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. 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 to 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.
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. 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.
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. 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. This has been explained by the presence of a "superplume", a very large mantle plume which also formed oceanic plateaus during the Cretaceous, with present-day volcanism at the Society and Macdonald volcanoes originating from secondary plumes that rise from the superplume to the crust.
The Austral Islands and the Cook Islands may have been formed by the Macdonald hotspot, 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, which is the presently active volcano on the Macdonald hotspot. 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. 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.
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. Additional younger ages in some volcanoes such as Rurutu have been explained by the presence of an additional system, the Arago hotspot, and some rocks from Tubuai and Raivavae 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. Other problems with using a hotspot to explain this volcanism is the highly variable composition of volcanism between various edifices, and that a number of Cook Islands are not located on the reconstructed path of the Macdonald hotspot. 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.
The high ratio of helium-3 to helium-4 has been used to infer a deep mantle origin of magmas of hotspot volcanoes. Helium samples taken from Macdonald support the contention 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.
Overall, the list of candidate volcanoes produced by the Macdonald hotspot is:
Arago hotspot is a hotspot in the Pacific Ocean, presently located below the Arago seamount close to the island of Rurutu, French Polynesia.
Arago is part of a family of hotspots in the southern Pacific, which include the Society hotspot and the Macdonald hotspot among others. These are structures beneath Earth's crust which generate volcanoes and which are in part formed by mantle plumes, although Arago itself might have a shallower origin. As the Pacific plate moves over the hotspots, new volcanoes form and old volcanoes are carried away; sometimes an older volcano is carried over the hotspot and is then uplifted as happened with Rurutu.
The Arago hotspot is responsible for the formation of Arago seamount and uplift on Rurutu; however reconstructions of the past positions of tectonic plates and geochemistry suggest that other islands and seamounts were constructed by the Arago hotspot during the past 120 million years. These potentially include Tuvalu, Gilbert Islands, the Ratak Chain of the Marshall Islands as well as part of the Austral Islands and Cook Islands.Austral Islands
The Austral Islands (French: Îles Australes, officially Archipel des Australes; Tahitian: Tuha'a Pae) are the southernmost group of islands in French Polynesia, an overseas country of the French Republic in the South Pacific. Geographically, they consist of two separate archipelagos, namely in the northwest the Tupua'i islands (French: Îles Tubuaï) consisting of the Îles Maria, Rimatara, Rūrutu, Tupua'i Island proper and Ra'ivāvae, and in the southeast the Bass Islands (French: Îles basses) composed of the main island of Rapa Iti and the small Marotiri (also known as Bass Rocks or Îlots de Bass). Inhabitants of the islands are known for their pandanus fiber weaving skills. The islands of Maria and Marotiri are not suitable for sustained habitation. Several of the islands have uninhabited islets or rocks off their coastlines. Austral Islands' population is 6,965 on almost 150 km2 (58 sq mi). The capital of the Austral Islands administrative subdivision is Tupua'i.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).Hawaii hotspot
The Hawaii hotspot is a volcanic hotspot located near the namesake Hawaiian Islands, in the northern Pacific Ocean. One of the best known and intensively studied hotspots in the world, the Hawaii plume is responsible for the creation of the Hawaiian–Emperor seamount chain, a 5,800-kilometre (3,600 mi) mostly undersea volcanic mountain range. Four of these volcanoes are active, two are dormant; more than 123 are extinct, most now preserved as atolls or seamounts. The chain extends from south of the island of Hawaiʻi to the edge of the Aleutian Trench, near the eastern coast of Russia.
While most volcanoes are created by geological activity at tectonic plate boundaries, the Hawaii hotspot is located far from plate boundaries. The classic hotspot theory, first proposed in 1963 by John Tuzo Wilson, proposes that a single, fixed mantle plume builds volcanoes that then, cut off from their source by the movement of the Pacific Plate, become increasingly inactive and eventually erode below sea level over millions of years. According to this theory, the nearly 60° bend where the Emperor and Hawaiian segments of the chain meet was caused by a sudden shift in the movement of the Pacific Plate. In 2003, fresh investigations of this irregularity led to the proposal of a mobile hotspot theory, suggesting that hotspots are mobile, not fixed, and that the 47-million-year-old bend was caused by a shift in the hotspot's motion rather than the plate's.
