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Definition: Seamount from The Seafaring Dictionary: Terms, Idioms and Legends of the Past and Present

A conical underwater mountain rising at least 500 fathoms above the sea bed, with its summit usually about the same distance below the surface. Cf. guyot.


Summary Article: SEAMOUNTS, GEOLOGY from Encyclopedias of the Natural World: Encyclopedia of Islands

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.

THE VOLCANIC ORIGIN OF SEAMOUNTS AND ISLANDS

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.

Intraplate Seamounts

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.

(A) Intraplate seamount formation over the Hawaii hotspot. On thick lithosphere, seamounts can grow very tall and even breach sea level to form islands. The volcano deforms the lithosphere, which responds by flexure. The hotspot feeds the active volcanoes by a network of feeder dikes; magma may pond beneath the crust as well. As plate motion carries the volcanoes away, they cease to be active and form a linear seamount chain. (B) Seamount formation near the East Pacific Rise. A thin plate cannot sustain large volcanoes, and typically only smaller cones are found. Excess magma is diverted into feeder dikes that reach the surface on the ridge flank, forming small volcanoes. (C) Island arc formation behind the Kermadec Trench. The subducting Pacific plate and its sediments will induce melt at depth, eventually erupting to create a volcanic arc that parallels the subduction zone. Note that the oldest part of the Louisville seamount chain (another intraplate chain) is currently being subducted.

Mid-Ocean Ridge Seamounts

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

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.

MORPHOLOGY AND THE EVOLUTION OF SEAMOUNTS

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.

THE DISTRIBUTION OF SEAMOUNTS

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.

Spatial Distribution

Distribution of seamounts inferred by satellite altimetry. Colors reflect seamount sizes (see legend). The majority of seamounts can be found in the Pacific basin, with the remainder divided between the Atlantic and Indian Oceans. Large igneous provinces (LIPs) are outlined in orange; these are often associated with seamount provinces.

The number of seamounts varies considerably between ocean basins (Fig. 2). Seamounts form both linear and random constellations, and their sizes and distributions provide invaluable information about their origins. Studies have found that the distribution of seamounts over a wide range of sizes is well approximated by an exponential or power-law model. Such models reflect the observation that most seamounts are small, and by extrapolating the power-law, there may be perhaps as many as 100,000— 200,000 seamounts reaching heights of i km or more (Fig. 3A). Extrapolation further down to the smallest seamount sizes observed (a few tens of meters) would predict seamount populations reaching into the millions, but the majority of such small seamounts will likely be buried, given the typical thickness (100—200 m) of sediment in the world's ocean basins. Consequently, the smallest sea-mounts are observed only on young sea floor with modest sediment cover. Seamount summit depths are normally distributed around a mean depth of ∼3 km. Notably, the shallow end seems to have an additional number of shallow seamounts and islands, possibly reflecting the ability of coral reef growth to keep up with subsidence for long periods of time (Fig. 3B).

(A) Seamount size-frequency distribution. The green circles indicate the number of seamounts taller than a given size. For seamounts greater than 2 km tall, the data are well explained by a scaling rule (solid line). For heights less than 2 km, the trend levels off because numerous smaller seamounts fall below the resolution of altimetry data. Only ç15,000 out of a potential of ç200,000 seamounts greater than 1 km in height may have been mapped. (B) Summit depths follow a normal distribution centered on a ∼3-km depth, with large and shallow seamounts appearing as outliers, possibly as a result of coral growth.

The abundance of seamounts has been shown to vary considerably among the ocean basins. The Pacific basin is host to nearly half of the seamounts that are large enough (greater than ∼ 2 km) to be mapped by satellite altimetry. The Atlantic and Indian Oceans combine to contain most of the remaining seamounts, with considerably fewer seamounts appearing on plate segments located at high latitudes (e.g., northern Atlantic on either the North American or Eurasian plates) or on relatively small plates (e.g., Cocos, Philippine Sea).

It is not clear what causes seamount abundances to vary spatially. One factor may be the underlying distribution of mantle plumes, which are found in higher numbers beneath plates with numerous seamounts. However, one would still expect excess magma at the spreading center to produce the smaller and more numerous sea-mounts. Another factor may be systematic variations in plate stresses, with smaller plates possibly being in a compressional stress state, which would not favor the intrusion of magma. Smaller plates are also less likely to have a directional regional stress dominating the state of stress. In contrast, the large Pacific plate, in particular the equatorial region, appears to be under tension from distant slab pull forces, as evidenced by widespread extensional volcanism associated with neither hotspots nor mid-ocean ridges. Finally, plates that move the fastest appear to have the highest seamount abundances, provided they share at least one spreading plate boundary.

