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.

Coordinates: 25°00′S 121°12′W / 25°S 121.2°W[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.

Geology and geomorphology

Regional

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 and the Line Islands, although a continuation through the Line Islands is problematic if it is assumed that the Easter hotspot generated this track[13] but more plausible if Crough seamount is supposed to be its own hotspot.[17] East of Crough, a series of even younger volcanic ridges continues until the East Pacific Rise[18] where the hotspot may be located.[13]

Local

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)[19] at 650 metres (2,130 ft).[20] 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,[19] having formerly hosted corals[21] and pteropods. Wave erosion that took place when Crough emerged above sea level truncated the seamount, turning it into a flat guyot.[22] Pillow lavas crop out between 1,400–950 metres (4,590–3,120 ft).[23] 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.[20]

A second seamount lies nearby and partly overlaps with Crough,[1] it is named Thomas Seamount[24] in honour of a geophysicist.[25] 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).[20]

Composition

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[26] and palagonite.[27] Hydrothermal iron crusts have also been found.[23]

Eruption history

Argon-argon dating has yielded ages of 8.4 to 7.6 million years ago for samples dredged from Crough,[28] while other geological indicators suggest an age of between 7 and 10 million years ago.[29] Other estimates of its age are 4[15]-3 million years.[30]

In 1955, a strong earthquake was recorded on the northern flank of Crough Seamount;[31] 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.[30] 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.[32]

References

  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. ^ Pockalny, R.A.; Barth, G.A.; Wertman, C. (December 2015). "A Double Hotspot Model for the Origin of Line Islands Ridge". AGU Fall Meeting Abstracts. 2015: V23B–3141. Bibcode:2015AGUFM.V23B3141P.
  18. ^ Binard et al. 1996, p. 34.
  19. ^ a b Hekinian et al. 1995, p. 380.
  20. ^ a b c Binard et al. 1996, p. 27.
  21. ^ Vacher & Quinn 1997, p. 407.
  22. ^ Binard et al. 1996, p. 31.
  23. ^ 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. doi:10.1080/10641199309379905.
  24. ^ Binard et al. 1996, p. 26.
  25. ^ Searle, Francheteau & Cornaglia 1995, p. 400.
  26. ^ Hekinian et al. 1995, p. 379.
  27. ^ Hekinian et al. 1995, p. 382.
  28. ^ O'Connor, Stoffers & McWilliams 1995, pp. 206-207.
  29. ^ Binard et al. 1996, p. 25.
  30. ^ a b Talandier & Okal 1987, p. 946.
  31. ^ Talandier & Okal 1987, p. 945.
  32. ^ Okal & Cazenave 1985, p. 108.

Sources

Ducie Island

Ducie Island is an uninhabited atoll in the Pitcairn Islands. It lies 575 kilometres (357 mi) east of Pitcairn Island, and 300 kilometres (190 mi) east of Henderson Island, and has a total area of 1.5 square miles (3.9 km2), which includes the lagoon. It is 1.5 miles (2.4 km) long, measured northeast to southwest, and about 1 mile (1.6 km) wide. The island is composed of four islets: Acadia, Pandora, Westward and Edwards.

Despite its sparse vegetation, the atoll is known as the breeding ground of a number of bird species. More than 90% of the world population of Murphy's petrel nests on Ducie, while pairs of red-tailed tropicbirds and fairy terns make around 1% of the world population for each species.

Ducie was first discovered in 1606 by Pedro Fernandes de Queirós, who named it Luna Puesta, and rediscovered by Edward Edwards, captain of HMS Pandora, who was sent in 1790 to capture the mutineers of HMS Bounty. He named the island Ducie in honour of Francis Reynolds-Moreton, 3rd Baron Ducie. In 1867, it was claimed by the United States under the Guano Islands Act, but the United Kingdom annexed it on 19 December 1902 as part of the Pitcairn Islands. Due to its inaccessibility and the distance from Pitcairn Island, Ducie is rarely visited today.

Geology of the Pacific Ocean

The Pacific Ocean evolved in the Mesozoic from the Panthalassic Ocean, which had formed when Rodinia rifted apart around 750 Ma. The first ocean floor which is part of the current Pacific Plate began 160 Ma to the west of the central Pacific and subsequently developed into the largest oceanic plate on Earth.The tectonic plates continue to move today. The slowest spreading ridge is the Gakkel Ridge on the Arctic Ocean floor, which spreads at less than 2.5 cm/year (1 in/year), while the fastest, the East Pacific Rise near Easter Island, has a spreading rate of over 15 cm/year (6 in/year).

Hotspot (geology)

In geology, the places known as hotspots or hot spots are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. Their position on the Earth's surface is independent of tectonic plate boundaries. There are two hypotheses that attempt to explain their origins. One suggests that hotspots are due to mantle plumes that rise as thermal diapirs from the core–mantle boundary. The other hypothesis is that lithospheric extension permits the passive rising of melt from shallow depths. This hypothesis considers the term "hotspot" to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. Well-known examples include the Hawaii, Iceland and Yellowstone hotspots.

Outline of oceanography

The following outline is provided as an overview of and introduction to Oceanography.

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