Ancient Hawaiians were the first to recognize the increasing age and weathered state of the volcanoes to the north as they progressed on fishing expeditions along the islands. The volatile state of the Hawaiian volcanoes and their constant battle with the sea was a major element in Hawaiian mythology, embodied in Pele, the deity of volcanoes. After the arrival of Europeans on the island, in 1880–1881 James Dwight Dana directed the first formal geological study of the hotspot's volcanics, confirming the relationship long observed by the natives. The Hawaiian Volcano Observatory was founded in 1912 by volcanologist Thomas Jaggar, initiating continuous scientific observation of the islands. In the 1970s, a mapping project was initiated to gain more information about the complex geology of Hawaii's seafloor.
The hotspot has since been tomographically imaged, showing it to be 500 to 600 km (310 to 370 mi) wide and up to 2,000 km (1,200 mi) deep, and olivine and garnet-based studies have shown its magma chamber is approximately 1,500 °C (2,730 °F). In its at least 85 million years of activity the hotspot has produced an estimated 750,000 km3 (180,000 cu mi) of rock. The chain's rate of drift has slowly increased over time, causing the amount of time each individual volcano is active to decrease, from 18 million years for the 76-million-year-old Detroit Seamount, to just under 900,000 for the one-million-year-old Kohala; on the other hand, eruptive volume has increased from 0.01 km3 (0.002 cu mi) per year to about 0.21 km3 (0.050 cu mi). Overall, this has caused a trend towards more active but quickly-silenced, closely spaced volcanoes—whereas volcanoes on the near side of the hotspot overlap each other (forming such superstructures as Hawaiʻi island and the ancient Maui Nui), the oldest of the Emperor seamounts are spaced as far as 200 km (120 mi) apart.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.Limalok
Limalok (formerly known as Harrie or Harriet) is a Cretaceous-Paleocene guyot/tablemount in the southeastern Marshall Islands, one of a number of seamounts (a type of underwater volcanic mountain) in the Pacific Ocean. It was probably formed by a volcanic hotspot in present-day French Polynesia. Limalok lies southeast of Mili Atoll and Knox Atoll, which rise above sea level, and is joined to each of them through a volcanic ridge. It is located at a depth of 1,255 metres (4,117 ft) and has a summit platform with an area of 636 square kilometres (246 sq mi).
Limalok is formed by basaltic rocks and was probably a shield volcano at first; the Macdonald, Rarotonga, Rurutu and Society hotspots may have been involved in its formation. After volcanic activity ceased, the volcano was eroded and thereby flattened, and a carbonate platform formed on it during the Paleocene and Eocene. These carbonates were chiefly produced by red algae, forming an atoll or atoll-like structure with reefs.
The platform sank below sea level 48 ± 2 million years ago during the Eocene, perhaps because it moved through the equatorial area, which was too hot or nutrient-rich to support the growth of a coral reef. Thermal subsidence lowered the drowned seamount to its present depth. After a hiatus lasting into the Miocene, sedimentation commenced on the seamount leading to the deposition of manganese crusts and pelagic sediments; phosphate accumulated in some sediments over time.List of seamounts in the Marshall Islands
The Marshall Islands are the site of a number of seamounts. These volcanoes form several groups, including the Ralik Chain, the Ratak Chain and some seamounts around Anewetak. These seamounts are in turn part of a larger province that extends from the South Pacific to the Mariana Trench and is characterized by unusually shallow ocean ground.These seamounts and volcanoes do not have simple hotspot-like age progressions, with some volcanoes being younger than one would expect from age progression and having more than one active episode. In some places, a middle Cretaceous and a late Cretaceous episode of volcanic activity have been determined by radiometric dating. Despite this, some hotspot-based genesis models have been formulated, often implying that French Polynesian hotspots are responsible for the formation of seamounts, with the Society hotspot, Rurutu hotspot, Rarotonga hotspot and the Macdonald hotspot being candidate hotspots responsible for the development of the Marshall Islands seamounts. Such linkages are in part supported by geochemical data. Some discrepancies between the age and position of such seamounts and the predictions of the hotspot model may reflect the activity of short-lived hotspots linked to large mantle plumes that produce more than one hotspot.Lo-En
Lo-En or Hess is an Albian–Campanian guyot in the Marshall Islands. One among a number of seamounts in the Pacific Ocean, it was probably formed by a hotspot in what is present-day French Polynesia. Lo-En lies southeast of Eniwetok which rises above sea level, and Lo-En is almost connected to it through a ridge.
