A mantle plume is a proposed mechanism of convection
within the Earth's mantle
. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspot
s, such as Hawaii
, and large igneous province
s such as the Deccan
and Siberian traps
. Some such volcanic regions lie far from tectonic plate boundaries
, while others represent unusually large-volume volcanism
near plate boundaries.
Mantle plumes were first proposed by J. Tuzo Wilson
and further developed by W. Jason Morgan
in 1971. A mantle plume is posited to exist where hot rock nucleates
at the core-mantle boundary
and rises through the Earth's mantle becoming a diapir
in the Earth's crust
In particular, the concept that mantle plumes are fixed relative to one another, and anchored at the core-mantle boundary, would provide a natural explanation for the time-progressive chains of older volcanoes seen extending out from some such hot spots, such as the Hawaiian–Emperor seamount chain
. However, paleomagnetic
data show that mantle plumes can be associated with Large Low Shear Velocity Provinces (LLSVPs)
and do move.
Two largely independent convective processes are proposed:
* the broad convective flow associated with plate tectonics, driven primarily by the sinking of cold plates of lithosphere
back into the mantle asthenosphere
* the mantle plume, driven by heat exchange across the core-mantle boundary carrying heat upward in a narrow, rising column, and postulated to be independent of plate motions.
The plume hypothesis was studied using laboratory experiments conducted in small fluid-filled tanks in the early 1970s. Thermal or compositional fluid-dynamical plumes produced in that way were presented as model
s for the much larger postulated mantle plumes. Based on these experiments, mantle plumes are now postulated to comprise two parts: a long thin conduit connecting the top of the plume to its base, and a bulbous head that expands in size as the plume rises. The entire structure is considered to resemble a mushroom. The bulbous head of thermal plumes forms because hot material moves upward through the conduit faster than the plume itself rises through its surroundings. 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 the head.
The sizes and occurrence of mushroom mantle plumes can be predicted easily by transient instability theory developed by Tan and Thorpe. The theory predicts mushroom shaped mantle plumes with heads of about 2000 km diameter that have a critical time of about 830 Myr for a core mantle heat flux
of 20 mW/m2
, while the cycle time is about 2 Gyr. The number of mantle plumes is predicted to be about 17.
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. It may then erupt onto the surface. Numerical modelling predicts that melting and eruption will take place over several million years. These eruptions have been linked to flood basalt
s, although many of those erupt over much shorter time scales (less than 1 million years). Examples include the Deccan traps
in India, the Siberian traps
of Asia, the Karoo-Ferrar
basalts/dolerites in South Africa and Antarctica, the Paraná and Etendeka traps
in South America and Africa (formerly a single province separated by opening of the South Atlantic Ocean), and the Columbia River basalts
of North America. Flood basalts in the oceans are known as oceanic plateaus, and include the Ontong Java plateau
of the western Pacific Ocean and the Kerguelen Plateau
of the Indian Ocean.
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". As the overlying tectonic plate (lithosphere) moves over this hotspot, the eruption of magma from the fixed conduit onto the surface is expected to form a chain of volcanoes that parallels plate motion. The Hawaiian Islands
chain in the Pacific Ocean is the type example. 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.
The eruption of continental flood basalts is often associated with continental rifting
and breakup. This has led to the hypothesis that mantle plumes contribute to continental rifting and the formation of ocean basins.
The current mantle plume theory is that material and energy from Earth's interior are exchanged with the surface crust in two distinct modes: the predominant, steady state plate tectonic regime driven by upper mantle convection
, and a punctuated, intermittently dominant, mantle overturn regime driven by plume convection.
This second regime, while often discontinuous, is periodically significant in mountain building and continental breakup.
Chemistry, heat flow and melting
Earth cross-section showing location of upper (3) and lower (5) mantle, ''D″''-layer (6), and outer (7) and inner (9) core
The chemical and isotopic composition of basalts found at hotspots differs subtly from mid-ocean-ridge basalts. These basalts, also called ocean island basalts (OIBs), are analysed in their radiogenic and stable isotope compositions. In radiogenic isotope systems the originally subducted material creates diverging trends, termed mantle components. Identified mantle components are DMM (depleted mid-ocean ridge basalt (MORB) mantle), HIMU (high U/Pb-ratio mantle), EM1 (enriched mantle 1), EM2 (enriched mantle 2) and FOZO (focus zone). This geochemical signature arises from the mixing of near-surface materials such as subducted slabs
and continental sediments, in the mantle source. There are two competing interpretations for this. In the context of mantle plumes, the near-surface material is postulated to have been transported down to the core-mantle boundary by subducting slabs, and to have been transported back up to the surface by plumes. In the context of the Plate hypothesis, subducted material is mostly re-circulated in the shallow mantle and tapped from there by volcanoes.
