Seamounts

[Christiana Hanaman]

Seamount

A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface (sea level), and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of 1,000–4,000 metres (3,281–13,123 ft) depth. They are defined by oceanographers as independent features that rise to at least 1,000 metres (3,281 ft) above the seafloor. The peaks are often found hundreds to thousands of metres below the surface, and are therefore considered to be within the deep sea.[1] There are an estimated 100,000 seamounts across the globe, with only a few having been studied. Seamounts come in all shapes and sizes, and follow a distinctive pattern of growth, activity, and death. In recent years, several active seamounts have been observed, for example Loihi in the Hawaiian Islands.

Because of their abundance, seamounts are one of the most common oceanic ecosystems in the world. Interactions between seamounts and underwater currents, as well as their elevated position in the water, attract plankton, corals, fish, and marine mammals alike. Their aggregational effect has been noted by the commercial fishing industry, and many seamounts support extensive fisheries. There are ongoing concerns on the negative impact of fishing on seamount ecosystems, and well-documented cases of stock decline, for example with the orange roughy (Hoplostethus atlanticus). 95% of ecological damage is done by bottom trawling, which literally scrapes whole ecosystems off seamounts.

Because of their large numbers, many seamounts remain to be properly studied, and even mapped. Bathymetry and satellite altimetry are two technologies working to close the gap. There have been instances where naval vessels have collided with uncharted seamounts; for example, Muirfield Seamount is named after the ship that struck it in 1973. However, the greatest danger from seamounts are flank collapses; as they get older, extrusions seeping in the seamounts put pressure on their sides, causing landslides that have the potential to generate massive tsunamis.

[Christiana Hanaman]

Bathymetric showing part of Davidson Seamount. The dots indicate significant coral nurseries.
   Littoral zone
   Intertidal zone
   Estuaries
   Kelp forests
   Coral reefs
   Ocean banks
   Continental shelf
   Neritic zone
   Straits
   Pelagic zone
   Oceanic zone
   Seamounts
   Hydrothermal vents
   Cold seeps
   Demersal zone
   Benthic zone

[Christiana Hanaman]

Geography

Seamounts can be found in every ocean basin in the world, distributed extremely widely both in space and in age. A seamount is technically defined as an isolated elevational rising of 1,000 m (3,281 ft) or more from the surrounding seafloor, and with a limited summit area,[2] a definition drafted in 1964. This definition is no longer strictly adhered to however, and some scientists recognize features as short as 100 m (328 ft) as seamounts. Under the strictest definition there are up to 100,000 seamounts in the oceans, and under the loosest there may be as many as 2 million;[3] however, there are many very small and very deep seamounts that are difficult to analyze, so the true number may never be known.[2]

Most seamounts are volcanic in origin, and thus tend to be found on oceanic crust near mid-ocean ridges, mantle plumes, and island arcs. Nearly half of the world's seamounts are found in the Pacific Ocean, and the rest are distributed mostly across the Atlantic and Indian oceans. Overall there is also a significant bias in distribution towards the southern hemisphere.[2]

Seamounts are often found in groupings or submerged archipelagos, a classic example being the Emperor Seamounts, an extension of the Hawaiian Islands. Formed millions of years ago by volcanism, they have since subsided far below sea level. This long chain of islands and seamounts extends thousands of kilometers northwest from the island of Hawaii. Isolated seamounts and those without clear volcanic origins are less common; examples include Bollons Seamount, Eratosthenes Seamount, Axial Seamount and Gorringe Ridge.[4] If all known seamounts were collected into one area, they would make a landform the size of Europe.[5] Their overall abundance makes them one of the most common, and least understood, marine structures and biomes on Earth,[6] a sort of exploratory frontier.[7]

[Christiana Hanaman]

A partial mapping of some of the world's major seamounts

[Christiana Hanaman]

