Macdonald Hotspot
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The Macdonald hotspot (also known as "Tubuai" or "Old Rurutu") is a
volcanic A volcano is commonly defined as a vent or fissure 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 fo ...
hotspot in the southern
Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...
. The hotspot was responsible for the formation of the Macdonald Seamount, and possibly the Austral-
Cook Islands The Cook Islands is an island country in Polynesia, part of Oceania in the South Pacific Ocean. It consists of 15 islands whose total land area is approximately . The Cook Islands' Exclusive Economic Zone (EEZ) covers of ocean. Avarua is its ...
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 ( officially ''Archipel des Australes;'' ) are the southernmost group of islands in French Polynesia, an overseas country of France, overseas country of the France, French Republic in the Oceania, South Pacific. Geographicall ...
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, an ...
, the
Gilbert Islands The Gilbert Islands (;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 name applied o ...
, 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 Kiribati, Republic ...
and several of the
Marshall Islands The Marshall Islands, officially the Republic of the Marshall Islands, is an island country west of the International Date Line and north of the equator in the Micronesia region of the Northwestern Pacific Ocean. The territory consists of 29 c ...
as well as several
seamount A seamount is a large submarine landform that rises from the ocean floor without reaching the water surface (sea level), and thus is not an island, islet, or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly 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 ho ...
s 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 Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...
, Bounty seamount at Pitcairn, Vailulu'u in
Samoa Samoa, officially the Independent State of Samoa and known until 1997 as Western Samoa, is an island country in Polynesia, part of Oceania, in the South Pacific Ocean. It consists of two main islands (Savai'i and Upolu), two smaller, inhabited ...
and Mehetia/ Teahitia in the
Society Islands The Society Islands ( , officially ; ) are an archipelago in the South Pacific Ocean that includes the major islands of Tahiti, Mo'orea, Moorea, Raiatea, Bora Bora and Huahine. Politically, they are part of French Polynesia, an overseas country ...
. 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 The Marquesas Islands ( ; or ' or ' ; Marquesan language, Marquesan: ' (North Marquesan language, North Marquesan) and ' (South Marquesan language, South Marquesan), both meaning "the land of men") are a group of volcano, volcanic islands 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 10,898 of a total population of 15,040. The Parliament of the Cook Islands, Coo ...
. 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 sur ...
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 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 o ...
s during the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
, with present-day volcanism at the Society and Macdonald volcanoes originating from secondary plumes that rise from the superplume to the crust. The association may explain the Hotspot highway of the South Pacific Ocean first described in 2010. An ultra-low velocity zone under Pitcairn extends to the Easter hotspot and the Macdonald hotspot.


Local geology

The
Austral Islands The Austral Islands ( officially ''Archipel des Australes;'' ) are the southernmost group of islands in French Polynesia, an overseas country of France, overseas country of the France, French Republic in the Oceania, South Pacific. Geographicall ...
and the
Cook Islands The Cook Islands is an island country in Polynesia, part of Oceania in the South Pacific Ocean. It consists of 15 islands whose total land area is approximately . The Cook Islands' Exclusive Economic Zone (EEZ) covers of ocean. Avarua is its ...
may have been formed by the Macdonald hotspot, as the Pacific plate 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 colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in group ...
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 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 Ātiu, also known as ʻEnuamanu (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 population of the island has ...
and Aitutaki may be explained by the long-range effect of
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 10,898 of a total population of 15,040. The Parliament of the Cook Islands, Coo ...
'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 Helium-3 (3He see also helion) is a light, stable isotope of helium with two protons and one neutron. (In contrast, the most common isotope, helium-4, has two protons and two neutrons.) Helium-3 and hydrogen-1 are the only stable nuclides with ...
to
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 consi ...
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 (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
s of hotspot volcanoes.
Helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
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 lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...
is possible. Seismic tomography has depicted a
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 ho ...
underneath the Macdonald hotspot.


