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Crough Seamount

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25°00′S 121°12′W / 25°S 121.2°W / -25; -121.2[1]Crough Seamount (named after the geologist Thomas Crough[2]) is a seamount in the Pacific Ocean, within the exclusive economic zone of Pitcairn.[3] It rises to a depth of 650 metres (2,130 ft) and is paired with a taller but overall smaller seamount to the east. This seamount has a flat top and probably formed an island in the past. It is about 7-8 million years old, although a large earthquake recorded at its position in 1955 may indicate a recent eruption.

Map
Crough Seamount

The seamount appears to be part of a long geological lineament with the neighbouring Henderson and Ducie islands, as well as the southern Tuamotus and Line Islands. Such a lineament may have been generated by a hotspot; the nearby Easter hotspot is a candidate hotspot.

Geology and geomorphology

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Regional

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The region lies between and around the islands of Pitcairn and Easter Island.[4] There, the East Pacific Rise is interrupted by a trapezoid microplate known as the Easter Microplate[5] about 400 kilometres (250 mi) wide. Seafloor spreading occurs at a rate of about 16 centimetres per year (6.3 in/year).[4]

There is a topographic swell that connects the two islands and continues eastward towards Sala y Gomez. The origin of this swell and the various volcanoes and seamounts associated with it has been variously explained as either being due to a mantle plume which forms volcanoes that are then carried away through plate motion or by a "hot line" where a number of simultaneously active volcanic centres develop.[4] This geological lineament may extend all the way to Tonga.[6]

Crough seamount was probably formed by the Easter hotspot that also generated Easter Island[7] albeit with the participation of a nearby fracture zone[8] that modified the trend of the hotspot path.[9] In this case the Easter Island-Sala y Gomez ridge and the Crough Seamount would be conjugate volcanic ridges paired across the East Pacific Rise.[10] although it is possible that two separate hotspots were active on the eastern and western side of the East Pacific Rise.[11][12] Another theory postulates that Crough was formed by its own hotspot, the Crough hotspot.[13]

Together with Ducie and Henderson Crough forms a 1,300 kilometres (810 mi) long westward trending lineament[14] with each volcano becoming older the farther west it lies,[15] and which may be a prolongation of the southern Tuamotus[16] which were generated by the same hotspot.[10] Even farther west the hotspot track may include Oeno, Minerve Reef, Marutea, Acton, Rangiroa, the Line Islands and the Mid-Pacific Mountains, although a continuation through the Line Islands is problematic if it is assumed that the Easter hotspot generated this track.[13][17] A different theory has Crough seamount as its own hotspot, that formed the seamounts and islands together with another hotspot ("Larson"),[18] and lesser contributions of the Society and Marquesas hotspot.[19] East of Crough, a series of even younger volcanic ridges continues until the East Pacific Rise[20] where the hotspot may be located.[13] The Crough hotspot may be a conjugate of the Easter hotspot,[21] and sourced from the middle mantle.[18]

Local

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Crough is an east-west trending seamount[5] which rises over 2 kilometres (1.2 mi) from the seafloor to a depth of less than 722 metres (2,369 ft)[22] at 650 metres (2,130 ft).[23] It has a flat top and the presence of coral sands indicates that Crough once emerged above sea level before subsiding to its present depth,[22] having formerly hosted corals[24] and pteropods. Wave erosion that took place when Crough emerged above sea level truncated the seamount, turning it into a flat guyot.[25] Pillow lavas crop out between 1,400–950 metres (4,590–3,120 ft).[26] Crough Seamount has a volume of 660 cubic kilometres (160 cu mi), comparable to that of other submarine volcanoes such as Macdonald seamount, Mehetia and Moua Pihaa.[23]

A second seamount lies nearby and partly overlaps with Crough,[1] it is named Thomas Seamount[27] in honour of a geophysicist.[28] This seamount is even shallower than Crough as it reaches a depth of 600 metres (2,000 ft) but has a smaller volume of 600 cubic kilometres (140 cu mi).[23]

Composition

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Dredging has yielded both vesicular and porphyritic basalt. Phenocrysts identified include clinopyroxene, olivine and plagioclase. Carbonates and hyaloclastites have also been found, and some samples were covered with manganese crusts[29] and palagonite.[30] Hydrothermal iron crusts have also been found.[26]

