In marine geology , a guyot ( / ˈ ɡ iː . oʊ , ɡ iː ˈ oʊ / ), also called a tablemount , is an isolated underwater volcanic mountain ( seamount ) with a flat top more than 200 m (660 ft) below the surface of the sea. The diameters of these flat summits can exceed 10 km (6 mi). Guyots are most commonly found in the Pacific Ocean , but they have been identified in all the oceans except the Arctic Ocean . They are analogous to tables (such as mesas ) on land.
31-530: The Anton Dohrn Seamount is a guyot in the Rockall Trough in the northeast Atlantic . It is 1.8 kilometres (1.1 mi) high and is topped with pinnacles, one of which reaches a depth of 530 metres (1,740 ft). Away from the flat top upon which the pinnacles rest, the slopes fall off steeply into the Rockall Trough and a moat in the sediment that surrounds the seamount. It appears to be
62-541: A Special Area of Conservation . The region is considered to be "the cradle of deep-sea biology" as Victorian -era scientists sampled the regional fauna. Ocean currents around Anton Dohrn Seamount are complicated and formed by various water masses. Internal tides at the seamount appear to be important for its ecosystem. The seamount has been impacted by deep water fishing . Lost fishing gear and trawl marks have been found on Anton Dohrn Seamount, and animals found at its foot have ingested microplastics . In October 2020
93-402: A dropstone from icebergs and such exotic rocks have been found in other dredge samples. Anton Dohrn Seamount is a former volcano. Radiometric dating of volcanic rocks dredged from it has yielded ages of 70 ± 1, 62 ± 1, 47 ± 1 and 41 ± 1 million years ago, indicating episodic activity over 29 million years. Pulses of volcanic activity of similar age have been identified at other volcanoes in
124-652: A volcano formed by basaltic lava and tuff . It formed during the Cretaceous and Paleogene and was proposed to be a source for bentonite layers across the British Isles . After the Cretaceous, subsidence and erosion lowered its top until it sank below sea level. The seamount was discovered in 1958. Anton Dohrn Seamount hosts a diverse ecosystem characterized by reefs formed by cold water corals , sponges and xenophyophorans , which themselves host
155-561: A flat-topped submerged mountain. Seamounts are made by extrusion of lavas piped upward in stages from sources within the Earth's mantle, usually hotspots , to vents on the seafloor. The volcanism invariably ceases after a time, and other processes dominate. When an undersea volcano grows high enough to be near or breach the ocean surface, wave action or coral reef growth tend to create a flat-topped edifice. However, all ocean crust and guyots form from hot magma or rock, which cools over time. As
186-682: A guyot or tablemount, they must stand at least 900 m (3,000 ft) tall. One guyot in particular, the Great Meteor Tablemount in the Northeast Atlantic Ocean, stands at more than 4,000 m (13,000 ft) high, with a diameter of 110 km (68 mi). However, there are many undersea mounts that can range from just less than 90 m (300 ft) to around 900 m (3,000 ft). Very large oceanic volcanic constructions, hundreds of kilometres across, are called oceanic plateaus . Guyots have
217-870: A mean area of 3,313 km (1,279 sq mi), which is much larger than typical seamounts, which have a mean area of 790 km (310 sq mi). There are 283 known guyots in the world's oceans, with the North Pacific having 119, the South Pacific 77, the South Atlantic 43, the Indian Ocean 28, the North Atlantic eight, the Southern Ocean six, and the Mediterranean Sea two; there are none known in
248-472: A number of animals. It has been affected by human fishing operations, however. Anton Dohrn Seamount is also known as Anton Dohrn Kuppe, a name used by German charts, and as Anton Dohrn bank. It was discovered on 22 September 1958 by the survey vessel Gauss during the Polarfront programme and later surveyed on 18–19 April 1959 by the fishery research vessel FFS Anton Dohrn . Anton Dohrn Seamount
279-461: A sediment cover or featuring gravelly sediments along with outcropping bedrock. There are cliffs , ridges and rockfalls but no gullies or canyons. Parasitic cones lie on the northwestern slope. A moat surrounds the seamount and reaches depths of about 2,300 metres (7,500 ft). It might have formed either through erosion of surrounding sediments by ocean currents or through isostatic subsidence . The crust underneath Anton Dohrn Seamount
310-673: Is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault . Typically a lineament will appear as a fault-aligned valley, a series of fault or fold-aligned hills, a straight coastline or indeed a combination of these features. Fracture zones , shear zones and igneous intrusions such as dykes can also be expressed as geomorphic lineaments. Lineaments are often apparent in geological or topographic maps and can appear obvious on aerial or satellite photographs. There are for example, several instances within Great Britain . In Scotland
