Plate theory (volcanism)
The main factors governing the evolution of the stress field are: Lithospheric extension enables pre-existing melt in the crust and mantle to escape to the surface.
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 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 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.
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 is difficult as the petrogenesis of magmas is extremely complex, rendering inferences from petrology or geochemistry to source temperatures unreliable.
Rather than introducing another extraneous theory, these explanations essentially expand the scope of plate tectonics in ways that can accommodate volcanic activity previously thought to be outside its remit.
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[14][15] and a textbook.
[7] 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 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),[16][17] 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),[18] and continental regions undergoing lithospheric delamination (e.g., New Zealand).
When extension is persistent and entirely compensated by magma from asthenospheric upwelling, oceanic crust is formed, and the rift becomes a spreading plate boundary.
More severe rifting occurred along the Caledonian Suture, a zone of pre-existing weakness where the Iapetus Ocean closed around 420 Ma.
[21] 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.[22] Diverse volcanic activity resulting from lithospheric extension has occurred throughout the western United States.
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.
[24] This means that melt is formed over a longer period, stored in reservoirs – most likely located at the lithosphere-asthenosphere boundary – and released by lithospheric extension.
There, thick lithosphere remained intact during large-volume magmatism, so decompression upwelling on the scale required can be ruled out, implying that large volumes of magma must have pre-existed.
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.
The northeast Atlantic formed in the early Cenozoic when, after an extensive period of rifting, Greenland separated from Eurasia as Pangaea began to break up.
The tectonic history of the western United States is heavily influenced by the subduction of the East Pacific Rise under the North American Plate beginning around 17 Ma.
This brought about widespread volcanism, commencing with the Columbia River Basalt Group which erupted through a 250-km-long zone of dikes that broadened the crust by several kilometres.
Compared with the others, the Yellowstone-Eastern Snake River Plain zone is considered unusual because of its time-progressive silicic volcano chain and striking geothermal features.
[27] This is further supported by analogous extension-driven silicic magmatism elsewhere in the Western United States, for example in the Coso Hot Springs[29] and Long Valley Caldera[30] in California.
It is thousands of kilometres from any major continental landmass and surrounded by deep ocean, very little of it is above sea level, and it is covered in thick basalt which obscures its deeper structure.
Observations that must be accounted for by any such theory include: The lack of any regional heatflow anomaly detected around the extinct islands and seamounts indicates that the volcanoes are local thermal features.
The plate's stress field evolved over the next 30 million years, causing the region of extension and consequent volcanism to migrate south-southeast.
Petrological and geochemical evidence suggests that this source may be old metamorphosed oceanic crust in the asthenosphere, highly fusible material which would produce far greater magma volumes than mantle rocks.
