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The Macdonald hotspot (also known as "Tubuai" or "Old Rurutu") 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.
Geology
Regional geology 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 made it initially 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. Later work after the establishment of a local seismic network allowed P wave studies confirming that the South Pacific superswell is linked to the Macdonald hotspot at top of its superplume at 400–500 km (250–310 mi) depth. The association may also 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 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. Seismic tomography has depicted a mantle plume 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:
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