Eruption history

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Argon-argon dating has yielded ages of 8.4 to 7.6 million years ago for samples dredged from Crough,[31] while other geological indicators suggest an age of between 7 and 10 million years ago.[32] Other estimates of its age are 4[15]-3 million years.[33]

In 1955, a strong earthquake was recorded on the northern flank of Crough Seamount;[34] the characteristics of the earthquake resemble these of volcanic processes and it is thus possible that Crough Seamount is still active. Such activity may constitute a post-shield stage of volcanism.[33] The earthquake has also been interpreted as a normal fault earthquake[2] which sometimes occur in young oceanic crust, but the 1955 Crough event was considerably stronger than other earthquakes of this type.[35]

References

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  1. ^ a b Spencer 1989, p. 3.
  2. ^ a b Okal & Cazenave 1985, p. 104.
  3. ^ Irving, Robert.; Dawson, Terence P. (2012). The marine environment of the Pitcairn Islands. Dundee: The Pew Environment Group. ISBN 9781845861612. OCLC 896746178.
  4. ^ a b c Hekinian et al. 1995, p. 376.
  5. ^ a b Hekinian et al. 1995, p. 377.
  6. ^ Spencer 1989, p. 6.
  7. ^ Hekinian et al. 1995, p. 389.
  8. ^ Spencer 1989, p. 5.
  9. ^ Searle, Francheteau & Cornaglia 1995, p. 397.
  10. ^ a b Searle, Francheteau & Cornaglia 1995, p. 417.
  11. ^ O'Connor, Stoffers & McWilliams 1995, p. 208.
  12. ^ Morgan & Morgan 2007, p. 51.
  13. ^ a b c Morgan & Morgan 2007, p. 71.
  14. ^ Binard et al. 1996, p. 24.
  15. ^ a b Bramwell, David; Caujapé-Castells, Juli (2011-07-21). The Biology of Island Floras. Cambridge University Press. p. 241. ISBN 9781139497800.
  16. ^ Vacher & Quinn 1997, p. 410.
  17. ^ Fletcher, Michael; Wyman, Derek A.; Zahirovic, Sabin (1 July 2020). "Mantle plumes, triple junctions and transforms: A reinterpretation of Pacific Cretaceous – Tertiary LIPs and the Laramide connection". Geoscience Frontiers. 11 (4): 1141. Bibcode:2020GeoFr..11.1133F. doi:10.1016/j.gsf.2019.09.003.
  18. ^ a b Pockalny et al. 2021, p. 1361.
  19. ^ Pockalny et al. 2021, p. 1360.
  20. ^ Binard et al. 1996, p. 34.
  21. ^ Fletcher, Michael; Wyman, Derek A.; Zahirovic, Sabin (26 September 2019). "Mantle plumes, triple junctions and transforms: A reinterpretation of Pacific Cretaceous – Tertiary LIPs and the Laramide connection". Geoscience Frontiers. 11 (4): 9. Bibcode:2020GeoFr..11.1133F. doi:10.1016/j.gsf.2019.09.003. ISSN 1674-9871.
  22. ^ a b Hekinian et al. 1995, p. 380.
  23. ^ a b c Binard et al. 1996, p. 27.
  24. ^ Vacher & Quinn 1997, p. 407.
  25. ^ Binard et al. 1996, p. 31.
  26. ^ a b Stoffers, P.; Glasby, G. P.; Stuben, D.; Renner, R. M.; Pierre, T. G.; Webb, J.; Cardile, C. M. (1993). "Comparative mineralogy and geochemistry of hydrothermal iron-rich crusts from the Pitcairn, Teahitia-mehetia, and Macdonald hot spot areas of the S. W. Pacific". Marine Georesources & Geotechnology. 11 (1): 47. Bibcode:1993MGG....11...45S. doi:10.1080/10641199309379905.
  27. ^ Binard et al. 1996, p. 26.
  28. ^ Searle, Francheteau & Cornaglia 1995, p. 400.
  29. ^ Hekinian et al. 1995, p. 379.
  30. ^ Hekinian et al. 1995, p. 382.
  31. ^ O'Connor, Stoffers & McWilliams 1995, pp. 206–207.
  32. ^ Binard et al. 1996, p. 25.
  33. ^ a b Talandier & Okal 1987, p. 946.
  34. ^ Talandier & Okal 1987, p. 945.
  35. ^ Okal & Cazenave 1985, p. 108.

Sources

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