Macdonald seamount

Coordinates: 28°58.7′S 140°15.5′W / 28.9783°S 140.2583°W / -28.9783; -140.2583[1]
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Macdonald seamount
Summit depth40m
HeightVaries
Location
LocationSouth Pacific Ocean
Coordinates28°58.7′S 140°15.5′W / 28.9783°S 140.2583°W / -28.9783; -140.2583[1]
Geology
TypeSeamount
Last eruption1987-89
History
Discovery date1967

Macdonald seamount (named after

hyperthermophilic
bacteria.

Macdonald seamount is the currently active volcano of the Macdonald hotspot, a volcanic hotspot that has formed this seamount and some other volcanoes. Eruptions occurred in 1967, 1977, 1979–1983 and 1987–1989, and earthquakes were recorded in 2007. The activity, which has produced basaltic rocks, has modified the shape of the volcano and may lead to the formation of an island in the future.

Discovery and name

Macdonald seamount was discovered in 1967, when hydrophones noted earthquake activity in the area.[2] The seamount was named in 1970 after Gordon A. Macdonald.[3] It is also known as Tamarii,[4] while MacDonald appears to be an incorrect capitalization.[3]

Geography and geology

Regional setting

Further information: Macdonald hotspot

The

atolls. This has led to the suggestion that they are formed by deep sources over which the Pacific Plate drifts and eventually carries the volcano away from its magma source.[5] These sources are known as "hotspots", and their total number has been estimated to be between 42 and 117.[6] Hotspots may also be formed by cracks propagating in the crust, and such hotspots would not necessarily show an age progression.[7]

Local setting

Macdonald seamount is located off the southeastern end of the Austral Islands.[8] The Austral Islands extend away from the southern Cook Islands to Îles Maria and eventually Marotiri southeastward,[5] including the islands Rimatara, Rurutu, Tubuai, Raivavae and Rapa.[9] A relatively large gap separates Marotiri from the Macdonald volcano.[10] The Ngatemato seamounts and Taukina seamounts lie north of Macdonald,[11] they are considerably older and appear to have a very different origin.[12] Even farther southeast lies the Foundation seamount chain,[13] and the associated hotspot may have generated some of the seamounts close to Macdonald.[14]

The seamount lies close to the southeastern end of an area of shallower ocean, which extends northwestward towards Marotiri,[15] and includes Annie seamount, Simone seamount and President Thiers Bank.[16] The 3,000 metres (9,800 ft) high Ra seamount (named after Polynesian term for "sun") rises 100 kilometres (62 mi) northwest of Macdonald to a depth of 1,040 metres (3,410 ft); it is apparently an extinct volcano and may have once emerged above sea level.[17] A smaller seamount, Macdocald, rises from the southern foot of Macdonald 850 metres (2,790 ft) to depths of 3,150 metres (10,330 ft).[18] Additional small seamounts that appear to have formed at the East Pacific Rise are also found in the area.[19] The crust beneath Macdonald is of Eocene age,[4] and away from the area of shallower ocean it is covered with hills and sediment.[20]

Macdonald seamount rises 4,200 metres (13,800 ft) from the seafloor to a depth of about 40 metres (130 ft) below sea level;

spatter cones.[23] Other sources indicate a surface area of 2.4 square kilometres (0.93 sq mi) for the summit plateau.[24] Ongoing volcanic activity may have modified the topography of the summit of Macdonald between surveys in 1975 and 1982, forming another elliptical pinnacle reaching a depth of 29 metres (95 ft) at the northwestern margin of the plateau and raising the summit plateau to depths of 50–34 metres (164–112 ft).[23] By the time of a new survey in 1986, the pinnacle had been replaced by a pile of rocks which only reached a depth of 42 metres (138 ft).[25]

The upper parts of the edifice are covered by 50 centimetres (20 in) thick

lava flow fronts form scarps which become particularly noticeable at depths of 620–1,000 metres (2,030–3,280 ft), except on the northern flank. Even deeper, pillow lavas predominate.[28]