Stable isotopes like Fe are used to track processes that the uprising material experiences during melting.
The processing of oceanic crust, lithosphere, and sediment through a subduction zone decouples the water-soluble trace elements (e.g., K, Rb, Th) from the immobile trace elements (e.g., Ti, Nb, Ta), concentrating the immobile elements in the oceanic slab (the water-soluble elements are added to the crust in island arc volcanoes). Seismic tomography
shows that subducted
oceanic slabs sink as far as the bottom of the mantle transition zone
at 650 km depth. Subduction to greater depths is less certain, but there is evidence that they may sink to mid-lower-mantle depths at about 1,500 km depth.
The source of mantle plumes is postulated to be the core-mantle boundary at 3,000 km depth.
Because there is little material transport across the core-mantle boundary, heat transfer must occur by conduction, with adiabatic gradients above and below this boundary. The core-mantle boundary is a strong thermal (temperature) discontinuity. The temperature of the core is approximately 1,000 degrees Celsius higher than that of the overlying mantle. Plumes are postulated to rise as the base of the mantle becomes hotter and more buoyant.
Plumes are postulated to rise through the mantle and begin to partially melt on reaching shallow depths in the asthenosphere by decompression melting
. This would create large volumes of magma. This melt rises to the surface and erupts to form "hot spots".
The lower mantle and the core
The most prominent thermal contrast known to exist in the deep (1000 km) mantle is at the core-mantle boundary at 2900 km. Mantle plumes were originally postulated to rise from this layer because the "hot spots" 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. There is no other known major thermal boundary layer in the deep Earth, and so the core-mantle boundary was the only candidate.
The base of the mantle is known as the D″ layer
, a seismological subdivision of the Earth. It appears to be compositionally distinct from the overlying mantle, and may contain partial melt.
Two very broad, large low-shear-velocity provinces
, exist in the lower mantle
under Africa and under the central Pacific. It is postulated that plumes rise from their surface or their edges. Their low seismic velocities were thought to suggest that they are relatively hot, although it has recently been shown that their low wave velocities are due to high density caused by chemical heterogeneity.
Evidence for the theory
Some common and basic lines of evidence cited in support of the theory are linear volcanic chains, noble gas
anomalies, and geochemistry
Linear volcanic chains
The age-progressive distribution of the Hawaiian-Emperor seamount chain
has been explained as a result of a fixed, deep-mantle plume rising into the upper mantle, partly melting, and causing a volcanic chain to form as the plate moves overhead relative to the fixed plume source.
Other "hot spots" with time-progressive volcanic chains behind them include Réunion
, the Chagos-Laccadive Ridge
, the Louisville Ridge
, the Ninety East Ridge
, and Yellowstone
An intrinsic aspect of the plume hypothesis is that the "hot spots" and their volcanic trails have been fixed relative to one another throughout geological time. Whereas there is evidence that the chains listed above are time-progressive, it has, however, been shown that they are not fixed relative to one another. The most remarkable example of this is the Emperor chain, the older part of the Hawaii system, which was formed by migration of volcanic activity across a geo-stationary plate.
Another example is the Canary Islands in the northeast Atlantic Ocean.
Many postulated "hot spots" are also lacking time-progressive volcanic trails, e.g., Iceland, the Galapagos, and the Azores. Mismatches between the predictions of the hypothesis and observations are commonly explained by auxiliary processes such as "mantle wind", "ridge capture", "ridge escape" and lateral flow of plume material.
Noble gas and other isotopes
Helium-3 is a primordial isotope that formed in the Big Bang
. Very little is produced, and little has been added to the Earth by other processes since then.
includes a primordial component, but it is also produced by the natural radioactive decay of elements such as uranium
. Over time, helium in the upper atmosphere is lost into space. Thus, the Earth has become progressively depleted in helium, and 3
He is not replaced as 4
He is. As a result, the ratio 3
He in the Earth has decreased over time.
