The plate theory is a model of volcanism that attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics. According to the plate theory, the principal cause of volcanism is extension of the lithosphere. Extension of the lithosphere is a function of the lithospheric stress field. The global distribution of volcanic activity at a given time reflects the contemporaneous lithospheric stress field, and changes in the spatial and temporal distribution of volcanoes reflect changes in the stress field. The main factors governing the evolution of the stress field are: Changes in the configuration of plate boundaries. Vertical motions. Thermal contraction.

Lithospheric extension enables pre-existing melt in the crust and mantle to escape to the surface. If extension is severe and thins the lithosphere to the extent that the asthenosphere rises, then additional melt is produced by decompression upwelling.

Origins of the plate theory

Developed during the late 1960s and 1970s, plate tectonics provided an elegant explanation for most of the Earth's volcanic activity. At spreading boundaries where plates move apart, the asthenosphere decompresses and melts to form new oceanic crust. At subduction zones, slabs of oceanic crust sink into the mantle, dehydrate, and release volatiles which lower the melting temperature and give rise to volcanic arcs and back-arc extensions. Several volcanic provinces, however, do not fit into this simple picture, and have traditionally been considered exceptional cases which require a non-plate-tectonic explanation.

Just prior to the development of plate tectonics in the early 1960s, the Canadian geophysicist John Tuzo Wilson suggested that chains of volcanic islands form from movement of the seafloor over relatively stationary hotspots in stable centres of mantle convection cells. In the early 1970s, Wilson's idea was revived by the American geophysicist W. Jason Morgan. In order to account for the long-lived supply of magma that some volcanic regions seemed to require, Morgan modified the hypothesis, shifting the source to a thermal boundary layer. Because of the perceived fixity of some volcanic sources relative to the plates, he proposed that this thermal boundary was deeper than the convecting upper mantle on which the plates ride and located it at the core-mantle boundary, 3,000 km beneath the surface. He suggested that narrow convection currents rise from fixed points at this thermal boundary and form conduits, which transport abnormally hot material to the surface. This, the mantle plume theory, became the dominant explanation for apparent volcanic anomalies for the remainder of the 20th century.

Testing the plume hypothesis, however, is beset with difficulties. A central tenet of the plume theory is that the source of melt is significantly hotter than the surrounding mantle, so the most direct test is to measure the source temperature of magmas. This is difficult, as the petrogenesis of magmas is extremely complex, rendering inferences from petrology or geochemistry to source temperatures unreliable. Seismic data used to provide additional constraints on source temperatures are highly ambiguous. In addition to this, predictions of the plume theory have proved unsuccessful at many locations purported to be underlain by mantle plumes, and there are also significant theoretical reasons to doubt the hypothesis.

The foregoing issues have inspired a growing number of geoscientists, led by American geophysicist Don L. Anderson and British geophysicist Gillian R. Foulger, to pursue other explanations for volcanic activity not easily accounted for by plate tectonics. Rather than introducing another exterior theory, these explanations essentially expand the scope of plate tectonics in ways that can accommodate volcanic activity previously thought to be outside its remit. The key modification to the basic plate-tectonic model here is a relaxation of the assumption that plates are rigid. This implies that lithospheric extension occurs not only at spreading plate boundaries but throughout plate interiors, a phenomenon that is well supported both theoretically and empirically.

Over the last two decades, the plate theory has developed into a cohesive research programme, attracting many adherents, and occupying researchers in several subdisciplines of Earth science. It has also been the focus of several international conferences and many peer-reviewed papers and is the subject of two major Geological Society of America edited volumes and a textbook.

Lithospheric extension

Global-scale lithospheric extension is a necessary consequence of the non-closure of plate motion circuits, and is equivalent to an additional slow-spreading boundary. Extension results principally from the following three processes: Changes in the configuration of plate boundaries, which can result from various processes including the formation or annihilation of plates and boundaries and slab rollback (vertical sinking of subducting slabs causing oceanward migration of trenches). Vertical motions resulting from delamination of the lower crust and mantle lithosphere and isostatic adjustment following erosion, orogeny, or melting of ice caps. Thermal contraction, which sums to the largest amount across large plates such as the Pacific.

Extension resulting from these processes manifests in a variety of structures including continental rift zones (e.g., the East African Rift), diffuse oceanic plate boundaries (e.g., Iceland), continental back-arc extensional regions (e.g., the Basin and Range Province in the Western United States), oceanic back-arc basins (e.g., the Manus Basin in the Bismarck Sea off Papua New Guinea), fore-arc regions (e.g., the western Pacific), and continental regions undergoing lithospheric delamination (e.g., New Zealand).

