Macdonald Seamount

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Macdonald-Seamount (Pacific Ocean)
Macdonald Seamount
Macdonald Seamount
Marotiri
Marotiri
Location of the Seamount Macdonald east-southeast of the Austral Islands in French Polynesia , especially Marotiri

The Macdonald Seamount is a volcanically active deep-sea mountain at the southeast end of the Austral Islands in French Polynesia .

Discovery and designation

The Macdonald Seamount was discovered in 1967 after the hydrophones reported seismic signals to an oceanographic survey campaign. In 1970 the newly discovered seamount was named after the American volcanologist Gordon A. Macdonald . The Polynesian name is Tamarii .

geography

The Macdonald Seamount is located at the southeast end of the Austral Islands, 338 kilometers east-southeast of Marotiri , the closest land mass. To the northwest, Marotiri, Rapa , Raivavae , Tubuai , Rurutu , Rimatara , the southern Cook Islands and finally the Îles Maria follow . There is quite a big gap between Marotiri and the Macdonald Seamount. North of the Macdonald Seamount are the Ngatemato Seamounts and the Taukina Seamounts , which are much older and are likely to have a completely different origin. The chain of Foundation seamounts , which can be traced back to the Foundation hotspot , follows clearly in the southeast . The Foundation hotspot may be responsible for some seamounts in the neighborhood of the Macdonald seamount.

At the Macdonald-Seamount ends a topographical high altitude that extends northwest to Marotiri and includes the Annie-Seamount , the Simone-Seamount and the President-Thiers-Bank . 100 kilometers to the northwest is the 3,000-meter-high Rà-Seamount (Rà is the Polynesian word for the sun ), which ends in 1,040 meters of water below sea level. Obviously, this is an extinct volcano that may have appeared before. At the southern foot of the Macdonald seamount, an 850 meter high, smaller seamount rises to a water depth of 3,150 meters. There are other small seamounts in the area that may have formed on the East Pacific Ridge .

description

The Macdonald Seamount, in the immediate vicinity of which there are other seamounts, rises 4,200 meters above the sea floor of the southern Pacific Ocean . Its tip is usually only 40 meters deep, but this depth can vary slightly depending on the volcanic activity. Since its discovery, the Macdonald Seamount has been in periodic activity, recognizable by gas rise and seismic signals . Eruptions occurred on the Seamount in 1967, 1977, 1979 to 1983 and 1987 to 1989. Smaller earthquakes were also recorded at the volcano in 2007.

The summit of the seamount is flattened to a 150 × 100 meter plateau (another source gives 2.4 square kilometers for the plateau), but carries several smaller volcanic cones (6 meter high and 3 meter wide weld cinder cones). It is assumed that the seamount sits on the Macdonald hotspot of the same name , which has created a number of other volcanoes in addition to the Macdonald seamount. The volcanic activities changed the morphology of the volcano considerably between 1975 and 1982 and it is therefore not unlikely that a new island will appear in the near future - a peak on the plateau reached 49 meters water depth in 1979, a jagged elliptical shape was in 1982 even only 29 meters deep and the plateau itself varied between 50 and 34 meters water depth; In 1986 the point was just a pile of stones in 42 meters of water.

The upper section of the seamount is covered by a 50 centimeter thick layer of lapilli , below which there are lava flows. Hydrothermal conversion products are also present. There is a thick layer of ash down to a depth of 2,000 meters. In addition to the lapilli, there are also scoria-rich lava flows. In deeper areas, the terms of the lava flows form terrain levels, which are particularly striking in water depths between 600 and 1,000 meters and are only missing on the north side. Further in the abyssal pillow lavas predominate.

Below the summit plateau, the volcano drops steeply to a depth of 600 meters, but then visibly flattens out. Apart from a rubble-covered ridge moving to the northwest, it has a circular shape with a diameter of 45 kilometers in 3,900 meters water depth. Its slopes are traversed by radially arranged topographical ridges that are either of tectonic origin (fracture systems) or represent isolated parasitic cones. The total volume of the seamount was estimated at 820 cubic kilometers. It also shows signs of mass movements , including tearing edges in the upper area and terrain smoothed by the sliding masses in the lower layers. Demolitions are suspected for the east, south, west and north-west flanks. There are also indications of turbidites and ripples on the sea floor .