Unusually high 3
He have been observed in some, but not all, "hot spots". This is explained by plumes tapping a deep, primordial reservoir in the lower mantle, where the original, high 3
He ratios have been preserved throughout geologic time.
Other elements, e.g. osmium
, have been suggested to be tracers of material arising from near to the Earth's core, in basalts at oceanic islands. However, so far conclusive proof for this is lacking.
The plume hypothesis has been tested by looking for the geophysical anomalies predicted to be associated with them. These include thermal, seismic, and elevation anomalies. Thermal anomalies are inherent in the term "hotspot". They can be measured in numerous different ways, including surface heat flow, petrology, and seismology. Thermal anomalies produce anomalies in the speeds of seismic waves, but unfortunately so do composition and partial melt. As a result, wave speeds cannot be used simply and directly to measure temperature, but more sophisticated approaches must be taken.
Seismic anomalies are identified by mapping variations in wave speed as seismic waves travel through Earth. A hot mantle plume is predicted to have lower seismic wave speeds compared with similar material at a lower temperature. Mantle material containing a trace of partial melt (e.g., as a result of it having a lower melting point), or being richer in Fe, also has a lower seismic wave speed and those effects are stronger than temperature. Thus, although unusually low wave speeds have been taken to indicate anomalously hot mantle beneath "hot spots", this interpretation is ambiguous.
The most commonly cited seismic wave-speed images that are used to look for variations in regions where plumes have been proposed come from seismic tomography. This method involves using a network of seismometers to construct three-dimensional images of the variation in seismic wave speed throughout the mantle.
generated by large earthquakes enable structure below the Earth's surface to be determined along the ray path. Seismic waves that have traveled a thousand or more kilometers (also called teleseismic waves
) can be used to image large regions of Earth's mantle. They also have limited resolution, however, and only structures at least several hundred kilometers in diameter can be detected.
Seismic tomography images have been cited as evidence for a number of mantle plumes in Earth's mantle. 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.
The mantle plume hypothesis predicts that domal topographic uplifts will develop when plume heads impinge on the base of the lithosphere. An uplift of this kind occurred when the north Atlantic Ocean opened about 54 million years ago. 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
. Current research has shown that the time-history of the uplift is probably much shorter than predicted, however. It is thus not clear how strongly this observation supports the mantle plume hypothesis.
Basalts found at oceanic islands are geochemically distinct from those found at mid-ocean ridge
s and volcanoes associated with subduction zones
(island arc basalts). "Ocean island basalt
" is also similar to basalts found throughout the oceans on both small and large seamounts (thought to be formed by eruptions on the sea floor that did not rise above the surface of the ocean). They are also compositionally similar to some basalts found in the interiors of the continents (e.g., the Snake River Plain
In major elements, ocean island basalts are typically higher in iron
(Fe) and titanium
(Ti) than mid-ocean ridge basalts at similar magnesium
(Mg) contents. In trace elements
, they are typically more enriched in the light rare-earth element
s than mid-ocean ridge basalts. Compared to island arc basalts, ocean island basalts are lower in alumina
) and higher in immobile trace elements (e.g., Ti, Nb
These differences result from processes that occur during the subduction of oceanic crust
and mantle lithosphere
. Oceanic crust (and to a lesser extent, the underlying mantle) typically becomes hydrated to varying degrees on the seafloor, partly as the result of seafloor weathering, and partly in response to hydrothermal circulation near the mid-ocean-ridge crest where it was originally formed. As oceanic crust and underlying lithosphere subduct, water is released by dehydration reactions, along with water-soluble elements and trace elements. This enriched fluid rises to metasomatize
the overlying mantle wedge and leads to the formation of island arc basalts. The subducting slab is depleted in these water-mobile elements (e.g., K
) and thus relatively enriched in elements that are not water-mobile (e.g., Ti, Nb, Ta) compared to both mid-ocean ridge and island arc basalts.
Ocean island basalts are also relatively enriched in immobile elements relative to the water-mobile elements. 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. This must have been recycled in the mantle, then re-melted and incorporated in the lavas erupted. Subducted slabs are postulated to have been subducted down as far as the core-mantle boundary, and transported back up to the surface in rising plumes.
In 2015, based on data from 273 large earthquakes, researchers compiled a model based on full waveform tomography
, requiring the equivalent of 3 million hours of supercomputer time.
