The Society hotspot is a volcanic hotspot in the south

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 ...

which is responsible for the formation of 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 ...

, an archipelago of fourteen volcanic islands and

atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop. Most ...

s spanning around of the ocean which formed between 4.5 and <1 Ma.

Formation

There are currently two main hypotheses concerning the cause of volcanic activity. The conventional view is that the hotspot is underlain by 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 ...

which has transported hot material from the lower mantle to the surface, creating the chain as the Pacific Plate has moved northwest over the plume. Several lines of evidence support this interpretation. Age progression along the chain is consistent with estimates of the velocity of plate motion.

Seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...

anomalies have been observed in the upper mantle and found to extend into the uppermost lower mantle, implying that the passage of hot material from the lower to upper mantle is not hindered by the transition zone. Magnetotelluric imaging has found higher conductivity in the upper mantle under the active area southeast of

Tahiti Tahiti (; Tahitian language, Tahitian , ; ) is the largest island of the Windward Islands (Society Islands), Windward group of the Society Islands in French Polynesia, an overseas collectivity of France. It is located in the central part of t ...

consistent with anomalously hot rising material. There are two competing versions of the mantle plume model. One version posits a narrow, discreet plume feeding only the Society hotspot. The other proposes a superplume with narrow conduits supplying several hotspots in the south Pacific. Evidence for the former model includes magnetotelluric imaging which finds conductivity anomalies of less than in radius indicating a plume of limited extent and seismic imaging of the transition zone under the Society hotspot which shows a thinned area of less than implying that the thermal flux from lower to upper mantle is on the scale of a plume rather than a superplume. Evidence for the latter model includes seismic imaging of the lower mantle which reveals a large-scale low-velocity anomaly from the base of the mantle to around depth, small-scale anomalies in the upper mantle which may be narrow plumes generated by the superplume and intermittent volcanic activity in south Pacific hotspots which contrasts with the persistent volcanism expected for individual plumes. Clouard and Bonneville 2001 have argued that certain features of the Society hotspot, such as the lack of an initial flood

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

at the old end of the chain, short-lived volcanic activity, and petrological and geochemical analysis of the lavas which reveals a number of shallow-source components, are inconsistent with the plume model and have proposed a tectonic origin. According to this model, the Society and other volcanic chains in the south Pacific result from a system of fissures caused by intraplate stresses related to thermal contraction 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 ...

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

-induced flow of the

asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between c. below the surface, and extends as deep as . However, the lower boundary of the asthenosphere i ...

, and changes in the configuration of plate boundaries which have enabled pre-existing melt in the crust and shallow mantle to escape to the surface. The timing of volcanic activity and orientation of the chain, both of which coincide closely with major alterations in plate boundary configurations and consequent changes in the lithospheric stress field and direction of asthenospheric counterflow, support this model. Some of the above features, however, can be accommodated by the plume model. The lack of initial flood basalt and short-lived activity, for example, are consistent with some versions of the superplume model which propose small-scale intermittent “plumelets” generated by the superplume, and the petrology and geochemistry of the lavas may be due to subducted

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

being sampled by the plume.

References

Volcanology {{Hotspots

Formation

See also

References