A geomagnetic survey revealed magnetization with normal polarity for the base of the volcano. However, an anomaly was encountered at a depth of 2 kilometers below the northern flank, which should correspond to the magma chamber. Gabbros produced during the eruptions also indicate that another magma chamber is to be expected at 5 kilometers (in the oceanic crust) below the Macdonald seamount.

Regional geological introduction

Hotspots in the Pacific Ocean

The Pacific Ocean is characterized by long chains of volcanic islands, which generally run parallel to the direction of movement of the Pacific plate in a southeast-northwest direction. As in the case of Hawai'i , these chains begin in the southeast with active volcanic structures, which are increasingly subject to erosion in the northwest and then end as atolls . For this reason, it has been assumed that the islands can be traced back to deep, stationary magma herds, over which the Pacific plate slides and moves the emerging volcanic structures similar to a conveyor belt. The magma herds are known as hotspots , of which there are likely to be between 42 and 117 all over the world. According to another theory, the island chains are formed by systems of cracks moving forward in the earth's crust and therefore show no clear age progression.

The oceanic crust below the Macdonald Seamount has an Eocene age and is dotted with sediment-covered elevations at lower elevations.

Petrology

The Macdonald seamount consists mainly of basalts with phenocrystals of clinopyroxene , olivine and plagioclase . Also basanites , Mugearite , Pikrite and Tephrite may occur. Overall, the volcanic rocks are alkaline .

Boulders recovered from the seamount that were formed by phreatomagmatic explosions also contain intrusive rocks such as gabbros, metadolerites , picrites and pyroxenites . The gabbros are due to the slow crystallization of a basaltic magma within a magma chamber. The crystallization process was followed by hydrothermal and thermal transformations in the low temperature range. Minerals such as amphibole , chlorite , epidote , phyllosilicates , pyrite , quartz and smectite were formed , as well as albite , biotite , labradorite and orthopyroxene .

Petrologically the volcanic rocks typical are ocean island basalts ( English ocean Iceland basalts or abbreviated OIB ). However, they differ in their alkaline character from other hotspot tholeiites, such as those found on Hawai'i , Iceland or Réunion . The latter hotspots also produced alkali rocks, but only in their post-shield stage of development. However, the Macdonald Seamount is clearly in the early stages of its development and therefore requires further geochemical studies. It is assumed that its magmas were created by partial melting of a spinel heart zeolite , whereby the melting process was influenced by fractional crystallization .

Outbreak events

The Macdonald Seamount is the only currently known active volcano in the Austral and Cook Islands. The first eruption recorded with hydrophones occurred in 1967, followed by further activities in 1977. As early as 1928 and 1936, floating pumice carpets had been sighted, possibly coming from the seamount. Other eruptions occurred between 1979 and 1983. The volcano was also active between June 1987 and December 1988. Swarms of earthquakes were recorded in 2007, but they were not necessarily related to volcanic eruptions. Eruptions, especially on the south side or inside the crater, may not have been registered at all.

The eruption style on the Macdonald-Seamount is predominantly phreatic and phreatomagmatic, recognizable by lapilli and lava bombs, but partly also effusive (lava flows). The volcanic activities are not regular, but show longer pauses between eruptions. The Macdonald Seamount is one of the most active underwater volcanoes in the world and certainly the most active on the ocean floor of the Pacific Ocean.

The events of 1989

Several eruptions occurred at Macdonald-Seamount in 1989 when it was being investigated by a scientific expedition. Over a distance of 1.6 kilometers, the sea water had become discolored and a propulsion bubble of hydrothermally changed water masses was accompanied by burning hydrogen and hydrogen sulfide gases . The research submarine Cyana observed intense bubble formation in one of the summit craters, while steam and water fountains could be seen on the surface of the water. In addition, gray swaths of silt formed on the surface of the water, composed of pyrite, sulfur and volcanic glass and containing small amounts of cinnabarite , cubanite and quenstedtite . As a result of these incidents, the pH of the seawater on the seamount changed and the methane concentration increased.

A new island emerges

At the time of the last glacial maximum , the Macdonald Seamount is likely to have jutted out of the sea as an island when the sea level was low. Even with today's sea ​​level rise , it is quite conceivable that the seamount will reappear due to continued eruption activity. The eruptions must then be very extensive and, above all, occur continuously so that the newly emerging island does not immediately succumb to erosion again.