Island arcs aside, the distribution of seamounts appears as a superpositioning of two separate processes: Divergent plate boundaries produce a near-steady stream of new, small seamounts, most of which exhibit no particular clustering pattern, whereas mantle plumes or hotspots generally create both small and large seamounts, which are often organized in linear groups by plate motions. Frequency-size analysis (Fig. 3A) of the combined sea- mount populations does not immediately separate out the two modes of production, but this possibly reflects the inability of satellite altimetry to detect smaller sea- mounts (less than 1 km) and the lack of significant spatial coverage of small-size seamount provinces using multi- beam techniques.

Temporal Distribution

Seamounts are among the youngest geologic features on Earth, reflecting the youthfulness of the oceans and the regenerative processes of plate tectonics. Only a few sea- mounts are currently volcanically active, and they tend to be restricted to (i) the very youngest volcanoes of hotspot island chains (such as Hawaii, Samoa, Réunion, and oth- ers), (2) various places along active island arcs, and (3) newly formed smaller seamounts associated with mid- ocean ridges. Many volcanic islands, but only a few sea- mounts, have been dated using radiometric techniques, yet the sparse age data, the underlying sea-floor age, and the size of seamounts imply that the production of seamounts is not steady-state. During the Cretaceous (146— 65 million years ago) the Pacific seamount production was almost twice as high, resulting in numerous large sea- mounts now residing in the western Pacific. This period also saw the formation of several large oceanic plateaus, such as the Ontong Java, the Manahiki, the Shatsky, and the Mid-Pacific Mountains; hence, plateau and seamount formation appear correlated.

THE IMPACT OF SEAMOUNTS
Geologic Impact

Seamounts are windows into the mantle that allow scientists to study the nature of erupting magma. Minor changes in the chemical and isotopic composition of basaltic lavas can be used to make inferences about magma source depth and composition. Seamounts represent a significant fraction of the entire crust production, perhaps as much as 5—i0%, and variations in this intraplate volcanic budget shed light on plate tectonics and how Earth gets rid of excess heat. The alignment of seamount chains provides a means to decode the motion of tectonic plates over long geologic intervals, enabling an understanding of the climatic changes experienced at islands that simply follow from latitudinal migration of plates carrying seamount provinces on their backs. Many seamounts have active hydrothermal convection systems that may have a significant effect on element cycles involving seawater, and they also participate in the dissipation of residual heat from the formation of both seamount and sea floor. Finally, seamounts and islands act as measuring sticks for relative sea-level variations, which can have both eustatic and tectonic components.

Oceanographic Impact

Bathymetry influences ocean circulation in several ways. The first-order features such as ridges and plateaus steer currents and, in places, act as barriers that prevent deep waters from mixing with warmer, shallower waters. Smaller-scale bathymetry, such as seamounts, may play a largely overlooked role in the turbulent mixing of the oceans. Measurements suggest that mixing around a shallow seamount is many orders of magnitude more vigorous than in areas far from seamounts. Understanding how the climate will evolve depends on how quickly heat and carbon dioxide can penetrate into the deep oceans, and assumed rates of vertical mixing can considerably affect model predictions.

Mineral Resources Impact

Older seamounts may accumulate a ferromanganese oxide crust enriched in the elements cobalt, copper, manganese, and sulfur, typically occurring at depths exceeding 3 km. The total cumulative amounts of such marine mineral resources might exceed the amounts currently available on land. So far, the cost of harvesting deep ocean nodules and crusts has been prohibitive. However, rising prices associated with depletion of terrestrial resources will likely make deep ocean resources more attractive, especially because the bulk of these are in international waters.

SEE ALSO THE FOLLOWING ARTICLES

Island Arcs / Plate Tectonics / Sea-Level Change / Seamounts, Biology

FURTHER READING
  • Batiza, R. 2001. Seamounts and off-ridge volcanism, in Encyclopedia ofocean sciences. Steele, J. H., Thorpe, S. A., and Turekian, K. K., eds. San Diego, CA: Academic Press, 2696-2708.
  • Keating, B. H. 1987. Seamounts, islands, and atolls. Washington, DC: AGU.
  • Macdonald, G. A. 1986. Volcanoes in the sea, 2nd ed. Honolulu: University of Hawaii Press.
  • Schmidt, R., and Schminke, H.-U.. 2000. Seamounts and island building, in Encyclopedia of volcanoes. Sigurdsson, H., ed. San Diego, CA: Academic Press, 383-402.
  • Wessel, P. 2001. Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry. Journal of Geophysical Research 106: 19, 431-419, 441.
  • White, S. M. 2005. Seamounts, in Encyclopedia of geology. Selley, R. C., et al., eds. London: Elsevier, 475-484.
PAUL WESSEL
University of Hawaii, Manoa
© 2009 by the Regents of the University of California

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