341-454: Is a vertical stratification, with Lophelia found at shallower depths than Solenosmilia . Corals such as antipatharians like Leiopathes sp. , small bamboo corals , large gorgonians and soft corals like as Anthomastus sp. have also been found at parasitic vents. The cold water coral cover can become so thick that the underground disappears underneath it. Dropframe camera surveys have seen anemones , anthozoans , ascidians ,
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#1732780511800372-528: Is formed by a pinnacle that protrudes from the c. 600 metres (2,000 ft) deep summit platform. A 100 metres (330 ft) thick layer of sediment covers the flat top and appears to be reworked by storms and sea currents. Mounds, slope breaks and other volcanic pinnacles are located on the flat top. The seamount tilts southeastward. Beyond the margin of the flat top, the slopes of Anton Dohrn Seamount drop down to 2,400 metres (7,900 ft) depth. The steep slopes have been variously described as either lacking
403-467: Is located in the northeast Atlantic Ocean west of Scotland , approximately halfway between St Kilda ( Hebrides ) and Rockall , about 155 kilometres (96 mi) west of the former. It lies in the Rockall Trough , an over 2,000 metres (6,600 ft) deep submarine depression of unclear origin. North-northeast lies the Rosemary Bank and Hebrides Terrace Seamount is found south-southeast from
434-649: Is much thinner than underneath the British Isles and the Rockall Plateau east and west of the seamount, respectively, and the Mohorovičić discontinuity is located at a shallower depth. It may be either stretched continental crust or oceanic crust , and is covered by sediments. At Anton Dohrn Seamount it appears to be unusually shallow, perhaps due to the Iceland plume 's buoyancy. The Iceland plume has uplifted terrain as far as 1,000 kilometres (620 mi) from
465-492: Is the first place in the United Kingdom where coral gardens have been discovered. The sandy and cobbly terrain of the slopes with occasional bedrock outcrops is populated by reefs that grow on bedrock or on cobbles. They mostly occur on the sides of the seamount, on mounds on the flat top and its margin, perhaps for hydrodynamic reasons or because substrates favourable for the development of the reefs are found there. There
496-809: The Great Glen Fault and Highland Boundary Fault give rise to lineaments as does the Malvern Line in western England and the Neath Disturbance in South Wales . The term 'megalineament' has been used to describe such features on a continental scale. The trace of the San Andreas Fault might be considered an example. The Trans Brazilian Lineament and the Trans-Saharan Belt , taken together, form perhaps
527-614: The Mid-Atlantic Ridge in the Atlantic Ocean, and deeper ocean at abyssal plains and oceanic trenches , such as the Mariana Trench . Thus, the island or shoal that will eventually become a guyot slowly subsides over millions of years. In the right climatic regions, coral growth can sometimes keep pace with the subsidence, resulting in coral atoll formation, but eventually the corals dip too deep to grow and
558-501: The asteroid (starfish) Henricia sp. , bamboo corals, caryophyllids , cerianthids , antipatharian corals with various shapes, the corals Desmophyllum dianthus , Lophelia pertusa and Solenosmilia variabilis , echinoderms including brisingids and crinoids , glass sponges , gorgonians , holothurians , the ophiuroids Ophiactis balli and Ophiomusium lymani , the pencil urchin Cidaris cidaris , pycnogonids ,
589-515: The scleractinian Madrepora oculata , the seapen Pennatula phosphorea , sea urchins , sea whips , serpulids , soft corals such as Gersemia sp. and Anthomastus sp. , lobose, large and encrusting sponges, stylasterids and xenophyophores . Decapods , fish including Lepidion eques , the eel Synaphobranchus kaupi and squat lobsters Munida sp. have also been encountered. Seamounts are considered to be biodiversity hotspots , and there are proposals to make Anton Dohrn Seamount
620-407: The 19th-century geographer Arnold Henry Guyot . Hess postulated they were once volcanic islands that were beheaded by wave action, yet they are now deep under sea level . This idea was used to help bolster the theory of plate tectonics . Guyots show evidence of having once been above the surface, with gradual subsidence through stages from fringed reefed mountain, coral atoll , and finally
651-547: The Arctic Ocean, though one is found along the Fram Strait off northeastern Greenland . Guyots are also associated with specific lifeforms and varying amounts of organic matter . Local increases in chlorophyll a , enhanced carbon incorporation rates and changes in phytoplankton species composition are associated with guyots and other seamounts . Lineament See also Line (geometry) A lineament
