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.
The hotspot is named after the Arago seamount 130 kilometres (81 mi) southeast of Rurutu. The seamount is named after the French Navy ship Arago, which discovered the seamount in 1993. The ship itself is named after astronomer François Arago. Polynesians knew of the existence of the shallow (27 metres (89 ft) beneath sea level) seamount and named it Tinomana. Before the link to Arago seamount was discovered, the hotspot was also known as "Rurutu hotspot", a name sometimes still used, which is a name also used for an older volcanic chain that starts at Raivavae or President Thiers Bank. Other names are "Young Rurutu" and "Atiu trend".
|Location in the southern Pacific Ocean|
The southern Pacific Ocean is the site of the South Pacific Superswell, an area where the ocean is anomalously shallow (by about 700 metres (2,300 ft)) and which covers an area of about 3,000 by 3,000 kilometres (1,900 mi × 1,900 mi). Underneath this superswell a large mantle plume might give rise to secondary plumes which in turn form the surface hotspots. Hotspots in the region are the Macdonald hotspot, Marquesas hotspot, Pitcairn hotspot and Society hotspot; of which the first and the last appear to be rooted deep in the mantle. The nature of the volcanism in the area is not completely understood.
Arago Seamount is part of the volcanic chain that forms the Austral Islands and Cook Islands. The 2,200 kilometres (1,400 mi) long chain consists of two separate trends that form two atolls and eleven islands; of these systems one (Macdonald seamount) is a still active volcano. The ages of these islands follow an approximate age progression typical of a hotspot volcano but the occurrence of younger ages on Aitutaki and Rurutu and the chemistry of these younger rocks indicated that there must be more than one hotspot involved. Recent models envisage the presence of a number of separate hotspot tracks in what has been dubbed a "hotspot highway". Further, some hotspots such as the Hawaii hotspot show evidence of movement but the Arago hotspot appears to be static.
The Arago and other hotspots probably are not deep mantle plumes but rather more shallow structures that are also influenced by the lithosphere; in the case of the Arago hotspot the absence of an oceanic plateau that could have been formed by the head of the mantle plume supports such a shallow origin. The upper mantle might be the source of the Arago hotspot. Data on the presence of seismic velocity anomalies and whether they are positive (higher) or negative (lower) beneath Arago are contradictory. Seismic imaging published in 2009 indicates only a slight seismic velocity anomaly shallower than 100 kilometres (62 mi), with no indication of a deep mantle root. More recent research however has endorsed a deep mantle origin for the Arago hotspot. Presently, Arago and the Macdonald hotspot are the two active hotspots of the Austral Islands, but a hotspot that formed Rarotonga may also still be active; additional hotspots in the area are Tubuai, Taukina and Ngatemato.
The eponymous Arago Seamount is a composite volcano with three rift zones, similar to Rurutu. The seamount was formed by three volcanoes with one overlapping the other two; potassium-argon dating on Arago has yielded ages of 230,000 ± 4,000 before present and an imprecise age of 0 years before present. There is some evidence of submarine landslide activity, a typical occurrence on ocean volcanoes, with one landslide scar each on the northern, eastern and western flank. This seamount is considered to be the present location of the hotspot, given its young age; however, unlike Macdonald, Arago Seamount has no recorded historical eruptions.
Hotspots other than the Arago hotspot may have contributed to the growth of the Arago Seamount; a hotspot associated with Raivavae and potentially the President Thiers Bank has been associated through isotope analysis with 8.2 million year old samples taken from Arago Seamount. Other volcanoes in the region also show evidence that they were built by more than one hotspot; this might indicate that their formation is controlled by lithospheric features.
As the Pacific Plate drifted over the hotspot several volcanoes were formed on the hotspot where weaknesses in the crust allowed the penetration of magma, and were subsequently carried away, at a rate of about 120 millimetres per year (4.7 in/year). Isotope ratios of lead in the volcanic rocks tie the younger volcanics of Rurutu to the Arago hotspot, the ratio in this case is characterized by high radiogenic lead isotope composition ("HIMU"). Some volcanic material from the Arago hotspot may have been recycled in the mantle and mixed into the magmas erupted in the northeastern Lau basin; rocks shed from seamounts created by the Arago hotspot may have been subducted in the Tonga trench which is close to the reconstructed path of the Arago hotspot and then erupted onto the Lau basin. HIMU xenoliths have been found in Tubuai just ahead of Arago Seamount as well.
