Darwin Rise

The Darwin Rise is broad triangular region in the north central Pacific Ocean where there is a concentration of atolls.

During his voyage across the globe Charles Darwin realised that vertical crustal motion must be responsible for the formation of continents and ocean basins, as well as isolated atolls in the Pacific. He deduced that the central basin of the Pacific had subsided while surrounding areas had risen. In 1964 U.S. geologist Henry Menard subsequently named the uplifted area in the Pacific after the English naturalist.[2]

Coordinates: 10°N 180°W / 10°N 180°W

Darwin Rise is located in Pacific Ocean
Darwin Rise
Darwin Rise
Location in the Pacific Ocean[1]

Geological context

Covering an area of 12,000 km × 3,000 km (7,500 mi × 1,900 mi), the Darwin Rise is limited to the east by the Izu-Bonin and Mariana trenches, to the west by the Line Islands.[3] Two major plateaux, the Shatsky Rise to the north and the Ontong Java Plateau to the south, border the Darwin Rise. How these plateaux relate to the rise remains disputed.[3]

There are six major chains of seamounts on the rise — the Japanese, Magellan, Wake, Marshall Islands, Line Islands seamounts and the Mid-Pacific Mountains — and numerous minor clusters. The ages of these seamounts in general follow the motion of the Pacific Plate from 130 to 180 Ma, as predicted by conventional hotspot theory, and decrease from west to east and north to south. This age-distance pattern is, however, not corroborated by the limited data available.[3]

Origin

In 1964, Henry Menard proposed that this was a superswell raised by volcanism during the Cretaceous (120-80 mya).[4] A problem with this conjecture is that this region actually has a sea floor at a normal depth that happens to possess an abundance of sea mounts.[5][6]

Instead this feature may have formed from diapirs or plumes rising from the Earth's upper mantle, which results in chains of sea mounts along the direction of the plate motion. However, this idea remains in dispute and an alternate hypothesis involving multiple "plumelets" has been proposed.[7]

In the 1980s it was proposed that the Darwin Rise was the South Pacific Superswell 100 Ma and that the volcanoes of the Darwin Rise erupted over the same mantle region as the volcanoes of French Polynesia today.[8] Hence, U.S. geologist Marcia McNutt proposed that the Darwin Rise is a palaeo-superswell.[9]

See also

References

Notes

  1. ^ Stein & Stein c. 1999, Figure 1: Map of the south Pacific showing place names
  2. ^ Stein & Stein 1993, Introduction, p. 53; Darwin 1845: When later dealing with other continents he became so fixed in his belief in vertical movements of the crust and the erosive power of the sea that he excluded other possible explanations[…] [1]; Menard 1964
  3. ^ a b c Janney & Castillo 1999, Setting and Age Constraints, pp. 10572, 10574
  4. ^ For an illustration of Menard's model of the evolution of the Darwin Rise see Stein & Stein 1993, Fig. 1, p. 54
  5. ^ DeLaughter, Stein & Stein 2005, p. 272
  6. ^ Foulger 2010, p. 220
  7. ^ Janney & Castillo 1999, Abstract
  8. ^ McNutt 1998, p. 213
  9. ^ McNutt 1998, Fig. 2, p. 212

Sources

  • Darwin, C. (1845). The Voyage of the Beagle.
  • DeLaughter, J. E.; Stein, C. A.; Stein, S. (2005). "Hotspots: A view from the swells". In Foulger, G. R.; Natland, J. H.; Presnall, D. C.; Anderson, D. L. (eds.). Plates, Plumes, and Paradigms (PDF). The Geological Society of America. doi:10.1130/0-8137-2388-4.257. ISBN 0-8137-2388-4. Retrieved 18 February 2017.
  • Foulger, G. R. (2010). Plates vs Plumes: A Geological Controversy. John Wiley and Sons. ISBN 1-4443-3679-7.
  • Janney, P. E.; Castillo, P. R. (1999). "Isotope geochemistry of the Darwin Rise seamounts and the nature of long-term mantle dynamics beneath the south central Pacific" (PDF). Journal of Geophysical Research. 104 (B5): 10571–10590. Bibcode:1999JGR...10410571J. doi:10.1029/1998JB900061. Retrieved 19 February 2017.
  • McNutt, M. K. (1998). "Superswells" (PDF). Reviews of Geophysics. 36 (2): 211–244. doi:10.1029/98RG00255. Retrieved 19 February 2017.
  • Menard, H. W. (1964). Marine Geology of the Pacific. New York: McGraw-Hill. OCLC 545637.
  • Stein, C. A.; Stein, S. (1993). "Constraints on Pacific midplate swells from global depth‐age and heat flow‐age models" (PDF). In Pringle, M. S.; Sager, W. W.; Sliter, W. V. (eds.). The Mesozoic Pacific: geology, tectonics, and volcanism. Geophysical Monograph. 77. Washington, D.C.: Geophysical Union. pp. 53–76. doi:10.1029/GM077p0053. Retrieved 19 February 2017.
  • Stein, Carol A.; Stein, Seth (c. 1999). "The Superswell and Darwin Rise: Thermal no longer?".
Allison Guyot