[ Due to computational limitations, high-frequency data still could not be used, and seismic data remained unavailable from much of the seafloor.] [ Nonetheless, vertical plumes, 400 C hotter than the surrounding rock, were visualized under many hotspots, including the Pitcairn, Macdonald, Samoa, Tahiti, Marquesas, Galapagos, Cape Verde, and Canary hotspots. They extended nearly vertically from the core-mantle boundary (2900 km depth) to a possible layer of shearing and bending at 1000 km.] [ They were detectable because they were 600–800 km wide, more than three times the width expected from contemporary models.] [ 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.
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. It is possible that there is a compositional difference between plumes and the surrounding mantle that slows them down and broadens them.]
Suggested mantle plume locations
Mantle plumes have been suggested as the source for flood basalts. These extremely rapid, large scale eruptions of basaltic magmas have periodically formed continental flood basalt provinces on land and oceanic plateaus in the ocean basins, such as the Deccan Traps, the Siberian Traps the Karoo-Ferrar flood basalts of Gondwana, and the largest known continental flood basalt, the Central Atlantic magmatic province (CAMP).
Many continental flood basalt events coincide with continental rifting.
This is consistent with a system that tends toward equilibrium: as matter rises in a mantle plume, other material is drawn down into the mantle, causing rifting. [
In parallel with the mantle plume model, two alternative explanations for the observed phenomena have been considered: the plate hypothesis and the impact hypothesis.
The plate hypothesis
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.
The plate hypothesis suggests that "anomalous" volcanism results from lithospheric extension that permits melt to rise passively from the asthenosphere beneath. It is thus the conceptual inverse of the plume hypothesis because the plate hypothesis attributes volcanism to shallow, near-surface processes associated with plate tectonics, rather than active processes arising at the core-mantle boundary.
Lithospheric extension is attributed to processes related to plate tectonics. These processes are well understood at mid-ocean ridges, where most of Earth's volcanism occurs. It is less commonly recognised that the plates themselves deform internally, and can permit volcanism in those regions where the deformation is extensional. Well-known examples are the Basin and Range Province in the western USA, the East African Rift valley, and the Rhine Graben. Under this hypothesis, variable volumes of magma are attributed to variations in chemical composition (large volumes of volcanism corresponding to more easily molten mantle material) rather than to temperature differences.
While not denying the presence of deep mantle convection and upwelling in general, the plate hypothesis holds that these processes do not result in mantle plumes, in the sense of columnar vertical features that span most of the Earth's mantle, transport large amounts of heat, and contribute to surface volcanism.
Under the umbrella of the plate hypothesis, the following sub-processes, all of which can contribute to permitting surface volcanism, are recognised:
*Fertility at mid-ocean ridges;
*Enhanced volcanism at plate boundary junctions;
*Small-scale sublithospheric convection;
*Oceanic intraplate extension;
*Slab tearing and break-off;
*Shallow mantle convection;
*Abrupt lateral changes in stress at structural discontinuities;
*Continental intraplate extension;
*Catastrophic lithospheric thinning;
*Sublithospheric melt ponding and draining.
The impact hypothesis
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. In the impact hypothesis, it is proposed that some regions of hotspot volcanism can be triggered by certain large-body oceanic impacts which are able to penetrate the thinner oceanic lithosphere, and flood basalt volcanism can be triggered by converging seismic energy focused at the antipodal point opposite major impact sites.
Impact-induced volcanism has not been adequately studied and comprises a separate causal category of terrestrial volcanism with implications for the study of hotspots and plate tectonics.
Comparison of the hypotheses
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.
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. For the Yellowstone hotspot, seismological evidence began to converge from 2011 in support of the plume model, as concluded by James et al., "we favor a lower mantle plume as the origin for the Yellowstone hotspot." Data acquired through Earthscope, a program collecting high-resolution seismic data throughout the contiguous United States has accelerated acceptance of a plume underlying Yellowstone.
Although there is strong evidence that at least two deep mantle plumes rise to the core-mantle boundary, confirmation that other hypotheses can be dismissed may require similar tomographic evidence for other hot spots.
Seismic-tomography image of Yellowstone mantle plume
Large Igneous Provinces Commission
mantleplumes.org - mantle-plume skeptic website managed and maintained by Gillian R. Foulger