Geochemistry and evolution

Most seamounts are built by one of two volcanic processes. Volcanoes near plate boundaries and mid-ocean ridges are built by decompression melting of rock in the mantle that then floats up to the surface, while volcanoes formed near subducting zones are created because the subducting plate adds volatiles to the rising plate that raise its melting point. Which process formed the seamount has a profound effect on its eruptive materials. Lava flows from mid-ocean ridge and plate boundary seamounts are mostly basaltic (both tholeiitic and alkalic), whereas flows from subducting ridge volcanoes are mostly calc-alkaline lavas. Compared to mid-ocean ridge seamounts, subduction zone seamounts generally have more sodium, alkali, and volatile abundances, and less magnesium, resulting in more explosive, viscous eruptions.[7]

All volcanic seamounts follow a particular pattern of growth, activity, subsidence and eventual extinction. The first stage of a seamount's evolution is its early activity, building its flanks and core up from the sea floor. This is followed by a period of intense volcanism, during which the new volcano erupts almost all (e.g. 98%) of its total magmatic volume. The seamount may even grow above sea level to become an oceanic island (for example, the 2009 eruption of Hunga Tonga). After a period of explosive activity near the ocean surface, the eruptions slowly die away. With eruptions becoming infrequent and the seamount losing its ability to maintain itself, the volcano starts to erode. After finally becoming extinct (possibly after a brief rejuvenated period), they are ground back down by the waves. Seamounts are built in a far more dynamic oceanic setting then their land counterparts, resulting in horizontal subsidation as the seamount moves on the grinding plate towards a subduction zone. Here it is subducted under the plate margin and ultimately destroyed, but it may leave evidence of its passage by carving an indentation into the opposing wall of the subduction trench. The majority of seamounts have already completed their eruptive cycle, so access to early flows by researchers is limited by late volcanic activity.[7]

[Christiana Hanaman]

Ocean-ridge volcanoes in particular have been observed to follow a certain pattern in terms of eruptive activity, first observed with Hawaiian seamounts but now shown to be the process followed by all seamounts of the ocean-ridge type. During the first stage the volcano erupts basalt of various types, caused by various degrees of mantle melting. In the second, most active stage of its life, ocean-ridge volcanoes erupt tholeiitic to mildly alkalic basalt as a result of a larger area melting in the mantle. This is finally capped by alkalic flows late in its eruptive history, as the link between the seamount and its source of volcanism is cut by crustal movement. Some seamounts also experience a brief "rejuvenated" period after a hiatus of 1.5 to 10 million years, the flows of which are highly alkalic and produce many xenoliths.[7]

In recent years, geologists have confirmed that a number of seamounts are active undersea volcanoes; two examples are Lo‘ihi in the Hawaiian Islands and Vailulu'u in the Manu'a Group (Samoa).[4]

[Christiana Hanaman]

Diagram of a submarine eruption. (key: 1. Water vapor cloud 2. Water 3. Stratum 4. Lava flow 5. Magma conduit 6. Magma chamber 7. Dike 8. Pillow lava) Click to enlarge.

[Christiana Hanaman]

Lava types

The most apparent lava flows at a seamount are the eruptive flows which cover their flanks, however igneous intrusions, in the forms of dikes and sills, are also an important part of seamount growth. The most common type of flow is pillow lava, named so after its unusual shape. Less common are sheet flows, which are glassy and marginal, and indicative of larger-scale flows. Volcaniclastic sedimentary rocks dominate shallow-water seamounts. They are the products of the explosive activity of seamounts that are near the water's surface, and can also form from mechanical wear of existing volcanic rock.[7]

[Christiana Hanaman]

Pillow lava, a type of basalt flow that originates from lava-water interactions during submarine eruptions.[8]

[Christiana Hanaman]

Structure

Seamounts can form in a wide variety of tectonic settings, resulting in a very diverse structural bank. Seamounts come in a wide variety of structural shapes, from conical to flat-topped to complexly shaped.[7] Some are built very large and very low, such as Koko Guyot[9] and Detroit Seamount;[10] others are built more steeply, such as Loihi Seamount[11] and Bowie Seamount.[12] Some seamounts also have a carbonate or sediment cap.[7]

Many seamounts show signs of intrusive activity, which is likely to lead to inflation, steepening of volcanic slopes, and ultimately, flank collapse.[7] There are also several sub-classes of seamounts. The first are guyots, seamount with a flat top. These tops must be 200 m (656 ft) or more below the surface of the sea; the diameters of these flat summits can be over 10 km (6.2 mi).[13] Knolls are isolated elevation spikes measuring less than 1,000 meters (3,281 ft). Lastly, pinnacles are small pillar-like seamounts.[2]