Below the summit area, the slopes fall down steeply to a depth of 600 metres (2,000 ft) and then flatten out.[17] Save for a debris-covered ridge to the northwest, Macdonald has a circular shape,[29] with a width of 45 kilometres (28 mi) at a depth of 3,900 metres (12,800 ft). The slopes of Macdonald display radial ridges which may reflect tectonically-controlled rift zones, as well as isolated parasitic cones.[17] The volume of the whole edifice has been estimated to be 820 cubic kilometres (200 cu mi).[24] Macdonald seamount bears traces of landslides, including collapse scars up on the edifice and smooth terrain formed by debris on its lower slopes;[30] collapses have been inferred on the eastern, southern, western and northwestern flank.[31] The seafloor further shows evidence of turbidity currents, including ripples.[30]

Geomagnetic analysis of the edifice has demonstrated the existence of a normally magnetized structure at the base of the volcano and an additional anomaly which seems to be the magma chamber at a depth of 2 kilometres (1.2 mi) within the edifice, close to the northern flank.[32] Data obtained in gabbroic rocks expelled by the volcano during its eruptions also suggest that another magma reservoir exists at depths of 5 kilometres (3.1 mi), that is within the crust beneath Macdonald.[33]

Composition

Macdonald has principally erupted

orthopyroxene.[40]

The vulcanites are typical

plate.[43]

Eruptions

Macdonald is the only known active volcano in the Cook Islands and Austral Islands,

seismic swarm probably unassociated with eruptions occurred in 2007.[47]

Eruptions at Macdonald include

lava flows.[34] Volcanic activity is not steady, with prolonged pauses observed between eruptions.[48] Macdonald seamount is among the most active submarine volcanoes in the world,[21] and the most active on the floor of the Pacific Ocean.[49]

Radiometric dating of rocks dredged from Macdonald has yielded two separate clusters of ages, one less than two million years old and the second about 30 million years.[50]

1989 events

Several eruptions occurred in 1989 when a scientific expedition was underway on the seamount. These eruptions were accompanied by the discolouration of the water over 1.6 kilometres (1 mi) of length, the release of burning hydrogen and hydrogen sulfide[21] accompanied by the formation of a plume of hydrothermally altered water.[8] The submarine Cyana observed activity directly in one summit crater in the form of intense bubbling,[51] while steam and water fountains were seen on the ocean surface.[21]

Grey-coloured slicks developed on the ocean surface,[51] which were formed by pyrite, sulfur and volcanic glass plus smaller amounts of cinnabar, cubatine and quenstedtite.[8] The events caused changes in the pH of the water on the seamount and increased methane concentrations.[1]

Future birth of an island

Macdonald likely formed an island during the

last glacial maximum when sea level was lower,[48] and future eruptions at Macdonald may lead to the birth of an island even with present-day sea levels. Such an eruption would have to be fairly large and continuous, otherwise the resulting island will likely be eroded away quickly.[52] Depending on how quickly erosion and other factors reduce its size, such an island will likely be temporary.[53]

Hydrothermal system

Macdonald seamount is hydrothermally active,[54] with several hydrothermal vents inferred to exist on the western flank.[55] A 2–3 metres (6 ft 7 in – 9 ft 10 in) wide eruption fissure was observed to be hydrothermally active in 1989.[56] Further, the volcano releases gases including carbon dioxide, methane and sulfur dioxide. Such release occurs in the summit area[57] in the so-called "Champagne Field",[37] but also from a second crater at 2,000 metres (6,600 ft) depth in the southeastern flank.[57] Macdonald volcano may be a major source of heavy metals for the area.[58] The methane appears to be partially of biological origin and partly abiogenic.[59]

Biology

heterotrophs and appear to be capable of long-range propagation, considering that relatives of the species found are known from Vulcano in Italy.[54]

Aside from hyperthermophiles, craniids,[60] corals,[61] polynoids[62] and sponges have been found in the summit area of Macdonald.[63]

