Macdonald hotspot

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The Macdonald hotspot is the Pacific Ocean, marked 24 on this map.

The Macdonald hotspot is a volcanic hotspot in the southern Pacific Ocean. The hotspot was responsible for the formation of the Macdonald Seamount, and possibly the Austral-Cook Islands chain.[1] It probably did not generate all of the volcanism in the Austral and Cook Islands as age data imply that several additional hotspots were needed to generate some volcanoes.

In addition of the volcanoes in the Austral Islands and Cook Islands, Tokelau, the Gilbert Islands, the Phoenix Islands and several of the Marshall Islands as well as several seamounts in the Marshall Islands may have been formed by the Macdonald hotspot.

Geology[edit]

Regional geology[edit]

Hotspots have been explained either by mantle plumes producing magma in the crust, reactivation of old lithospheric structures such as fractures or spreading of the crust through tectonic tension.[2] Aside from Macdonald seamount, active volcanoes which are considered hotspots in the Pacific Ocean include Hawaii, Bounty seamount at Pitcairn, Vailulu'u in Samoa and Mehetia/Teahitia in the Society Islands.[3]

Volcanism in the southern Pacific Ocean has been associated with the "South Pacific Superswell", a region where the seafloor is abnormally shallow. It is the site of a number of often short-lived volcanic chains, including the previously mentioned hotspots as well as the Arago hotspot, Marquesas Islands and Rarotonga. Beneath the Superswell, a region of upwelling has been identified in the mantle, although the scarcity of seismic stations in the regions make it difficult to reliably image it.[4] In the case of Macdonald, it seems like a low velocity anomaly in the mantle rises from another anomaly at 1,200 kilometres (750 mi) depth to the surface.[5] This has been explained by the presence of a "superplume", a very large mantle plume which also formed oceanic plateaus during the Cretaceous,[6] with present-day volcanism at the Society and Macdonald volcanoes originating from secondary plumes that rise from the superplume to the crust.[7]

Local geology[edit]

The Austral Islands and the Cook Islands may have been formed by the Macdonald hotspot,[8] as the Pacific Plate was carried above the hotspot at a rate of 10–11 centimetres per year (3.9–4.3 in/year). A 500–300 metres (1,640–980 ft) high swell underpins the Austral Islands as far as Macdonald seamount,[9] which is the presently active volcano on the Macdonald hotspot.[10] They fit the pattern of linear volcanism, seeing as they are progressively less degraded southeastward (with the exception of Marotiri, which unprotected by coral reefs unlike the other more equatorial islands has been heavily eroded) and the active Macdonald volcano lies at their southeastern end.[11] However, there appear to be somewhat older guyots in the area as well, some of which show evidence that secondary volcanoes formed on them. It is possible that the guyots are much older and that lithospheric anomalies were periodically reactivated and triggered renewed volcanism on the older guyots.[12]

In addition, dating of the various volcanoes in the Cook-Austral chain indicates that there is no simple age progression away from Macdonald seamount and that the chain appears to consist of two separate alignments. While the younger ages of Atiu and Aitutaki may be explained by the long-range effect of Rarotonga's growth, Rarotonga itself is about 18-19 million years younger than would be expected if it was formed by Macdonald.[13][14] Additional younger ages in some volcanoes such as Rurutu have been explained by the presence of an additional system, the Arago hotspot,[15] and some rocks from Tubuai and Raivavae[14] as well as deeper samples taken on other volcanoes appear to be too old to be explained by the Macdonald hotspot. These ages may indicate that some volcanoes were originally formed by the Foundation hotspot.[16] Other problems with using a hotspot to explain this volcanism is the highly variable composition of volcanism between various edifices,[17] and that a number of Cook Islands are not located on the reconstructed path of the Macdonald hotspot.[18] Some of these discrepancies may be due to the presence of multiple hotspots or the reactivation of dead volcanism by the passage nearby of another hotspot.[19]

The high ratio of helium-3 to helium-4 has been used to infer a deep mantle origin of magmas of hotspot volcanoes.[20] Helium samples taken from Macdonald support the contention[21] and have been used to rule out the notion that such magmas may be derived from the crust, although an origin in primitive-helium-enriched sectors of the lithosphere is possible.[22]

Candidate edifices[edit]

Overall, the list of candidate volcanoes produced by the Macdonald hotspot is:

See also[edit]

References[edit]