Allison Guyot (formerly known as Navoceano Guyot) is a tablemount (guyot) in the underwater Mid-Pacific Mountains of the Pacific Ocean. It is a trapezoidal flat mountain rising 1,500 metres above the seafloor to a depth of less than 1,500 m, with a summit platform 35 by 70 kilometres wide. The Mid-Pacific Mountains lie west of Hawaii and northeast of the Marshall Islands, but at the time of their formation were located in the Southern Hemisphere.

The tablemount was probably formed by a hotspot in the present-day Southern Pacific before plate tectonics moved it to its current location. Several hotspots, including the Easter, Marquesas and Society hotspots, may have been involved in the formation of the Mid-Pacific Mountains. Volcanic activity is dated to have occurred circa 111–85 million years ago and formed a volcanic island. Subsequently, carbonate deposition commenced as Allison Guyot subsided and eventually buried the island, forming an atoll-like structure and a carbonate platform. Among other animals, crocodilians lived on Allison Guyot.

The platform emerged above sea level during the Albian and Turonian ages. It drowned about 99 ± 2 million years ago for unknown reasons; possibly a phase of renewed emergence damaged the reefs, or it was located in unfavourable waters. Later, pelagic sedimentation commenced on the seamount and led to the deposition of sediments including limestone, ooze and sand, which bear traces of climatic events and ocean currents.

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).

Horizon Guyot

Horizon Guyot is a presumably Cretaceous guyot (tablemount) in the Mid-Pacific Mountains, Pacific Ocean. It is an elongated ridge, over 300 kilometres (190 mi) long and 4.3 kilometres (2.7 mi) high, that stretches in a northeast-southwest direction and has two flat tops; it rises to a minimum depth of 1,443 metres (4,730 ft). The Mid-Pacific Mountains lie west of Hawaii and northeast of the Line Islands.

It was probably formed by a hotspot, but the evidence is conflicting. Volcanic activity occurred during the Turonian-Cenomanian eras 100.5–89.8 million years ago and another stage has been dated to have occurred 88–82 million years ago. Between these volcanic episodes, carbonate deposition from lagoonal and reefal environments set in and formed limestone. Volcanic islands developed on Horizon Guyot as well and were colonised by plants.

Horizon Guyot became a seamount during the Coniacian-Campanian period. Since then, pelagic ooze has accumulated on the seamount, forming a thick layer that is further modified by ocean currents and by various organisms that live on the seamount; sediments also underwent landsliding. Ferromanganese crusts were deposited on exposed rocks.

Izu–Bonin–Mariana Arc

The Izu–Bonin–Mariana (IBM) arc system is a tectonic-plate convergent boundary. The IBM arc system extends over 2800 km south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, Bonin Islands, and Mariana Islands; much more of the IBM arc system is submerged below sealevel. The IBM arc system lies along the eastern margin of the Philippine Sea Plate in the Western Pacific Ocean. It is the site of the deepest gash in Earth's solid surface, the Challenger Deep in the Mariana Trench.

The IBM arc system formed as a result of subduction of the western Pacific plate. The IBM arc system now subducts mid-Jurassic to Early Cretaceous lithosphere, with younger lithosphere in the north and older lithosphere in the south, including the oldest (~170 million years old, or Ma) oceanic crust. Subduction rates vary from ~2 cm (1 inch) per year in the south to 6 cm (~2.5 inches) in the north.