Hydrothermal processes

The Macdonald Seamount is hydrothermally active and it is believed that there are several openings in front of all on the western flank. In 1989 hydrothermal eruptions had occurred from a 2 to 3 meter wide fissure. The volcano emits gas, including carbon dioxide , methane and sulfur dioxide . The emissions originate from the summit crater, but also flow from a secondary crater on the south-east flank at a depth of 2,000 meters. The Macdonald Seamount is possibly a major supplier of heavy metals for its surroundings . The methane emitted is partly biogenic and partly abiogenic in origin.

Age

Radiometric age dating found two age groups for rock samples from the Macdonald Seamount: an age group at 30 million years ( Oligocene , Rupelian ) and young ages at 2 million years ( Pliocene , Gelasian ).

Living world

At Macdonald Seamount were hyper thermophilic bacteria discovered, including the taxa Archaeoglobus , Pyrococcus , Pyrodictium and Thermococcus and as yet unknown species. The communities include both hydrogen- and sulfur-consuming autotrophic bacteria, but heterotrophic bacteria also occur. Obviously they have a huge range, as related species are also found on Vulcano in Italy.

In the summit region of the Macdonald Seamount, in addition to the hyperthermophilic bacteria, corals , Craniidae belonging to the Brachiopods, Polynoidae belonging to the Polynoidae and sponges were found.

