Intraplate volcanism

[7] Two largely independent convective processes are proposed: The plume hypothesis was studied using laboratory experiments conducted in small fluid-filled tanks in the early 1970s.

When a plume head encounters the base of the lithosphere, it is expected to flatten out against this barrier and to undergo widespread decompression melting to form large volumes of basalt magma.

The narrow vertical pipe, or conduit, postulated to connect the plume head to the core-mantle boundary, is viewed as providing a continuous supply of magma to a fixed location, often referred to as a "hotspot".

It has recently been discovered that the volcanic locus of this chain has not been fixed over time, and it thus joined the club of the many type examples that do not exhibit the key characteristic originally proposed.

[3] Some common and basic lines of evidence cited in support of the theory are linear volcanic chains, noble gases, geophysical anomalies, and geochemistry.

Some scientists have linked this to a mantle plume postulated to have caused the breakup of Eurasia and the opening of the north Atlantic, now suggested to underlie Iceland.

This, and other observations, have been interpreted as indicating that the distinct geochemical signature of ocean island basalts results from inclusion of a component of subducted slab material.

[35] Many of these plumes are in the large low-shear-velocity provinces under Africa and the Pacific, while some other hotspots such as Yellowstone were less clearly related to mantle features in the model.

[37] The unexpected size of the plumes leaves open the possibility that they may conduct the bulk of the Earth's 44 terawatts of internal heat flow from the core to the surface, and means that the lower mantle convects less than expected, if at all.

The main factors governing the evolution of the stress field are: Beginning in the early 2000s, dissatisfaction with the state of the evidence for mantle plumes and the proliferation of ad hoc hypotheses drove a number of geologists, led by Don L. Anderson, Gillian Foulger, and Warren B. Hamilton, to propose a broad alternative based on shallow processes in the upper mantle and above, with an emphasis on plate tectonics as the driving force of magmatism.

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.

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.

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.

[57] Since 2003, discussion and development of the plate theory has been fostered by the Durham University(UK)-hosted website mantleplumes.org, a major international forum with contributions from geoscientists working in a wide variety of specialties.

When extension is persistent and entirely compensated by magma from asthenospheric upwelling, oceanic crust is formed, and the rift becomes a spreading plate boundary.

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.[69] Diverse volcanic activity resulting from lithospheric extension has occurred throughout the western United States.

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.

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.

[74] This is further supported by analogous extension-driven silicic magmatism elsewhere in the Western United States, for example in the Coso Hot Springs[76] and Long Valley Caldera[77] 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.

[83][84] In addition to these processes, impact events such as ones that created the Addams crater on Venus and the Sudbury Igneous Complex in Canada are known to have caused melting and volcanism.

[86] For the Hawaii hotspot, long-period seismic body wave diffraction tomography provided evidence that a mantle plume is responsible, as had been proposed as early as 1971.

A superplume generated by cooling processes in the mantle (LVZ= low-velocity zone ) [ 2 ]
Hydrodynamic simulation of a single "finger" of the Rayleigh–Taylor instability , a possible mechanism for plume formation. [ 18 ] In the third and fourth frame in the sequence, the plume forms a "mushroom cap". Note that the core is at the top of the diagram and the crust is at the bottom.
Earth cross-section showing location of upper (3) and lower (5) mantle, D″ -layer (6), and outer (7) and inner (9) core
Calculated Earth's temperature vs. depth. Dashed curve: Layered mantle convection ; Solid curve: Whole mantle convection. [ 25 ]
Diagram showing a cross section though the Earth's lithosphere (in yellow) with magma rising from the mantle (in red). The crust may move relative to the plume, creating a track .
An example of plume locations suggested by one recent group. [ 38 ] Figure from Foulger (2010). [ 3 ]
Schematic of the plate theory. Mid-blue: lithosphere; light-blue/green: inhomogeneous upper mantle; yellow: lower mantle; orange/red: core-mantle boundary. Lithospheric extension enables pre-existing melt (red) to rise. [ 46 ]
An illustration of competing models of crustal recycling and the fate of subducted slabs. The plume hypothesis invokes deep subduction (right), while the plate hypothesis focuses on shallow subduction (left).
Regional map of the North East Atlantic. Bathymetry shown in colour; land topography in grey. RR: Reykjanes Ridge; KR: Kolbeinsey Ridge; JMMC: Jan Mayen Microcontinent; AR: Aegir Ridge; FI: Faroe Islands. Red lines: boundaries of the Caledonian orogen and associated thrusts, dashed where extrapolated into younger Atlantic Ocean. [ 65 ]
Geological map of northwest USA showing Basin and Range faults and basalts and rhyolites <17 Ma. Blue lines represent approximate age contours of silicic volcanic centres across the Eastern Snake River Plain and a contemporaneous trend of oppositely propagating silicic volcanism across central Oregon. [ 73 ]