Rurutu already existed before the interaction with the Arago hotspot, having been formed by an older volcanic episode; when it moved over the Arago hotspot a volcanic episode occurred and emplaced lava flows that are formed by basanite and hawaiite. Also, the island and surrounding coral reef were uplifted, and these uplifted coral reefs (known as makatea) caught the attention of early geologists, who were speculating as to what might have lifted the reefs out of the sea already in 1840. Other uplifted atolls occur northwest from Rurutu and may have formed in the same way when they passed over the Arago hotspot.
The following volcanics are at least tentatively attributed to the Arago hotspot:
The oldest volcanic structures potentially formed by the Arago hotspot are 120 million years old. If their attribution is correct, the Arago hotspot may be the oldest still active hotspot in the Pacific Ocean, ahead of the Hawaii hotspot and the Louisville hotspot. A contrasting viewpoint believes that Arago is a short-lived hotspot with few dated volcanoes along its predicted path.
The island of Tubuai is located ahead of the hotspot, and the island will be transported over it in a few million years. As with Rurutu, this interaction will lead to uplift in Tubuai and possibly to renewed volcanism.
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).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.Louisville hotspot
The Louisville hotspot is a volcanic hotspot responsible for the volcanic activity that has formed the Louisville Ridge in the southern Pacific Ocean.Macdonald hotspot
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.Marquesas hotspot
The Marquesas hotspot is a volcanic hotspot in the central Pacific Ocean. It is responsible for the Marquesas Islands, a group of 12 volcanic islands and one of the five archipelagos of French Polynesia.Pitcairn hotspot
The Pitcairn hotspot is a volcanic hotspot located in the south-central Pacific Ocean. Over the past 11 million years, it has formed the Pitcairn-Gambier hotspot chain. It is responsible for creating the Pitcairn Islands and two large seamounts named Adams and Bounty, as well as atolls at Moruroa, Fangataufa and the Gambier Islands. The hotspot is currently located at Adams and Bounty, which are ~60 kilometers East-Southeast of Pitcairn Island.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.Samoa hotspot
The Samoa hotspot is a volcanic hotspot located in the south Pacific Ocean.
The hotspot model describes a hot upwelling plume of magma through the Earth's crust as an explanation of how volcanic islands are formed. The hotspot idea came from J. Tuzo Wilson in 1963 based on the Hawaii volcanic island chain.
In theory, the Samoa hotspot is based on the Pacific Tectonic Plate traveling over a fixed hotspot located deep underneath the Samoan Islands.
The Samoa hotspot includes the Samoan Islands (American Samoa and Samoa), and extends to the islands of Uvea or Wallis Island (Wallis and Futuna) and Niulakita (Tuvalu), as well as the submerged Pasco banks and Alexa Bank.As the Pacific Plate moves slowly over the hotspot, thermal activity builds up and is released in magma plume spewing through the Earth's crust, forming each island in a chain. The Samoa islands generally lie in a straight line, east to west, in the same direction of the tectonic plate 'drifting' over the hotspot.
A characteristic of a “classic” hotspot, like the Hawaii hotspot, results in islands located further from the hotspot being progressively older with newer and younger islands closest to the fixed hotspot, like the Loihi Seamount, the only submarine volcano which has been studied in detail by scientists. The scientific research from Loihi has resulted in a 'Hawaii' model for hotspots primarily limited to the information gathered from the Hawaii islands.However, the Samoa hotspot is currently an enigma for scientists. In the Samoa Islands, the easternmost island of Ta'u and the westernmost island of Savai'i have both erupted in the past 150 years. The most recent eruption on Sava'i occurred with Mount Matavanu (1905–1911) and on Ta'u in 1866.Society hotspot
The Society hotspot is a volcanic hotspot located in the Pacific Ocean, and is responsible for the creation of the Society Islands.