  1. ^ W. J. Morgan (1971). "Convection Plumes in the Lower Mantle". Nature. 230 (5288): 42. Bibcode:1971Natur.230...42M. doi:10.1038/230042a0. 
  2. ^ Binard et al. 2004, p. 158.
  3. ^ Binard et al. 2004, p. 157.
  4. ^ Tanaka et al. 2009, p. 268.
  5. ^ Tanaka et al. 2009, p. 276.
  6. ^ Suetsugu & Hanyu 2013, p. 260.
  7. ^ Suetsugu & Hanyu 2013, p. 267.
  8. ^ Talandier & Okal 1984, p. 813.
  9. ^ Bideau & Hekinian 2004, p. 309.
  10. ^ Bideau & Hekinian 2004, p. 312.
  11. ^ Johnson & Malahoff 1971, p. 3284.
  12. ^ Johnson & Malahoff 1971, p. 3289.
  13. ^ Thompson, G. M.; Malpas, J.; Smith, Ian E. M. (2010). "Volcanic geology of Rarotonga, southern Pacific Ocean". New Zealand Journal of Geology and Geophysics. 41 (1): 95. doi:10.1080/00288306.1998.9514793. 
  14. ^ a b DALRYMPLE, G. BRENT; JARRARD, R. D.; CLAGUE, D. A. (1 October 1975). "K-Ar ages of some volcanic rocks from the Cook and Austral Islands". GSA Bulletin. 86 (10): 1466. doi:10.1130/0016-7606(1975)86<1463:KAOSVR>2.0.CO;2. ISSN 0016-7606. 
  15. ^ Bonneville et al. 2002, p. 1024.
  16. ^ McNutt et al. 1997, p. 480.
  17. ^ McNutt et al. 1997, p. 482.
  18. ^ a b Fleitout, L.; Moriceau, C. (1 July 1992). "Short-wavelength geoid, bathymetry and the convective pattern beneath the Pacific Ocean". Geophysical Journal International. 110 (1): 13. doi:10.1111/j.1365-246X.1992.tb00709.x. ISSN 0956-540X. 
  19. ^ a b c Morgan & Morgan 2007, p. 59.
  20. ^ Moreira & Allègre 2004, p. 984.
  21. ^ Moreira & Allègre 2004, p. 986.
  22. ^ Moreira & Allègre 2004, p. 987.
  23. ^ Chauvel et al. 1997, p. 127.
  24. ^ Chauvel et al. 1997, p. 133.
  25. ^ Woodhead, Jon D. (1996). "Extreme HIMU in an oceanic setting: the geochemistry of Mangaia Island (Polynesia), and temporal evolution of the Cook—Austral hotspot". Journal of Volcanology and Geothermal Research. 72 (1-2): 16. doi:10.1016/0377-0273(96)00002-9. 
  26. ^ a b c Morgan & Morgan 2007, p. 60.
  27. ^ Bonneville et al. 2002, p. 1025.
  28. ^ Sipkin, Stuart A.; Jordan, Thomas H. (10 April 1975). "Lateral heterogeneity of the upper mantle determined from the travel times of". Journal of Geophysical Research. 80 (11): 1479. doi:10.1029/JB080i011p01474. 
  29. ^ Jackson, Matthew G.; Hart, Stanley R.; Konter, Jasper G.; Koppers, Anthony A. P.; Staudigel, Hubert; Kurz, Mark D.; Blusztajn, Jerzy; Sinton, John M. (December 2010). "Samoan hot spot track on a "hot spot highway": Implications for mantle plumes and a deep Samoan mantle source". Geochemistry, Geophysics, Geosystems. 11 (12): 19. doi:10.1029/2010GC003232. 
  30. ^ Konter, J. G.; Koppers, A. A.; Staudigel, H.; Hanan, B. B.; Blichert-Toft, J. (2004-12-01). "Intermittent Volcanism in the S Pacific: Tracking Persistent Geochemical Sources". AGU Fall Meeting Abstracts. 51. 
  31. ^ Jarrard & Clague 1977, p. 67.
  32. ^ Jarrard & Clague 1977, p. 68.
  33. ^ Bergersen 1995, p. 609.
  34. ^ Lincoln, Pringle & Silva 1993, p. 303.
  35. ^ Bergersen 1995, p. 610.
  36. ^ Bergersen 1995, p. 612.
  37. ^ Bergersen 1995, p. 611.
  38. ^ Staudigel, Hubert; Park, K.-H.; Pringle, M.; Rubenstone, J.L.; Smith, W.H.F.; Zindler, A. (1991). "The longevity of the South Pacific isotopic and thermal anomaly". Earth and Planetary Science Letters. 102 (1): 34. doi:10.1016/0012-821x(91)90015-a. 
  39. ^ a b c d Lincoln, Pringle & Silva 1993, p. 300.

Sources[edit]