Hebrides Terrace Seamount

The seamount lies in the southern Rockall Trough,[2] at the foot of the Scottish continental slope and 166 kilometres (103 mi) west-southwest of Barra Head, Hebrides.[1] It is the smallest of three seamounts in the Rockall Trough;[3] the other two are Anton Dohrn Seamount and Rosemary Bank both north of Hebrides Terrace Seamount,[4] and farther west is the Rockall Bank.[5] It straddles the political border between the United Kingdom and Ireland.[6]

The Hebrides Terrace Seamount is a volcanic[7] guyot, a seamount with a flat top and steep slopes,[1] that rises to a depth of about 980 metres (3,220 ft).[8] It is about 28 by 37 kilometres (17 mi × 23 mi) wide at its basis[3] and its southern and western flanks feature canyons, escarpments and gullies. The surrounding terrain at 2,300 metres (7,500 ft) depth is covered by debris of Plio-Pleistocene age; the debris deposits are almost 1 kilometre (0.62 mi) thick.[2] The Barra and Donegal fans, two sediment fans that form a larger complex, border the Hebrides Terrace Seamount to the north and south, respectively.[9] The seamount has diverted sediment flows,[10] leading to the formation of these two fans[11] and the accumulation of sediments east of Hebrides Terrace Seamount.[12]

Rocks dredged from Hebrides Terrace Seamount have a tholeiitic[13] basaltic composition and define two separate suites, one aluminium-rich and the other aluminium-poor. They contain phenocrysts of augite, olivine, pigeonite, plagioclase and titanomagnetite, which together with ilmenite and pyroxene also occur in the groundmass.[14] The seamount is the location of a positive gravity anomaly that is thought to indicate the presence of a 17 kilometres (11 mi) thick igneous body.[15]

The formation of the Hebrides Terrace Seamount began after the Cretaceous-Palaeogene extinction event. In the early Cenozoic, three pulses of volcanic activity generated the seamount.[2] The volcanism may be correlative to the early Cenozoic Hebridean volcanic province.[16] Ages of 67-60 million years have been obtained on Hebrides Terrace Seamount,[17] as well as 62±1, 51±1 and 48±1 million years ago which coincide with activity at Anton Dohrn Seamount.[18] These dates have been interpreted as indicating fluctuations of the Iceland hotspot.[19]

After the end of volcanic activity, its top was flattened perhaps by Paleocene erosion that has been recognized in the region[7] although other processes may be involved as well.[16] This along with subsidence during the Eocene, Oligocene and in a lesser measure during the Miocene and later time lowered its summit below sea level.[20]

Evidence of more recent seismic activity is found along its southern slopes.[2] On 13 April 1980 a M_L 3.5 earthquake[21] occurred just west of Hebrides Terrace Seamount;[22] it was probably not an underwater explosion and may have been instead caused by movements along a local fault.[23] Isostatic processes may have caused the earthquake.[24] Another M_L 3.1 earthquake occurred south of the seamount in 1986.[25]

Xenophyophores (mainly Syringammina fragilissima), sponges and corals (such as Solenosmilia variabilis) occur on Hebrides Terrace Seamount,[26] principally on its slopes,[27] and form so-called "coral gardens".[28] These also host black corals, crinoids, egg cases of elasmobranchs, glass sponges and ophiuroids.[26] The faunal communities vary depending on the depth, the substrate (bedrock, cobbles, corals, gravel and sand), the nature of the watermass surrounding them[27] and internal tides which modulate the nutrient supply.[29] The complicated oceanographic regime may facilitate the concentration of nutrients at the seamount, enabling the development of rich biological communities.[2]

Beaked whales, fin whales, harbour porpoises, long-finned pilot whales have been sighted on the seamount.[2] Fish species encountered at Hebrides Terrace Seamount include Atlantic codling, false boarfish and roundnose grenadier.[30] Overfishing has decimated orange roughy spawning aggregations that formerly occurred at Hebrides Terrace Seamount.[2] Sharks and other fish have also declined since the 1970s.[31]

The Hebrides Terrace Seamount, together with a nearby seabed feature known as the Barra Fan, are together designated by the Scottish Government as a Nature Conservation Marine Protected Area named the Barra Fan and Hebrides Terrace Seamount Marine Protected Area.[32]