurally less resistant to erosion, which begins to shape the islands, augmented by catastrophic submarine landslides. The combined effect of rift zones, erosion, and landslides is to modify the basic circular form of seamounts into stellate forms.
Once the island is well established, the volcano enters the shield-building stage, during which large flows of 'a'a and pahoehoe lava are extruded. When active construction finally wanes, the island no longer regenerates to keep up with the destructive forces of erosion, which combine with long-term thermal subsidence of the sea floor and isostatic adjustments to bring the summit area back to sea level, where wave erosion forms a flat-topped guyot. Coral growth may keep up with the subsidence rate, capping many volcanic islands with a thick coral reef layer before subsidence eventually drowns the seamount. Complex interplays between eustatic sea-level changes, vertical isostatic adjustments, and latitude changes caused by plate motion result in a wide variety of seamounts, some with fringing reefs, others with lagoons with calcareous sediments, and others that never developed a coral cap and may have drowned long ago.
Seamounts are distributed both in space (geographically) and time (temporally), and studies of these variations have provided key insights into several factors that control the formation of seamounts.
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- Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry. Journal of Geophysical Research 106: 19, 431-419, 441. 2001.
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