Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.
Mechanisms.
Mechanisms that have been proposed to explain intraplate volcanism include mantle plumes; non-rigid motion within tectonic plates (the plate model); and impact events. It is likely that different mechanisms accounts for different cases of intraplate volcanism.
Plume model.
A mantle plume is a proposed mechanism of convection of abnormally hot rock within the Earth's mantle. Because the plume head partly melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, and large igneous provinces 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.
The hypothesis of mantle plumes has required progressive hypothesis-elaboration leading to variant propositions such as mini-plumes and pulsing plumes.
Concepts.
Mantle plumes were first proposed by J. Tuzo Wilson in 1963 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 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 models 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 basalts, 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. In the context of the alternative "Plate model", continental breakup is a process integral to plate tectonics, and massive volcanism occurs as a natural consequence when it starts.
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.
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. The plume hypothesis postulates that 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.
Various lines of evidence have been cited in support of mantle plumes. There is some confusion regarding what constitutes support, as there has been a tendency to re-define the postulated characteristics of mantle plumes after observations have been made.
Some common and basic lines of evidence cited in support of the theory are linear volcanic chains, noble gases, geophysical 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 Kerguelen, Tristan, 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.
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. Helium-4 includes a primordial component, but it is also produced by the natural radioactive decay of elements such as uranium and thorium. Over time, helium in the upper atmosphere is lost into space. Thus, the Earth has become progressively depleted in helium, and 3He is not replaced as 4He is. As a result, the ratio 3He/4He in the Earth has decreased over time.
Unusually high 3He/4He have been observed in some, but not all, "hot spots". In mantle plume theory, 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. In the context of the Plate hypothesis, the high ratios are explained by preservation of old material in the shallow mantle. Ancient, high 3He/4He ratios would be particularly easily preserved in materials lacking U or Th, so 4He was not added over time. Olivine and dunite, both found in subducted crust, are materials of this sort.
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.
Geophysical anomalies.
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