Alexa Bank

Alexa Bank is a seamount in Samoa, northwest of Rotuma.[1] The seamount reaches a depth of 18–21 metres (59–69 ft) below sea level and has the appearance of an atoll with a flat top and steep slopes. Some active coral growth takes place at its top, but if it ever was an active atoll it has now drowned. It was probably formed by the Samoa hotspot 24 million years ago, although older volcanism about 40 million years ago has also been identified.

Alexa Bank
Summit depth18–21 metres
Location
LocationRotuma
GroupMelanesian Border Plateau
Coordinates
CountrySamoa
Geology
TypeSeamount
Age of rock37-24 million years

Geology and geomorphology

Local

Alexa Bank rises to depths of 18–21 metres (59–69 ft),[2] has an eastward elongated flat top[1] and is 150 kilometres (93 mi) long and 20–65 kilometres (12–40 mi) wide, widening towards the west;[3] such dimensions resemble these of Savaii and Upolu in Samoa.[4] The flat top has some features of an atoll including a raised rim, and it may indeed once have been an atoll.[5] The rim is surrounded by a narrow terrace and in turn encloses a much wider terrace, that surrounds the flat top[2] at an average depth of 42 metres (138 ft).[6] The northern and southern flanks are formed by steep scarps, with the eastern slope featuring a terrace at 750 metres (2,460 ft) depth and the western slope extending as a 150 kilometres (93 mi) long volcanic ridge.[3]

The upper 1,600–800 metres (5,200–2,600 ft) of the Bank are formed by carbonate sediments, while the eastern slope terrace may be part of the original volcanic edifice that was carried to this depth by thermal subsidence of the oceanic crust;[7] seismic data may support the existence of a volcano within the carbonate sediments that form the bulk of Alexa Bank as well.[2] Such thicknesses of the sediments are comparable to these of Bikini Atoll and Eniwetok.[5]

Regional

The seafloor around Alexa Bank has a rough relief, with additional seamounts known as the Louisa Bank, Morton Bank, Penguin Bank and Turpie Bank nearby;[8] the last two are often considered to be part of Alexa Bank.[4] The 4,000 metres (13,000 ft) deep Alexa Trough lies west and south of Alexa Bank[9] and is part of the Vitiaz Trench,[10] connecting it with the Tonga Trench.[11] The bank itself is part of the "Melanesian Border Plateau", a group of seamounts and islands along the Vitiaz Trench that are mostly of volcanic origin.[12]

Alexa Bank appears to be the westernmost extension of the Samoa hotspot trail, which has covered a length of 1,700 kilometres (1,100 mi) at least.[13] The hotspot trail is in part coincident with a large group of islands and seamounts which were probably formed by the same hotspot but also featured substantial postshield volcanism, probably owing to tectonic phenomena triggered by the Tonga Trench close by.[14] The seafloor is part of the Pacific Plate.[15]

Composition

Samples dredged from the Alexa Bank include basaltic rocks[16] and have alkali basalt to tholeiitic composition.[12] Isotope data display affinities to these from other Samoan volcanoes.[17] Other rocks dredged from the seamount are argillite, breccia, shale and siltstone[16] but they may not have originated on Alexa Bank proper.[18]

Geologic history

Rocks from Alexa Bank have been dated to be 37[16]–36.9 million years old[12] and may be a product of Eocene volcanism.[10] More recent age estimates are 24 million years ago,[13] while fossil data have been inferred to justify a Cretaceous age.[19]

Alexa Bank has been considered to be a product of the Samoa hotspot, but its age may be too high to be a product of this hotspot.[12] The newer age measurements however are consistent with the association with the Samoa hotspot.[20] Some tectonic activity may have continued until later times, leading to the development of segmentation and tilting.[21]

The bank was probably an atoll during the Pleistocene and then drowned.[3] It is possible that unusually rapid geological subsidence of Alexa Bank outpaced the ability of its reefs to keep up.[22] Originally, the flat surface of Alexa was explained by wave erosion during sea level lowstands.[2]