The volcanic islands that comprise these island arcs are thought to have been formed from the release of volatiles (steam from trapped water, and other gases) being released from the subducted plate, as it reached sufficient depth for the temperature to cause release of these materials. The associated trenches are formed as the oldest (most western) part of the Pacific plate crust increases in density with age, and because of this process finally reaches its lowest point just as it subducts under the crust to the west of it.

The IBM arc system is an excellent example of an intra-oceanic convergent margin (IOCM). IOCMs are built on oceanic crust and contrast fundamentally with island arc built on continental crust, such as Japan or the Andes. Because IOCM crust is thinner, denser, and more refractory than that beneath Andean-type margins, study of IOCM melts and fluids allows more confident assessment of mantle-to-crust fluxes and processes than is possible for Andean-type convergent margins. Because IOCMs are far removed from continents they are not affected by the large volume of alluvial and glacial sediments. The consequent thin sedimentary cover makes it much easier to study arc infrastructure and determine the mass and composition of subducted sediments. Active hydrothermal systems found on the submarine parts of IOCMs give us a chance to study how many of earth's important ore deposits formed.

MIT Guyot

MIT Guyot is a guyot in the Pacific Ocean that rises to a depth of 1,323 metres (4,341 ft). It has a 20-kilometre-long (12 mi) summit platform and formed during the Cretaceous in the region of present-day French Polynesia through volcanic eruptions.

The volcano was eventually covered by a carbonate platform resembling that of a present-day atoll which was colonized by a number of animals. A major volcanic episode disrupted this platform, which subsequently redeveloped until it drowned in the late Albian.

Mid-Pacific Mountains

The Mid-Pacific Mountains (MPM) is a large oceanic plateau located in the central North Pacific Ocean or south of the Hawaiian–Emperor seamount chain. Of volcanic origin and Mesozoic in age, it is located on the oldest part of the Pacific Plate and rises up to 2 km (1.2 mi) (Darwin Rise) above the surrounding ocean floor and is covered with several layers of thick sedimentary sequences that differ from those of other plateaux in the North Pacific. About 50 seamounts are distributed over the MPM. Some of the highest points in the range are above sea level which include Wake Island and Marcus Island.

The ocean floor of the MPM dates back to the Jurassic-Cretaceous, some of the oldest oceanic crust on Earth.The MPM is a range of guyots with a lava composition similar to those found in Iceland and the Galapagos Islands, and they probably formed similarly at or near a rift system.

In the Cretaceous, they formed large tropical islands located closer to the Equator that began to sink in the late Mesozoic.The MPM formed in the Early Cretaceous (at c. 110 Ma) over a hotspot that uplifted the ocean floor of the still young Pacific Plate. Reefs developed on the subsiding islands and renewed volcanism in the Late Cretaceous helped maintain some of eastern islands but inevitably the guyots sank to their present depth.

It has been proposed that the MPM has crossed over several hotspots, and the MPM guyots are indeed older on the western MPM than the eastern part, but the guyots do not form chains that can be traced to any known hotspots. The MPM, nevertheless, must have originated over the South Pacific Superswell. Among the guyots in the Mid-Pacific Mountains are Allison Guyot, Horizon Guyot, Resolution Guyot and Darwin Guyot.The western half of the Easter hotspot chain, a lineament that includes the Line Islands and Tuamotu archipelago, begins near the eastern part of the MPM. The formation of the MPM thus probably occurred at the Pacific-Farallon Ridge and the Easter hotspot, or where the Easter Microplate is now located.

Outline of oceanography

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

Resolution Guyot

Resolution Guyot (formerly known as Huevo) is a guyot (tablemount) in the underwater Mid-Pacific Mountains in the Pacific Ocean. It is a circular flat mountain, rising 500 metres (1,600 ft) above the seafloor to a depth of about 1,320 metres (4,330 ft), with a 35 kilometres (22 mi) wide summit platform. The Mid-Pacific Mountains lie west of Hawaii and northeast of the Marshall Islands, but at the time of its formation the guyot was located in the Southern Hemisphere.

