The Macdonald hotspot (also known as "Tubuai" or "Old Rurutu") is a

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plat ...

hotspot in the southern

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the conti ...

. The hotspot was responsible for the formation of the

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 seamounts. It rises from t ...

, and possibly the Austral-

Cook Islands ) , image_map = Cook Islands on the globe (small islands magnified) (Polynesia centered).svg , capital = Avarua , coordinates = , largest_city = Avarua , official_languages = , lan ...

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 The Austral Islands (french: Îles Australes, officially ''Archipel des Australes;'' ty, Tuha'a Pae) are the southernmost group of islands in French Polynesia, an overseas country of the French Republic in the South Pacific. Geographically, t ...

and Cook Islands,

Tokelau Tokelau (; ; known previously as the Union Islands, and, until 1976, known officially as the Tokelau Islands) is a dependent territory of New Zealand in the southern Pacific Ocean. It consists of three tropical coral atolls: Atafu, Nukunonu, a ...

, the

Gilbert Islands The Gilbert Islands ( gil, Tungaru;Reilly Ridgell. ''Pacific Nations and Territories: The Islands of Micronesia, Melanesia, and Polynesia.'' 3rd. Ed. Honolulu: Bess Press, 1995. p. 95. formerly Kingsmill or King's-Mill IslandsVery often, this n ...

, the

Phoenix Islands The Phoenix Islands, or Rawaki, are a group of eight atolls and two submerged coral reefs that lie east of the Gilbert Islands and west of the Line Islands in the central Pacific Ocean, north of Samoa. They are part of the Republic of Kiri ...

and several of the

Marshall Islands The Marshall Islands ( mh, Ṃajeḷ), officially the Republic of the Marshall Islands ( mh, Aolepān Aorōkin Ṃajeḷ),'' () is an independent island country and microstate near the Equator in the Pacific Ocean, slightly west of the Inte ...

as well as several

seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise a ...

s in the Marshall Islands may have been formed by the Macdonald hotspot.

Geology

Regional geology

Hotspots have been explained either by

mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...

s producing magma in the crust, reactivation of old

lithospheric A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years or ...

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 Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...

, Bounty seamount at

Pitcairn The Pitcairn Islands (; Pitkern: '), officially the Pitcairn, Henderson, Ducie and Oeno Islands, is a group of four volcanic islands in the southern Pacific Ocean that form the sole British Overseas Territory in the Pacific Ocean. The four i ...

, Vailulu'u in

Samoa Samoa, officially the Independent State of Samoa; sm, Sāmoa, and until 1997 known as Western Samoa, is a Polynesian island country consisting of two main islands ( Savai'i and Upolu); two smaller, inhabited islands ( Manono and Apolima); ...

and Mehetia/

Teahitia Teahitia ("Standing Fire") is a submarine volcano, located northeast of the southeast tip of Tahiti of the Society Islands in the Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends ...

in the

Society Islands The Society Islands (french: Îles de la Société, officially ''Archipel de la Société;'' ty, Tōtaiete mā) are an archipelago located in the South Pacific Ocean. Politically, they are part of French Polynesia, an overseas country of the F ...

. 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 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 ...

Marquesas Islands The Marquesas Islands (; french: Îles Marquises or ' or '; Marquesan: ' (North Marquesan) and ' ( South Marquesan), both meaning "the land of men") are a group of volcanic islands in French Polynesia, an overseas collectivity of France in ...

and

Rarotonga Rarotonga is the largest and most populous of the Cook Islands. The island is volcanic, with an area of , and is home to almost 75% of the country's population, with 13,007 of a total population of 17,434. The Cook Islands' Parliament buildings a ...

. Beneath the Superswell, a region of

upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted ...

has been identified in the

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

, 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 depth to the surface. This has been explained by the presence of a "superplume", a very large mantle plume which also formed

oceanic plateau An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides. There are 184 oceanic plateaus in the world, covering an area of or about 5.11% of the ...

s during the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

, with present-day volcanism at the Society and Macdonald volcanoes originating from secondary plumes that rise from the superplume to the crust.

Local geology

The

and the

Cook Islands ) , image_map = Cook Islands on the globe (small islands magnified) (Polynesia centered).svg , capital = Avarua , coordinates = , largest_city = Avarua , official_languages = , lan ...

may have been formed by the Macdonald hotspot, as the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and I ...

was carried above the hotspot at a rate of . A 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 reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...

s 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

guyot In marine geology, a guyot (pronounced ), also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than below the surface of the sea. The diameters of these flat summits can exceed .Atiu Atiu, also known as Enuamanu (meaning ''land of the birds''), is an island of the Cook Islands archipelago, lying in the central-southern Pacific Ocean. Part of the Nga-pu-Toru, it is northeast of Rarotonga. The island's population has dropped b ...

and

Aitutaki Aitutaki, also traditionally known as Araura and Utataki, is the second most-populated island in the Cook Islands, after Rarotonga. It is an "almost atoll", with fifteen islets in a lagoon adjacent to the main island. Total land area is , and the ...

may be explained by the long-range effect of

'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 Rūrutu 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 south of Tahiti. Its land area is .Arago hotspot 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 ...