The seamount is formed by basaltic rocks that probably formed a shield volcano first. It is believed that a number of hotspots such as the Macdonald hotspot, the Rarotonga hotspot and the Rurutu hotspot may have been involved in the formation of Lo-En. After volcanic activity, by the Turonian the seamount was submerged although it is possible that a carbonate platform formed during the Albian. After a hiatus, sedimentation commenced on the seamount in Oligocene time and led to the deposition of manganese crusts and pelagic sediments including limestone, some of which were later modified by phosphate.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.Macdonald seamount
Macdonald seamount (named after Gordon A. MacDonald) is a seamount in Polynesia, southeast of the Austral Islands and in the neighbourhood of a system of seamounts that include the Ngatemato seamounts and the Taukina seamount. It rises 4,200 metres (13,800 ft) from the seafloor to a depth of about 40 metres (130 ft) and has a flat top, but the height of its top appears to vary with volcanic activity. There are some subsidiary cones such as Macdocald seamount. The seamount was discovered in 1967 and has been periodically active with gas release and seismic activity since then. There is hydrothermal activity on Macdonald, and the vents are populated by 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.Musicians Seamounts
Musicians Seamounts are a chain of seamounts in the Pacific Ocean, north of the Hawaiian Ridge. There are about 65 seamounts, some of which are named after musicians. These seamounts exist in two chains, one of which has been attributed to a probably now-extinct hotspot called the Euterpe hotspot. Others may have formed in response to plate tectonics associated with the boundary between the Pacific Plate and the former Farallon Plate.
The seamounts were constructed on young oceanic crust during the Cretaceous, but a second phase of volcanic activity took place during the Eocene. Deep sea coral reefs occur on the seamounts.Outline of oceanography
The following outline is provided as an overview of and introduction to Oceanography.President Thiers Bank
President Thiers Bank is a broad guyot, which lies northwest of Rapa and 200 kilometres (120 mi) southeast of Raivavae, in the Austral Islands. Its summit reaches a depth of 33 metres (108 ft). It may have been created by the Macdonald hotspot. Another theory sees in the seamount the endpoint of an alignment that starts with Aitutaki and also involves one volcanic phase at Raivavae.Rarotonga hotspot
The Rarotonga hotspot is a volcanic hotspot in the southern Pacific Ocean. The hotspot was responsible for the formation of Rarotonga and some volcanics of Aitutaki.
In addition to these volcanoes in the Cook Islands, the composition of volcanic rocks in Samoa and in the Lau Basin may have been influenced by the Rarotonga hotspot, and some atolls and seamounts in the Marshall Islands may have formed on the hotspot as well.Rurutu
Rurutu is the northernmost island in the Austral archipelago of French Polynesia, and the name of a commune consisting solely of that island. It is situated 572 km (355 mi) south of Tahiti. Its land area is 32.7 km2 (12.6 sq mi). It is 10.8 km long and 5.3 km wide. Its highest point (Manureva) is 389 m (1,276 ft). At the 2017 census it had a population of 2,466.Geologically, Rurutu was initially formed 12 million years ago by the Macdonald hotspot, a hotspot associated with the Macdonald seamount. Over the next 10 million years, erosion shrank the island until it was almost an atoll. Then, just over a million years ago, Rurutu passed over the Arago hotspot, which lifted it roughly 150 meters. Steep sea cliffs of ancient coral lifted by the event — called makatea — now largely encircle the island. These are riddled with caves filled with concretions — indeed, Rurutu is largely unique among islands in French Polynesia in that its historic inhabitants were cave-dwelling.
Because it is endowed with a fringing reef, Rurutu has in recent years become known for whale watching: Humpback whales come and reproduce here between July and October within easy sighting distance from the beach.
Although its tiny community still subsists primarily on fishing and basic agriculture, tourism has been a growing industry, especially since François Mitterrand's visit in 1990. Whale watching season sees the bulk of tourists, but the largely untouched native culture, the white sand beaches, and the lush tropical flora draw small numbers of tourists year-round.Taukina seamounts
Taukina seamounts are a series of seamounts on the Pacific Plate. The Macdonald hotspot and the Ngatemato seamounts are located nearby. The Taukina and Ngatemato seamounts were discovered in 1996 by the RV Maurice Ewing and both are named after families in Rapa Iti.The Taukina seamounts are formed by small volcanoes, with heights of 1,500–1,000 metres (4,900–3,300 ft) and widths of 6–10 kilometres (3.7–6.2 mi). They often feature a caldera on their summit. Tholeiitic rocks make up the seamounts.The shape of the Taukina seamounts resembles that of the seamounts that form on the East Pacific Rise. An alternate theory of origin is that the Ngatemato seamounts deformed the Pacific plate enough with their weight to trigger the eruption of magma.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.