Mantle plume

However, paleomagnetic data show that mantle plumes can also be associated with Large Low Shear Velocity Provinces (LLSVPs)[7][8] and do move relative to each other.

In the late 1980s and early 1990s, experiments with thermal models showed that as the bulbous head expands it may entrain some of the adjacent mantle into itself.

The size and occurrence of mushroom mantle plumes can be predicted by the transient instability theory of Tan and Thorpe.

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 conduit, postulated to connect the plume head to the core-mantle boundary, is viewed as providing a continuous supply of magma to 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.

In radiogenic isotope systems the originally subducted material creates diverging trends, termed mantle components.

[22][23] This geochemical signature arises from the mixing of near-surface materials such as subducted slabs and continental sediments, in the mantle source.

Seismic tomography shows that subducted oceanic slabs sink as far as the bottom of the mantle transition zone at 650 km depth.

Mantle plumes were originally postulated to rise from this layer because the hotspots that are assumed to be their surface expression were thought to be fixed relative to one another.

This required that plumes were sourced from beneath the shallow asthenosphere that is thought to be flowing rapidly in response to motion of the overlying tectonic plates.

[28][29] Some common and basic lines of evidence cited in support of the theory are linear volcanic chains, noble gases, geophysical anomalies, and geochemistry.

The most remarkable example of this is the Emperor chain, the older part of the Hawaii system, which was formed by migration of the hotspot in addition to the plate motion.

This is explained by plumes tapping a deep, primordial reservoir in the lower mantle, where the original, high 3He/4He ratios have been preserved throughout geologic time.

Thus, although unusually low wave speeds have been taken to indicate anomalously hot mantle beneath hotspots, this interpretation is ambiguous.

This method involves using a network of seismometers to construct three-dimensional images of the variation in seismic wave speed throughout the mantle.

[37] Seismic waves generated by large earthquakes enable structure below the Earth's surface to be determined along the ray path.

[38] There is, however, vigorous on-going discussion regarding whether the structures imaged are reliably resolved, and whether they correspond to columns of hot, rising rock.

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

[41] 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.

[43] 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.

[56][57] The reason for this is that the mantle-plume hypothesis has not been suitable for making reliable predictions since its introduction in 1971 and has therefore been repeatedly adapted to observed hotspots depending on the situation.

[57] Over time, with the growing number of models, the concept of a plume developed into a weakly defined hypothesis, which as a general term is currently neither provable nor refutable.

[56][57]The 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.

[56] The plate hypothesis suggests that "anomalous" volcanism results from lithospheric extension that permits melt to rise passively from the asthenosphere beneath.

Well-known examples are the Basin and Range Province in the western USA, the East African Rift valley, and the Rhine Graben.

In 1997 it became possible using seismic tomography to image submerging tectonic slabs penetrating from the surface all the way to the core-mantle boundary.

[59] 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 ) [ 1 ]
Hydrodynamic simulation of a single "finger" of the Rayleigh–Taylor instability , a possible mechanism for plume formation. [ 19 ] 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. [ 26 ]
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. [ 44 ] Figure from Foulger (2010). [ 36 ]
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).