References

  1. ^ a b Huber et al. 1990, p. 180.
  2. ^ a b Talandier & Okal 1984, p. 813.
  3. ^ a b Morgan & Morgan 2007, p. 59.
  4. ^ a b c Rubin & Macdougall 1989, p. 50.
  5. ^ a b Johnson & Malahoff 1971, p. 3282.
  6. ^ Stoffers et al. 1989, p. 101.
  7. ^ Jarrard & Clague 1977, p. 74.
  8. ^ a b c Chemine'e et al. 1991, p. 319.
  9. ^ Johnson & Malahoff 1971, p. 3283.
  10. ^ Johnson & Malahoff 1971, p. 3289.
  11. ^ Bonneville et al. 2002, p. 1024.
  12. ^ McNutt et al. 1997, p. 480.
  13. doi:10.1016/0012-821x(92)90135-i
    .
  14. ^ Morgan & Morgan 2007, p. 58.
  15. ^ Stoffers et al. 1989, p. 103.
  16. ^ Binard et al. 2004, p. 176.
  17. ^ a b c d Stoffers et al. 1989, p. 104.
  18. ^ Binard et al. 2004, p. 178.
  19. ^ Binard et al. 2004, p. 196.
  20. ^ Hekinian et al. 1991, p. 2112.
  21. ^ a b c d e Chemine'e et al. 1991, p. 318.
  22. ^ a b Talandier & Okal 1984, p. 814.
  23. ^ a b Talandier & Okal 1984, p. 815.
  24. ^ a b Binard et al. 2004, p. 160.
  25. ^ Talandier 2004, p. 65.
  26. ^ a b Binard et al. 2004, p. 177.
  27. ^ Bideau & Hekinian 2004, p. 315.
  28. ^ a b Stoffers et al. 1989, p. 107.
  29. ^ Johnson & Malahoff 1971, p. 3285.
  30. ^ a b Clouard & Bonneville 2004, p. 222.
  31. ^ Clouard & Bonneville 2004, p. 223.
  32. ^ Johnson & Malahoff 1971, p. 3286.
  33. ^ Bideau & Hekinian 2004, p. 344.
  34. ^ a b c Stoffers et al. 1989, p. 109.
  35. ^ Hekinian et al. 1991, p. 2115.
  36. ^ a b Hekinian et al. 1991, p. 2136.
  37. ^ a b c Stoffers 1993, p. 20.
  38. ^ Stoffers et al. 1989, p. 111.
  39. ^ Bideau & Hekinian 2004, p. 343.
  40. ^ Hekinian et al. 1991, p. 2117.
  41. ^ Chauvel et al. 1997, p. 133.
  42. ^ Suetsugu & Hanyu 2013, p. 268.
  43. ^ Stoffers 1993, p. 18.
  44. ^ Bonneville et al. 2002, p. 1023.
  45. ^ Stoffers 1993, p. 16.
  46. ^ "Macdonald". Global Volcanism Program. Smithsonian Institution.
  47. doi:10.5479/si.gvp.bgvn200701-333060
    .
  48. ^ a b Bideau & Hekinian 2004, p. 313.
  49. ^ Binard et al. 2004, p. 175.
  50. doi:10.1016/j.chemgeo.2003.08.002
    .
  51. ^ a b Chemine'e et al. 1991, p. 322.
  52. ^ Talandier 2004, p. 69.
  53. ^ Bideau & Hekinian 2004, p. 312.
  54. ^ a b Huber et al. 1990, p. 181.
  55. doi:10.1016/0016-7037(92)90162-c
    .
  56. ^ Stoffers et al. 1989, p. 104,107.
  57. ^ a b Bideau & Hekinian 2004, p. 311.
  58. ^ Rubin & Macdougall 1989, p. 51.
  59. ISBN 978-3-642-62290-8
    .
  60. ISSN 0024-4082.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  61. ^ Stoffers et al. 1989, p. 108.
  62. ^ Molodtsova, T.; Budaeva, N. (2007-11-01). "Modifications of corallum morphology in black corals as an effect of associated fauna". Bulletin of Marine Science. 81 (3): 478.
  63. ^ Binard et al. 2004, p. 173.

Sources

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