Candidate edifices

The hotspot may have been active for 70 million years (exceeded by the Arago hotspot), possibly forming volcanoes like: * Macdonald seamount. * Rá seamount is located on the path of Macdonald but is too old to have been formed by this hotspot. * Marotiri, Rapa, Raivavae, Tubuai and the older volcanics of Rurutu and of the Arago seamount, and the correlation is in part supported by isotope data, although a change in isotopic composition between Raivavae and Rapa appears to have occurred, possibly as a consequence of the hotspot crossing the 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. *
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 10,898 of a total population of 15,040. The Parliament of the Cook Islands, Coo ...
during the
Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
but with more recent volcanism as well. * Rose Atoll and Malulu seamount in
Samoa Samoa, officially the Independent State of Samoa and known until 1997 as Western Samoa, is an island country in Polynesia, part of Oceania, in the South Pacific Ocean. It consists of two main islands (Savai'i and Upolu), two smaller, inhabited ...
, 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 Arago hotspot instead, and Moki has an appropriate composition. Malulu and Papatua may have been formed either by the Macdonald hotspot or the Arago hotspot. *
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, an ...
, based on plate reconstructions and
isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
data. *
Gilbert Islands The Gilbert Islands (;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 name applied o ...
, although such a track would require a bend in the path of the hotspot. A bend exists in the Hawaii-Emperor seamount chain but whether one exists at Macdonald is unclear. It is predicted to occur close to where the Samoa hotspot is today. *
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 Kiribati, Republic ...
, 43–66 million years ago. * The northern
Marshall Islands The Marshall Islands, officially the Republic of the Marshall Islands, is an island country west of the International Date Line and north of the equator in the Micronesia region of the Northwestern Pacific Ocean. The territory consists of 29 c ...
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 (also known as the Rurutu hotspot, Young Rurutu hotspot or Atiu hotspot) is a Hotspot (geology), hotspot in the Pacific Ocean, presently located below the Arago seamount close to the island of Rurutu, French Polynesia. Arago is pa ...
, the Society hotspot and the
Rarotonga hotspot The Rarotonga hotspot is a volcanic hotspot in the southern Pacific Ocean. The hotspot is claimed to be responsible for the formation of Rarotonga and some volcanics of Aitutaki but an alternative explanation for these islands most recent volca ...
leading to a complex history of volcanism and uplift. ** The guyot Aean-Kan during the mid-
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
. ** The northern Ralik Chain 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, although Arago hotspot passed even closer to Erikub. ** Late
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
volcanism of Lokkworkwor and Lomjenaelik seamounts. **
Aptian The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...
-
Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...
volcanism at Lobbadede and Lewa guyots, followed by renewed activity at Lobbadede 82.4 million years ago probably linked to the
Rurutu hotspot Arago hotspot (also known as the Rurutu hotspot, Young Rurutu hotspot or Atiu hotspot) is a Hotspot (geology), hotspot in the Pacific Ocean, presently located below the Arago seamount close to the island of Rurutu, French Polynesia. Arago is pa ...
. **
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 (geology), hotspot in what is present-day French Polynesia. Lo-En lies southeast of ...
seamount during the
Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...
. **
Aptian The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...
-
Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...
volcanism at Wōdejebato and Ruwitūntūn seamounts. Later these seamounts were further affected by the
Rurutu hotspot Arago hotspot (also known as the Rurutu hotspot, Young Rurutu hotspot or Atiu hotspot) is a Hotspot (geology), hotspot in the Pacific Ocean, presently located below the Arago seamount close to the island of Rurutu, French Polynesia. Arago is pa ...
, at the same time as volcanism occurred at
Bikini A bikini is a two-piece swimsuit primarily worn by women that features one piece on top that covers the breasts, and a second piece on the bottom: the front covering the pelvis but usually exposing the navel, and the back generally covering ...
and
Rongelap Rongelap Atoll ( ; , ) is an uninhabited coral atoll of 61 islands (or motu (geography), 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 ...
. * The Magellan Seamounts may have been influenced by magmas and melts ascending from the Macdonald and other hotspots of the South Pacific. In particular, Ita Mai Tai has compositions resembling the Rarotonga and (to a lesser degree) Macdonald hotspot.


See also

* Arago hotspot


References


Sources

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