[Christiana Hanaman]

Ecological role of seamounts

Seamounts are exceptionally important to their biome ecologically, but their role in their environment is little understood. Because they project out above the surrounding sea floor, they disturb standard water flow, causing eddies and associated hydrological phenomena that ultimately result in water movement in an otherwise still ocean bottom. Currents have been measured at up to 0.9 knots, or 48 centimeters per second. Because of this upwelling seamounts often carry above-average plankton populations, seamounts are thus centers where the fish that feed on them aggregate, in turn falling prey to further predation, making seamounts important biological hotspots.[2]

Seamounts provide habitats and spawning grounds for these larger animals, including numerous fish. Some species, including black oreo (Allocyttus niger) and blackstripe cardinalfish (Apogon nigrofasciatus), have been shown to occur more often on seamounts then anywhere else on the ocean floor. Marine mammals, sharks, tuna, and cephalopods all congregate over seamounts to feed, as well as some species of seabirds when the features are particularly shallow.[2]

[Christiana Hanaman]

Seamounts often project upwards into shallower zones more hospitable to sea life, providing habitats for marine species that are not found on or around the surrounding deeper ocean bottom. Because seamounts are isolated from each other they form "undersea islands" creating the same biogeographical interest. As they are formed from volcanic rock, the substrate is much harder than the surrounding sedimentary deep sea floor. This causes a different type of fauna to exist than on the seafloor, and leads to a theoretically higher degree of endemism.[15] However, recent research especially centered at Davidson Seamount suggests that seamounts may not be especially endemic, and discussions are ongoing on the effect of seamounts on endemicity. They have, however, been confidently shown to provide a habitat to species that difficulty surviving elsewhere.[16][17]

[Christiana Hanaman]

The volcanic rocks on the slopes of seamounts are heavily populated by suspension feeders, particularly corals, which capitalize on the strong currents around the seamount to supply them with food. This is in sharp contrast with the typical deep-sea habitat, where deposit-feeding animals rely on food they get off the ground.[2] In tropical zones extensive coral growth results in the formation of coral atolls late in the seamount's life.[3][17]

In addition soft sediments tend to accumulate on seamounts, which are typically populated by polychaetes (annelid marine worms) oligochaetes (microdrile worms), and gastropod mollusks (sea slug[disambiguation needed]s). Xenophyophores have also been found. They tend to gather small particulates and thus form beds, which alters sediment deposition and creates a habitat for smaller animals.[2] Many seamounts also have hydrothermal vent communities, for example Suiyo[18] and Loihi seamounts.[19] This is helped by geochemical exchange between the seamounts and the ocean water.[7]

Seamounts may thus be vital stopping points for some migratory animals, specifically whales. Some recent research indicates whales may use such features as navigational aids throughout their migration.[20] For a long time it has been surmised that many pelagic animals visit seamounts as well, to gather food, but proof of this aggregating effect has been lacking. The first demonstration of this conjecture has recently[update] been published.[21]

[Christiana Hanaman]

Grenadier fish (Coryphaenoides sp.) and bubblegum coral (Paragorgia arborea) on the crest of Davidson Seamount. These are two species attracted to the seamount; Paragorgia arborea in particular grows in the surrounding area as well, but nowhere near as profusely.[14]

[Christiana Hanaman]

Fishing

The effect that seamounts have on fish populations has not gone unnoticed by the commercial fishing industry. Seamounts were first extensively fished in the second half of the 20th century, due to poor management practices and increased fishing pressure seriously depleting stock numbers on the typical fishing ground, the continental shelf. Seamounts have been the site of targeted fishing since that time.[22]

Nearly 80 species of fish and shellfish are commercially harvested from seamounts, including spiny lobster (Palinuridae), mackerel (Scombridae and others), red king crab (Paralithodes camtschaticus), red snapper (Lutjanus campechanus), tuna (Scombridae), Orange roughy (Hoplostethus atlanticus), and perch (Percidae).[2]