Continental breakup begins with rifting. When extension is persistent and entirely compensated by magma from asthenospheric upwelling, oceanic crust is formed, and the rift becomes a spreading plate boundary. If extension is isolated and ephemeral, it is classified as intraplate. Rifting can occur in both oceanic and continental crust, and ranges from minor to amounts approaching those seen at spreading boundaries. All forms can give rise to magmatism.

Various extensional styles are seen in the northeast Atlantic. Continental rifting began in the late Paleozoic, and was followed by catastrophic destabilisation in the late Cretaceous and early Paleocene. The latter was possibly caused by rollback of the Alpine slab, which generated extension throughout Europe. More severe rifting occurred along the Caledonian Suture, a zone of pre-existing weakness where the Iapetus Ocean closed around 420 Ma. As extension became localised, oceanic crust began to form around 54 Ma, with diffuse extension persisting around Iceland.

Some intracontinental rifts are essentially failed continental breakup axes, and some of these form triple junctions with plate boundaries. The East African Rift, for example, forms a triple junction with the Red Sea and the Gulf of Aden, both of which have progressed to the seafloor spreading stage. Likewise, the Mid-American Rift constitutes two arms of a triple junction along with a third, which separated the Amazonian Craton from Laurentia around 1.1 Ga.

Diverse volcanic activity resulting from lithospheric extension has occurred throughout the western United States. The Cascade Volcanoes are a back-arc volcanic chain extending from British Columbia to Northern California. Back-arc extension continues to the east in the Basin and Range Province, with small-scale volcanism distributed throughout the region.

The Pacific Plate is the largest tectonic plate on Earth, covering about one third of Earth's surface. It undergoes considerable extension and shear deformation due to thermal contraction of the lithosphere. Shear deformation is greatest in the area between Samoa and the Easter Microplate, an area replete with volcanic provinces such as the Cook-Austral chain, the Marquesas and Society Islands, the Tuamotu Archipelago, the Fuca and Pukapuka ridges and Pitcairn Island.

Magma source

The volume of magma that is intruded and/or erupted in a given area of lithospheric extension depends on two variables: the availability of pre-existing melt in the crust and mantle, and the amount of additional melt supplied by decompression upwelling. The latter depends on three factors: lithospheric thickness, the amount of extension, and the fusibility and temperature of the source.

There is abundant pre-existing melt throughout both the crust and the mantle. In the crust, melt is stored under active volcanoes in shallow reservoirs which are fed by deeper ones. In the asthenosphere, a small amount of partial melt is thought to provide a weak layer that acts as lubrication for the movement of tectonic plates. The presence of pre-existing melt means that magmatism can occur even in areas where lithospheric extension is modest, such as the Cameroon and Pitcairn-Gambier volcanic lines.

The rate of magma formation from decompression of the asthenosphere depends on how high the asthenosphere can rise, which in turn depends on the thickness of the lithosphere. From numerical modelling, it is evident that the formation of melt in the largest flood basalts cannot be concurrent with its emplacement. This means that melt is formed over a longer period, stored in reservoirs, most likely located at the lithosphere-asthenosphere boundary, and then released by lithospheric extension. That large volumes of magma are stored at the base of the lithosphere is shown by observations of large magmatic provinces such as the Great Dyke in Zimbabwe and the Bushveld Igneous Complex in South Africa. There, thick lithosphere remained intact during large-volume magmatism, so decompression upwelling on the scale required for that activity can be ruled out, implying that large volumes of magma must have pre-existed.

If extension is severe and results in significant thinning of the lithosphere, the asthenosphere can rise to shallow depths, inducing decompression melting and producing larger volumes of melt. At mid-ocean ridges, where the lithosphere is thin, decompression upwelling produces a modest rate of magmatism. The same process can also produce small-volume magmatism on or near slowly extending continental rifts. Beneath continents, the lithosphere is up to 200 km thick. If lithosphere this thick undergoes severe and persistent extension, it can rupture, and the asthenosphere can upwell to the surface, producing tens of millions of cubic kilometres of melt along axes hundreds of kilometres long. This occurred, for example, during the opening of the North Atlantic Ocean, when the asthenosphere rose from base of the Pangaean lithosphere to the surface.

Examples

The vast majority of volcanic provinces which are thought to be anomalous in the context of rigid plate tectonics can be explained using the plate theory. The type examples of this kind of volcanic activity are Iceland, Yellowstone, and Hawaii. Iceland is the type example of a volcanic anomaly situated on a plate boundary. Yellowstone, together with the Eastern Snake River Plain to its west, is the type example of an intra-continental volcanic anomaly. Hawaii, along with the related Hawaiian-Emperor seamount chain, is the type example of an intra-oceanic volcanic anomaly.

Iceland

Iceland is a 1 km high, 450x300 km basaltic shield on the mid-ocean ridge in the northeast Atlantic Ocean. It comprises over 100 active or extinct volcanoes, and has been extensively studied by earth scientists for several decades.