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#1732780511800682-503: The Earth's mantle beneath the lithosphere. There are thought to be up to an estimated 50,000 seamounts in the Pacific basin. The Hawaiian–Emperor seamount chain is an excellent example of an entire volcanic chain undergoing this process, from active volcanism, to coral reef growth, to atoll formation, to subsidence of the islands and becoming guyots. The steepness gradient of most guyots is about 20 degrees. To technically be considered
713-669: The Eocene and continued afterwards. Barnacles and brachiopods grow on the top of the seamount, and echinoderms , corals and cirripedes also occur there. On the sandy or gravelly substrate serpulids and sponges are found. The seamount may be a shark nursery. Finally, the bivalve Xylophaga anselli has been found at Anton Dohrn Seamount and the Hebrides slope. A number of ecosystems have been found on Anton Dohrn Seamount, including coral gardens , cold water coral reefs and sponge and xenophyophore communities; this seamount
744-407: The island becomes a guyot. The greater the amount of time that passes, the deeper the guyots become. Seamounts provide data on movements of tectonic plates on which they ride, and on the rheology of the underlying lithosphere . The trend of a seamount chain traces the direction of motion of the lithospheric plate over a more or less fixed heat source in the underlying asthenosphere , the part of
775-406: The lithosphere that the future guyot rides on slowly cools, it becomes denser and sinks lower into Earth's mantle, through the process of isostasy . In addition, the erosive effects of waves and currents are found mostly near the surface: the tops of guyots generally lie below this higher-erosion zone. This is the same process that gives rise to higher seafloor topography at oceanic ridges, such as
806-815: The plume. A 100 kilometres (62 mi) long crustal lineament known as the Anton Dohrn Lineament crosses through the seamount; it may extend into Scotland and Rockall Bank and runs in northwest–southeast direction. Anton Dohrn Seamount is probably formed mostly by basaltic lava and tuffs which define a transitional to alkaline suite . The rocks contain feldspar and olivine phenocrysts as well as plagioclase . They are covered with ferromanganese crusts and vesicles contain carbonates , clay and zeolites which formed through alteration. Chalks of Maastrichtian age, Eocene nearshore conglomerates and Miocene muds and sands have also been recovered. A granite rock has been dredged as well; it may be
837-632: The region and may reflect fluctuations of the Iceland plume. The onset of volcanic activity may have been the consequence of crustal extension in the region. The activity during the Cretaceous implies that rifting in the North Atlantic was already underway at that time. At that time, the Rockall Trough was at least 1 kilometre (0.62 mi) deep. Xenoliths found in volcanic rocks indicate that at Anton Dohrn volcanic activity involved interactions between magma and sediments, resulting in phreatomagmatic eruptions that could have dispersed volcanic ash in
868-597: The region. This volcanic ash erupted by Anton Dohrn may be the source of post- Cenomanian bentonites of the British Isles but the age and composition of the bentonites do not support this theory. The seamount was once proposed to be the source of Turonian tephra deposits in Western Europe before its Maastrichtian age was established. During the Cretaceous the seamount was about 2 kilometres (1.2 mi) higher than present, perhaps even reaching 2,000 metres (6,600 ft) height above sea level; presumably it
899-554: The seamount was made part of the West of Scotland Marine Protected Area by the Scottish Government in attempt to protect the area's ecology. Guyot Guyots were first recognized in 1945 by Harry Hammond Hess , who collected data using echo-sounding equipment on a ship he commanded during World War II . His data showed that some undersea mountains had flat tops. Hess called these undersea mountains "guyots", after
930-455: The seamount. The seamount is located inside the exclusive economic zone of the United Kingdom. Anton Dohrn Seamount is a 1.8 kilometres (1.1 mi) high and about 45 kilometres (28 mi)–40 kilometres (25 mi) wide circular guyot with a flat top at 1,100–530 metres (3,610–1,740 ft) depth. Flat-topped seamounts are unusual in the North Atlantic. The shallowest point of the seamount lies at about 530 metres (1,740 ft) depth and
961-474: Was then eroded during the Paleocene when a wave of erosion took place in western Britain and stripped much of the volcanic centres of northwest Scotland. An episode of crustal subsidence in the Cretaceous - Oligocene also played a role in lowering Anton Dohrn Seamount. The pinnacles on the seamount may be leftover volcanic conduits that resisted erosion. Sedimentation covered the seamount and its flanks in