Individual evidence

  1. ↑ a b Talandier, Jacques and Okál, Emile A .: New surveys of MacDonald Seamount, south central Pacific, Following volcanoseismic activity, 1977-1983 . In: Geophysical Research Letters . tape 11 (9) , 1984, ISSN  1944-8007 , pp. 813-816 , doi : 10.1029 / GL011i009p00813 .
  2. ^ Rubin, KH and Macdougall, JD: Submarine magma degassing and explosive magmatism at Macdonald (Tamarii) seamount . In: Nature . tape 341 (6237) , 1989, pp. 50-52 , doi : 10.1038 / 341050a0 .
  3. ↑ McNutt, MK, Caress, DW, Reynolds, J., Jordahl, KA and Duncan, RA: Failure of plume theory to explain midplate volcanism in the southern Austral islands . In: Nature . tape 389 (6650) , 1997, ISSN  0028-0836 , pp. 479-48 , doi : 10.1038 / 39013 .
  4. ↑ Mammerickx, J .: The Foundation Seamounts: tectonic setting of a newly discovered seamount chain in the South Pacific . In: Earth and Planetary Science Letters . tape 113 (3) , 1992, pp. 293-306 , doi : 10.1016 / 0012-821x (92) 90135-i .
  5. ^ Morgan, W. Jason and Morgan, Jason Phipps: Plate velocities in hotspot reference frame: electronic supplement . 2007.
  6. ↑ Stoffers, P. et al .: Geology of Macdonald Seamount region, Austral Islands: Recent hotspot volcanism in the south Pacific . In: Marine Geophysical Researches . tape 11 (2) , 1989, ISSN  0025-3235 , pp. 101-112 , doi : 10.1007 / BF00285661 .
  7. ↑ Clouard, V. and Bonneville, A .: Submarine Landslides in French Polynesia . In: Oceanic Hotspots . Springer, Berlin, Heidelberg 2004, ISBN 978-3-642-62290-8 , pp. 209-238 .
  8. ↑ Bideau, D. and Hekinian, R .: Intraplate Gabbroic rock debris Ejected from the Magma Chamber of the Macdonald seamount (Austral Hotspot): Comparison with Other Provinces . In: Oceanic Hotspots . Springer, Berlin, Heidelberg 2004, pp. 309-348 , doi : 10.1007 / 978-3-642-18782-7_11 .
  9. ↑ Johnson, Rockne H. and Malahoff, Alexander: Relation of Macdonald Volcano to migration of volcanism along the Austral Chain . In: Journal of Geophysical Research . tape 76 (14) , 1971, ISSN  2156-2202 , pp. 3282-3290 , doi : 10.1029 / JB076i014p03282 .
  10. ^ Jarrard, Richard D. and Clague, David A .: Implications of Pacific Island and seamount ages for the origin of volcanic chains . In: Reviews of Geophysics . tape 15 (1) , 1977, pp. 57 , doi : 10.1029 / RG015i001p00057 .
  11. ↑ Hekinian, Roger et al. a .: Submarine intraplate volcanism in the South Pacific: Geological setting and petrology of the society and the austral regions . In: Journal of Geophysical Research . 96 (B2), 1991, pp. 2109 , doi : 10.1029 / 90JB02139 .
  12. ↑ Chauvel, C., McDonough, W., Guille, G., Maury, R. and Duncan, R .: Contrasting old and young volcanism in Rurutu Island, Austral chain . In: Chemical Geology . tape 139 (1-4) , 1997, pp. 125-143 , doi : 10.1016 / s0009-2541 (97) 00029-6 .
  13. ↑ Suetsugu, Daisuke and Hanyu, Takeshi: Origin of hotspots in the South Pacific: Recent advances in seismological and geochemical models . In: Geochemical Journal . tape 47 (2) , 2013, p. 259-284 , doi : 10.2343 / geochemj.2.0229 .
  14. ^ Bonneville, Alain et al.: Arago Seamount: The missing hotspot found in the Austral Islands . In: Geology . tape 30 (11) , 2002, ISSN  0091-7613 , pp. 1023-1026 , doi : 10.1130 / 0091-7613 (2002) 030 <1023: ASTMHF> 2.0.CO; 2 .
  15. ↑ Cheminée, J.-L., Stoffers, P., McMurtry, G., Richnow, H., Puteanus, D. and Sedwick, P .: Gas-rich submarine exhalations during the 1989 eruption of Macdonald Seamount . In: Earth and Planetary Science Letters . tape 107 (2) , 1991, pp. 318-327 , doi : 10.1016 / 0012-821X (91) 90079-W .
  16. ↑ Talandier, J .: Seismicity of the Society and Austral Hotspots in the South Pacific: Seismic Detection, Monitoring and Interpretation of Underwater Volcanism . In: Oceanic Hotspots . Springer, Berlin, Heidelberg 2004, ISBN 978-3-642-62290-8 , pp. 29-71 .
  17. ↑ Huber, R., Stoffers, P., Cheminee, J.-L., Richnow, HH and Stetter, KO: Hyperthermophilic archaebacteria within the crater and open-sea plume of erupting Macdonald Seamount . In: Nature . tape 345 (6271) , 1990, pp. 179-182 , doi : 10.1038 / 345179a0 .
  18. ↑ Stüben, Doris, Stoffers, Peter, Cheminée, Jean-Luc, Hartmann, Martin, McMurtry, Gary M., Richnow, Hans-Hermann, Jenisch, Angela and Michaelis, Walter: Manganese, methane, iron, zinc, and nickel anomalies in hydrothermal plumes from Teahitia and Macdonald volcanoes . In: Geochimica et Cosmochimica Acta . tape 56 (10) , 1992, pp. 3703 , doi : 10.1016 / 0016-7037 (92) 90162-c .
  19. ↑ Thießen, O., Schmidt, M., Botz, R., Schmitt, M. and Stoffers, P .: Methane Venting into the Water Column Above the Pitcairn and the Society - Austral Seamounts, South Pacific . In: Oceanic Hotspots . Springer, Berlin, Heidelberg 2004, pp. 426 , doi : 10.1007 / 978-3-642-18782-7_14 .
  20. ↑ Lassiter, J. C, Blichert-Toft, J., Hauri, E. H and Barsczus, H. G: Isotope and trace element variations in lavas from Raivavae and Rapa, Cook – Austral islands: constraints on the nature of HIMU- and EM-mantle and the origin of mid-plate volcanism in French Polynesia . In: Chemical Geology . tape 202 (1-2) , 2003, pp. 116 , doi : 10.1016 / j.chemgeo.2003.08.002 .
  21. ^ L. Cohen Bernard, Anne Kaulfuss and Carsten Lüter: Craniid brachiopods: aspects of clade structure and distribution reflect continental drift (Brachiopoda: Craniiformea) . In: Zoological Journal of the Linnean Society . tape 171 (1) , 2014, ISSN  0024-4082 , p. 144 , doi : 10.1111 / zoj12121 .
  22. ↑ Molodtsova, T. and Budaeva, N .: Modifications of corallum morphology in black corals as an effect of associated fauna . In: Bulletin of Marine Science . tape 81 (3) , 2007, pp. 478 .
  23. ↑ Binard, N., Hekinian, R., Stoffers, P. and Cheminée, J.-L .: South Pacific Intraplate Volcanism: Structure, Morphology and Style of Eruption . In: Oceanic Hotspots . Springer, Berlin, Heidelberg 2004, pp. 157-207 , doi : 10.1007 / 978-3-642-18782-7_6 .

Coordinates: 28 ° 59 ′  S , 140 ° 15 ′  W