Intraplate magmatism plumes

A more careful look from our satellite at oceanic segments of plates reveals tens of thousands of small isolated volcanic seamounts (Batiza, 1982; Wessel and Lyons, 1997), but also, within the plates, large volcanoes, volcanic islands and even chains of volcanoes and volcanic islands. A classical example is the ~ 5000-km-long Hawaiian-Emperor chain on the Pacific plate. The Meiji seamount, at the extreme north-western end of this chain and about to descend into the Aleutian subduction zone, is 85 Myr old, and at its south-eastern end the Hawaiian islands have highly active present-day volcanism. Between these extremes, the intermediate ages correlate with the distance from Hawaii. If the track of this chain (from older to younger) is compared with the direction of motion of the Pacific plate (to the north-west), the first impression is that the plate has been persistently punctured by silicate magma as it moved over a stationary "plume" of hot material upwelling from the mantle. Even though seismological observations show that some plumes are not stationary, their displacement is much slower and the tracks are shorter than those typical of plate motion (e.g. Molnar and Stock, 1987; Steinberger and O'Connell, 1998).

Because their sizes, structures and compositions are variable, plumes are thought to originate at different depths in the mantle and from different source materials. Moreover, there is no strict definition of what constitutes a plume, and therefore no consensus on their total number; estimates vary from ~ 10 to ~ 100 (e.g. Malamud and Turcotte, 1999). The highest-temperature Hawaiian, Tahiti and Iceland plumes, for instance, are derived from great depth: their roots are detected well below ~ 600 km (Section 24.6). Their upwelling velocities reach ~ 300 km Myr-1 in relatively narrow jets, so they are able to cross the mantle in geologically short times ~ 10 Myr (Larsen et al., 1999). In contrast, the Azores plume relates to a shallow (possibly crustal) source. At present the total rate of magma generation by plumes is a factor ~ 10 less than the MOR rate (Crisp, 1984); however, plume materials bear important signatures of deep-mantle sources and their origin and evolution. Moreover, in earlier geological epochs plumes may have played a major role in both tectonics and magmatism, and have been powerful triggers for the rifting of plates and the break-up of large continents as well as possibly playing a role in the generation of the continental crust and subcontinental lithosphere.

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