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.

Geology

Oceanic plateaus and linear volcanic chains dot the floor of the Pacific Ocean. Their formation has been explained with mantle plumes which rise from the core-mantle boundary and spread out when they rise, forming a large "head" that causes intense volcanic activity once it hits the crust. This volcanism is responsible for the formation of the oceanic plateaus. Later, the remnant "tail" of the plume is still rising and induces the formation of volcano chains as the crust moves over the plume tail, thus forming the linear chains.

A number of hotspots are or were active in the Pacific Ocean and some of these may be the product of mantle plumes. Other hotspots such as Rarotonga appear to have been active only for short time periods; many of these are located in French Polynesia where there is a superswell. Such hotspot volcanism may be the product of shallow processes. Later research has suggested however that the Macdonald hotspot, the Rarotonga hotspot and the Rurutu hotspot are long lived hotspots that were active as far back as the Cretaceous; they may be over 100 million years old and in such case the oldest still active hotspots in the Pacific. These hotspots together may have built the Cook-Austral Islands together, resulting in overlapping ages of the volcanoes.

Seismic tomography has found slow velocity anomalies underneath the Rarotonga hotspot, down to depths of about with more recent research indicating that they root at about depth. The anomaly lies at over depth with no evidence of shallower anomalies, however. The Rarotonga hotspot and other regional hotspots appear to be anchored to a deep mantle structure called a large low shear-wave velocity province.

Products

The Rarotonga hotspot is reliably linked only to the formation of Rarotonga and to volcanism on Aitutaki, potential volcanic structures between the Tonga Trench and Rarotonga that may have been formed by the same hotspot are poorly studied. Rarotonga itself is young but there is little indication of volcanism either southeast or northwest from it and no evidence of its current position.

Other candidate volcanoes/structures formed by the Rarotonga hotspot or influenced by it are:
* Rarotonga.
* The young volcanics of Aitutaki. An origin of the young volcanics as rejuvenated volcanism cannot be ruled out, however.
* Rose Atoll and Malulu Seamount may have been formed by the Rarotonga hotspot, but other hotspots are also candidates. The connection to Rarotonga is supported by geochemical traits.
* Uo Mamae seamount in Samoa share geochemical traits with the Rarotonga hotspot and plate motion reconstructions indicate that the hotspot track passed through it. Potentially, the hotspot formed Uo Mamae and local tectonic processes later (940,000 years ago) triggered rejuvenated volcanism.
* The composition of rejuvenated volcanism in Samoa may bear traces of the influence of the Rarotonga hotspot, which passed across Samoa in the past.
* Reconstructions of the path of the Rarotonga hotspot imply that part of its output was subducted into the Tonga Trench; back-arc magmas may thus ended up entraining material formerly produced by the Rarotonga hotspot. Backarc volcanic rocks in the Lau Basin bear traces of such influence.
* The Marshall Islands underwent vigorous volcanic and geological activity while they passed over the Rarotonga hotspot and neighbouring hotspots.
** Geochemical traits and plate reconstruction links the Ralik Chain to the Rarotonga hotspot less than 80 million years ago.
** Limalok guyot was close to the Rarotonga and Rurutu hotspots 62 million years ago. The plate reconstructions point towards Rurutu being the origin of Limalok, while geochemical traits match Rarotonga best.
** Lo-En guyot volcanic activity
** Eniwetok was located close to the Rarotonga hotspot about 76.9 million years ago; this date corresponds to the a radiometric age obtained on the upper volcano.
** A cluster of volcanoes close to Eniwetok and Ujlan may be the product of the Rarotonga hotspot.
** Volcanic activity at Wōdejebato coincides with a period where the Rarotonga hotspot, the Rurutu hotspot and the Tahiti hotspot were all three located close to the seamount.
* Geochemical traits and plate reconstruction links the Magellan Seamounts to the Rarotonga hotspot less than 80 million years ago.
* The Western Pacific Seamount Province has been argued to be the Cretaceous path of the Rarotonga hotspot, but its older members appear to be offset slightly north of the reconstructed path. Some seamounts on the reconstructed path of the Rarotonga hotspot share geochemical traits with the hotspot, but with different lead isotope ratios.
* Hemler Guyot has similar isotope ratios as Rarotonga and its reconstructed position match those of the Rarotonga hotspot.

References

Sources

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{{Hotspots
Geology of the Pacific Ocean
Hotspots of the Pacific Ocean
Rarotonga