Plate theory (volcanism)

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:

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