Biology

Dead corals and foraminifera occur on Alexa Bank. Some corals are still alive[23] and form pinnacles on the surface of the bank.[2] Algal rhodoliths[24] and stromatoliths have been found on Alexa as well.[25] The Japanese snake blenny has been found at Alexa Bank.[15] Radiolarians in Eocene-Quaternary sediments south of Alexa Bank probably originate there.[23]

References
  1. Pelletier & Auzende 1996, p. 325.
  2. Fairbridge & Stewart 1960, p. 108.
  3. Pelletier & Auzende 1996, p. 320.
  4. Brocher, Thomas M. (1985). "On the Age Progression of the Seamounts West of the Samoan Islands, SW Pacific". Investigations of theNorthern Melanesian Borderland, Earth Science Series: 181.
  5. Ladd, Harry S. (1961). "Reef Building". Science. 134 (3481): 712. doi:10.1126/science.134.3481.703. JSTOR 1707367. PMID 17795280.
  6. Daly, Reginald A. (1915). "The Glacial-Control Theory of Coral Reefs". Proceedings of the American Academy of Arts and Sciences. 51 (4): 190. doi:10.2307/20025572. JSTOR 20025572. S2CID 21702277.
  7. Pelletier & Auzende 1996, p. 331.
  8. Pelletier & Auzende 1996, pp. 316, 320.
  9. Pelletier & Auzende 1996, p. 310.
  10. Pelletier & Auzende 1996, p. 309.
  11. Pelletier & Auzende 1996, p. 305.
  12. Pelletier & Auzende 1996, p. 308.
  13. Price, Allison A.; Jackson, Matthew G.; Blichert-Toft, Janne; Kurz, Mark D.; Gill, Jim; Blusztajn, Jerzy; Jenner, Frances; Brens, Raul; Arculus, Richard (March 2017). "Geodynamic implications for zonal and meridional isotopic patterns across the northern Lau and North Fiji Basins" (PDF). Geochemistry, Geophysics, Geosystems. 18 (3): 23. Bibcode:2017GGG....18.1013P. doi:10.1002/2016gc006651. hdl:1912/8979. ISSN 1525-2027.
  14. Hart et al. 2004, p. 38.
  15. Smith-Vaniz, William F. (1987). "The Saber-Toothed Blennies, Tribe Nemophini (Pisces: Blenniidae): An Update". Proceedings of the Academy of Natural Sciences of Philadelphia. 139 (1): 5. JSTOR 4064893.
  16. Pelletier & Auzende 1996, p. 329.
  17. Hart et al. 2004, p. 46.
  18. Brocher, Thomas M.; Wirasantosa, Sugiarta; Theyer, Fritz; Mato, Chris (1985). "Regional Sedimentation Patterns Along the Northern Melanesian Borderland". Investigations of theNorthern Melanesian Borderland, Earth Science Series: 88.
  19. Lonsdale, Peter (1975). "Sedimentation and Tectonic Modification of Samoan Archipelagic Apron". AAPG Bulletin. 59 (5): 782. ISSN 0149-1423.
  20. Hart et al. 2004, p. 41.
  21. Fairbridge & Stewart 1960, p. 109.
  22. Winterer, Edward L.; Metzler, Christopher V. (10 November 1984). "Origin and subsidence of Guyots in Mid-Pacific Mountains". Journal of Geophysical Research: Solid Earth. 89 (B12): 9975. Bibcode:1984JGR....89.9969W. doi:10.1029/jb089ib12p09969. ISSN 0148-0227.
  23. Chase, Clement G. (1971). "Tectonic History of the Fiji Plateau". Geological Society of America Bulletin. 82 (11): 3094. doi:10.1130/0016-7606(1971)82[3087:THOTFP]2.0.CO;2. ISSN 0016-7606.
  24. Bosellini, Alfonso; Ginsburg, Robert N. (November 1971). "Form and Internal Structure of Recent Algal Nodules (Rhodolites) from Bermuda". The Journal of Geology. 79 (6): 670. Bibcode:1971JG.....79..669B. doi:10.1086/627697. ISSN 0022-1376.
  25. McMaster, Robert L.; Conover, J. Towne (September 1966). "Recent Algal Stromatolites from the Canary Islands". The Journal of Geology. 74 (5, Part 1): 647. Bibcode:1966JG.....74..647M. doi:10.1086/627198. ISSN 0022-1376.

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