, and some rocks from

Tubuai Tubuai or Tupuai is the main island of the Austral Island group, located south of Tahiti. In addition to Tubuai, the group of islands include Rimatara, Rurutu, Raivavae, Rapa and the uninhabited Îles Maria. They are part of the Austral Isla ...

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 Foundation Seamounts are a series of seamounts in the southern Pacific Ocean. Discovered in 1992, these seamounts form a long chain which starts from the Pacific-Antarctic Ridge. Some of these seamounts may have once emerged from the ocean. T ...

. 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 Helium-3 (3He see also helion) is a light, stable isotope of helium with two protons and one neutron (the most common isotope, helium-4, having two protons and two neutrons in contrast). Other than protium (ordinary hydrogen), helium-3 is the ...

helium-4 Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and cons ...

has been used to infer a deep mantle origin of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

s of hotspot volcanoes.

Helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...

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 A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years ...

is possible.

Seismic tomography Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on ...

has depicted a

underneath the Macdonald hotspot.

Candidate edifices

Overall, the list of candidate volcanoes produced by the Macdonald hotspot is: *

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 seamounts. It rises from t ...

. * Rá seamount is located on the path of Macdonald but is too old to have been formed by this hotspot. *

Marotiri Marotiri is a group of four uninhabited volcanic rocks protruding from the sea (and several submerged rocks), forming the southeastern end of the Austral Islands of French Polynesia. Marotiri is also known as Bass Rocks (''Îlots de Bass'' in Fre ...

, Rapa, Raivavae,

and the older volcanics of

Rurutu Rūrutu 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 south of Tahiti. Its land area is .Austral Fracture Zone. Older ages at Marotiri may indicate a separate volcanic event, generated by the same source as Rá seamount. * ZEP2-7 seamount near Rurutu. * The Neilson Bank is on the path of Macdonald, but the only age is much older than predicted and of questionable accuracy. * ZEP2-19 seamount may be 8.8 million years old. *

Mangaia Mangaia (traditionally known as A'ua'u Enua, which means ''terraced'') is the most southerly of the Cook Islands and the second largest, after Rarotonga. It is a roughly circular island, with an area of , from Rarotonga. Originally heavily popula ...

. *

during the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but t ...

but with more recent volcanism as well. *

Rose Atoll Rose Atoll, sometimes called Rose Island or Motu O Manu ("Bird Island") by people of the nearby Manu'a Islands, is an oceanic atoll within the U.S. territory of American Samoa. An uninhabited wildlife refuge, it is the southernmost point belo ...

and Malulu seamount in

, if they are about 40 million years old. Moki, Dino and Malulu seamounts are more plausible products of the Macdonald hotspot, as Rose Atoll has been linked to the

instead, and Moki has an appropriate composition. Malulu and Papatua may have been formed either by the Macdonald hotspot or the Arago hotspot. *

, based on plate reconstructions and

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...

data. *

, although such a track would require a bend in the path of the hotspot. A bend exists in the Hawaii-Emperor seamount chain but there is no clear evidence of a bend in the Macdonald hotspot track. It is predicted to occur close to where the Samoa hotspot is today. *

, 43–66 million years ago. * The northern

were above the Macdonald hotspot 100–150 million years ago. Later some of these seamounts and atolls were influenced by the

Rurutu hotspot 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 th ...

, the Society hotspot and the

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 ...

leading to a complex history of volcanism and uplift. ** The

guyot In marine geology, a guyot (pronounced ), also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than below the surface of the sea. The diameters of these flat summits can exceed .Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

. ** The northern

Ralik Chain The Ralik Chain ( Marshallese: , ) is a chain of islands within the island nation of the Marshall Islands. Ralik means "sunset". It is west of the Ratak Chain. In 1999 the total population of the Ralik islands was 19,915. Christopher Loeak, who b ...

may also have been formed by the Macdonald hotspot, although uncertainties in plate motions from before about 90 million years ago make any such reconstruction uncertain. **

Erikub Atoll Erikub Atoll ( Marshallese: , ) is an uninhabited coral atoll of fourteen islands in the Pacific Ocean, located in the Ratak Chain of the Marshall Islands. Its total land area is only , but it encloses a lagoon with an area of . It is located sli ...

, although

passed even closer to Erikub. ** Late

volcanism of Lokkworkwor and Lomjenaelik seamounts. **

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ag ...

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0 ...

volcanism at Lobbadede and Lewa guyots, followed by renewed activity at Lobbadede 82.4 million years ago probably linked to the

. ** Lo-En seamount during the

. **

volcanism at

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 so ...

and Ruwitūntūn seamounts. Later these seamounts were further affected by the

, at the same time as volcanism occurred at

Bikini A bikini is a two-piece swimsuit primarily worn by women that features two triangles of fabric on top that cover the breasts, and two triangles of fabric on the bottom: the front covering the pelvis but exposing the navel, and the back coverin ...

and

Rongelap Rongelap Atoll ( Marshallese: , ) is a coral atoll of 61 islands (or motus) in the Pacific Ocean, and forms a legislative district of the Ralik Chain of the Marshall Islands. Its total land area is . It encloses a lagoon with an area of . ...

References

Sources

* * * * * * * * * * * * * * * * * * {{Hotspots Hotspots of the Pacific Ocean Geology of the Pacific Ocean Geography of the Cook Islands Geography of the Austral Islands