Seamount - WikiHQ

[[Bear Seamount|thumb]]

A seamount is a large submarine landform that rises from the ocean floor without reaching the water surface (sea level), and thus is not an island, islet, or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly and are usually found rising from the seafloor to in height. They are defined by oceanographers as independent features that rise to at least above the seafloor, characteristically of conical form. The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea. During their evolution over geologic time, the largest seamounts may reach the sea surface where wave action erodes the summit to form a flat surface. After they have subsided and sunk below the sea surface, such flat-top seamounts are called "guyots" or "tablemounts". of which 9,951 seamounts and 283 guyots, covering a total area of , have been mapped but only a few have been studied in detail by scientists. Seamounts and guyots are most abundant in the North Pacific Ocean, and follow a distinctive evolutionary pattern of eruption, build-up, subsidence and erosion. In recent years, several active seamounts have been observed, for example Kamaʻehuakanaloa (formerly Lōʻihi) in the Hawaiian Islands.

Because of their abundance, seamounts are one of the most common marine 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 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.

Geography

thumb|Bathymetric mapping of part of [[Davidson Seamount. The dots indicate significant coral nurseries.]]

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 rise in elevation of or more from the surrounding seafloor, and with a limited summit area, Seamount chains occur in all three major ocean basins, with the Pacific having the most number and most extensive seamount chains. These include the Hawaiian (Emperor), Mariana, Gilbert, Tuomotu and Austral Seamounts (and island groups) in the north Pacific and the Louisville and Sala y Gomez ridges in the southern Pacific Ocean. In the North Atlantic Ocean, the New England Seamounts extend from the eastern coast of the United States to the mid-ocean ridge. Craig and Sandwell Otherwise, seamounts tend not to form distinctive chains in the Indian and Southern Oceans, but rather their distribution appears to be more or less random.

Isolated seamounts and those without clear volcanic origins are less common; examples include Bollons Seamount, Eratosthenes Seamount, Axial Seamount and Gorringe Ridge.

If all known seamounts were collected into one area, they would make a landform the size of Europe. Their overall abundance makes them one of the most common, and least understood, marine structures and biomes on Earth, Volcanoes near plate boundaries and mid-ocean ridges are built by decompression melting of rock in the upper mantle. The lower density magma rises through the crust to the surface. Volcanoes formed near or above subducting zones are created because the subducting tectonic plate adds volatiles to the overriding plate that lowers its melting point. Which of these two process involved in the formation of a 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.

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.]]

The most apparent lava flows at a seamount are the eruptive flows that 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 distinctive 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. and Detroit Seamount; others are built more steeply, such as Kamaʻehuakanaloa Seamount and Bowie Seamount. Some seamounts also have a carbonate or sediment cap. Knolls are isolated elevation spikes measuring less than . Lastly, pinnacles are small pillar-like seamounts.

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 than 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.]]

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. 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 have difficulty surviving elsewhere.

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. These coral are therefore host to numerous other organisms in a commensal relationship, for example brittle stars, who climb the coral to get themselves off the seafloor, helping them to catch food particles, or small zooplankton, as they drift by. This is in sharp contrast with the typical deep-sea habitat, where deposit-feeding animals rely on food they get off the ground. and Kamaʻehuakanaloa seamounts. This is helped by geochemical exchange between the seamounts and the ocean water. 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 was published in 2008.

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.

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).

thumb|left|[[Coral (precious)|Coral earrings of this type are often made from coral harvested off seamounts.]]

Corals from seamounts are also vulnerable, as they are highly valued for making jewellery and decorative objects. Significant harvests have been produced from seamounts, often leaving coral beds depleted.

Possibly the best ecologically studied seamount in the world is Davidson Seamount, with six major expeditions recording over 60,000 species observations. The contrast between the seamount and the surrounding area was well-marked. Much of what is known about seamounts ecologically is based on observations from Davidson.

Exploration

thumb|right|Graph showing the rise in global sea level (in mm) as measured by the [[NASA/CNES oceanic satellite altimeter TOPEX/Poseidon (left) and its follow-on mission Jason-1]]

The study of seamounts has been hindered for a long time by the lack of technology. Although seamounts have been sampled as far back as the 19th century, their depth and position meant that the technology to explore and sample seamounts in sufficient detail did not exist until the last few decades. Even with the right technology available, only a scant 1% of the total number have been explored,

Before seamounts and their oceanographic impact can be fully understood, they must be mapped, a daunting task due to their sheer number.

Observations from CryoSat-2 combined with data from other satellites has shown thousands of previously uncharted seamounts, with more to come as data is interpreted.

Deep-sea mining

Seamounts are a possible future source of economically important metals. Even though the ocean makes up 70% of Earth's surface area, technological challenges have severely limited the extent of deep sea mining. But with the constantly decreasing supply on land, some mining specialists see oceanic mining as the destined future, and seamounts stand out as candidates.

Seamounts are abundant, and all have metal resource potential because of various enrichment processes during the seamount's life. An example for epithermal gold mineralization on the seafloor is Conical Seamount, located about 8 km south of Lihir Island in Papua New Guinea. Conical Seamount has a basal diameter of about 2.8 km and rises about 600 m above the seafloor to a water depth of 1050 m. Grab samples from its summit contain the highest gold concentrations yet reported from the modern seafloor (max. 230 g/t Au, avg. 26 g/t, n=40). Iron-manganese, hydrothermal iron oxide, sulfide, sulfate, sulfur, hydrothermal manganese oxide, and phosphorite (the latter especially in parts of Micronesia) are all mineral resources that are deposited upon or within seamounts. However, only the first two have any potential of being targeted by mining in the next few decades. More recently, the submarine USS San Francisco ran into an uncharted seamount in 2005 at a speed of , sustaining serious damage and killing one seaman.

One major seamount risk is that often, in the late of stages of their life, extrusions begin to seep in the seamount. This activity leads to inflation, over-extension of the volcano's flanks, and ultimately flank collapse, leading to submarine landslides with the potential to start major tsunamis, which can be among the largest natural disasters in the world. In an illustration of the potent power of flank collapses, a summit collapse on the northern edge of Vlinder Seamount resulted in a pronounced headwall scarp and a field of debris up to away. far too high for a normal wave to reach. The date corresponded with a massive flank collapse at the nearby Mauna Loa, and it was theorized that it was a massive tsunami, generated by the landslide, that deposited the fossils.

References

Bibliography

Geology

Keating, B.H., Fryer, P., Batiza, R., Boehlert, G.W. (Eds.), 1987: Seamounts, islands and atolls. Geophys. Monogr. 43:319–334.
Menard, H.W. (1964). Marine Geology of the Pacific. International Series in the Earth Sciences. McGraw-Hill, New York, 271 pp.

Ecology

Pitcher, T.J., Morato, T., Hart, P.J.B., Clark, M.R., Haggan, N. and Santos, R.S. (eds) (2007). "Seamounts: Ecology, Fisheries and Conservation". Fish and Aquatic Resources Series 12, Blackwell, Oxford, UK. 527pp.

External links

Geography and geology

Earthref Seamount Catalogue. A database of seamount maps and catalogue listings.
Volcanic History of Seamounts in the Gulf of Alaska.
The giant Ruatoria debris avalanche on the northern Hikurangi margin, New Zealand . Aftermath of a seamount carving into the far side of a subduction trench.
Evolution of Hawaiian volcanoes. The life cycle of seamounts was originally observed off of the Hawaiian arc.
How Volcanoes Work: Lava and Water. An explanation of the different types of lava-water interactions.