The guyot was probably formed by a hotspot in today's French Polynesia before plate tectonics shifted it to its present-day location. The Easter, Marquesas, Pitcairn and Society hotspots, among others, may have been involved in the formation of Resolution Guyot. Volcanic activity has been dated to have occurred 107–129 million years ago and formed a volcanic island that was subsequently flattened by erosion. Carbonate deposition commenced, forming an atoll-like structure and a carbonate platform.

The platform emerged above sea level at some time between the Albian and Turonian ages before eventually drowning for reasons unknown between the Albian and the Maastrichtian. Thermal subsidence lowered the drowned seamount to its present depth. After a hiatus, sedimentation commenced on the seamount and led to the deposition of manganese crusts and pelagic sediments, some of which were later modified by phosphate.

Superswell

A superswell is a large area of anomalously high topography and shallow ocean regions. These areas of anomalous topography are byproducts of large upwelling of mantle material from the core–mantle boundary, referred to as superplumes. Two present day superswells have been identified: the African superswell and the South Pacific superswell. In addition to these, the Darwin Rise in the south central Pacific Ocean is thought to be a paleosuperswell, showing evidence of being uplifted compared to surrounding ancient ocean topography.

Takuyo-Daini

Takuyo-Daini is a seamount in the Pacific Ocean.

Takuyo-Daini is part of the so-called "Seiko" cluster or the "Geisha Guyots" in the Japanese Seamounts; it lies just west of Takuyo-Daisan seamount with which it forms a pair. Takuyo-Daini rises from a depth of 5,195 metres (17,044 ft) to a minimum depth of 1,420 metres (4,660 ft) and has a regular round shape with a small volume of 2,237 cubic kilometres (537 cu mi). Both seamounts are guyots and together with two other guyots known as Winterer and Isakov have been interpreted as being part of a hotspot track.The Western Pacific Ocean contains a large number of seamounts which often from clusters or groups. Many of them have flat tops 1–2 kilometres (0.62–1.24 mi) below sea level. A number of these formed during a large-scale volcanic episode in the Albian-Aptian era of the Cretaceous; this includes Takuyo-Daini, where radiometric dating has yielded ages of 118.6 million years ago. At the time of its formation this seamount was located in the central Pacific Ocean. Fossils of rudist bivalves have been found on Takuyo-Daini; the seamount once featured rudist reefs that ceased growing during the Albian. The rudist genera Magallanesia was discovered on Takuyo-Daini and on Cebu in the Philippines.

Takuyo-Daisan

Takuyo-Daisan is a guyot in the Western Pacific Ocean off Japan. It is 1,409 metres (4,623 ft) deep and has a square-shaped flat top surrounded by a perimeter ridge. Several other seamounts lie nearby.

The seamount formed as a volcanic island during the Cretaceous in the area currently occupied by French Polynesia. Subsequently reefs developed around the volcanic island and generated a carbonate platform which drowned during the Albian along with several other such platforms in the world.

Wōdejebato

Wōdejebato (formerly known as Sylvania) is a Cretaceous guyot or tablemount in the northern Marshall Islands, Pacific Ocean. Wōdejebato is probably a shield volcano and is connected through a submarine ridge to the smaller Pikinni Atoll 74 kilometres (46 mi) southeast of the guyot; unlike Wōdejebato, Pikinni rises above sea level. The seamount rises for 4,420 metres (14,500 ft) to 1,335 metres (4,380 ft) depth and is formed by basaltic rocks. The name Wōdejebato refers to a sea god of Pikinni.

It was probably formed by a hotspot in what is present-day French Polynesia before plate tectonics moved it to its present-day location. The Macdonald, Rarotonga, Rurutu and Society hotspots may have been involved in its formation. The first volcanic phase took place in the Cenomanian and was followed by the formation of a carbonate platform that quickly disappeared below the sea. A second volcanic episode between 85 and 78.4 million years ago (in the Campanian) led to the formation of an island. This island was eventually eroded and rudist reefs generated an atoll or atoll-like structure, covering the former island with carbonates and thus a second carbonate platform.

The second carbonate platform drowned about 68 million years ago (in the Maastrichtian), perhaps because at that time it was moving through the equatorial area which may have been too hot or too nutrient-rich to support the growth of a coral reef. Thermal subsidence lowered the drowned seamount to its present depth. After a hiatus, sedimentation commenced on the seamount and led to the deposition of manganese crusts and pelagic sediments, some of which were later modified by phosphate.

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