Mantle plume
Mantle plume ( briefly also Plume, from the English / French for " bushy feather" or " plume ") is a geoscientific term used to describe a hot upflow rock material from the deep mantle. Mantle plumes have in depth on a slender, tubular shape and widen helmet bush -like or mushroom-shaped when reaching the rigid lithosphere. In German-speaking countries, the term Manteldiapir (or short diapir ) will be used. In places where the aufströmende rock material can make a way up to the surface to mantle plumes cause a particular form of volcanism, is not bound to plate boundaries and is referred to as a hotspot volcanism.
- 3.1 Current state of research
Emergence of the plume concept
The conceptual model of mantle plumes emerged in the mid -1960s. Volcanism is a geoscience phenomenon whose occurrence is usually linked to active plate margins, so to subduction and rift zones. The origin of the magma takes place here in the uppermost mantle and is physically completely explained by plate tectonics. Incomprehensible but remained the so-called intraplate volcanoes that occur independently of plate boundaries at arbitrary locations and could not be explained by the concept of plate tectonics. Intraplate volcanoes were soon referred to as hotspots (English for " hot spot" ), since their occurrence is apparently due to locally increased temperatures in the Earth's mantle.
Most hotspot volcanoes are observed in oceanic areas, often in conjunction with linear chains of islands. One of the best known examples is the archipelago of Hawaii, which form the eastern end of the Hawaiian - Emperor chain of islands, a number of seamounts in the middle of the Pacific. Age dating of rocks showed that the islands with increasing distance from the active center of volcanism today are continuously older. John Tuzo Wilson led in 1963 from this observation, a correlation between the volcanism and the drift of the plates from and concluded that the source region of the magma must lie much deeper in the Earth than ordinary volcanoes. The deep source supplies therefore the active volcano, which is, however, carried away with the lithospheric plate, on which it is located by the plate motion until he can no longer be fed by the stationary deep magma source. Instead, a new volcano, which, in turn, after some time disappears, even though he has moved too far away from the source region. Over geological time, this creates the parallel to the plate motion chain of islands.
The concept was expanded in 1971 and improved by the geophysicist W. Jason Morgan. Morgan postulated that hotspots are caused by aufströmende Plumes, which are an expression of convection processes in the lower mantle. With this assumption, he could simultaneously explain another observation, namely that the basalts, which are promoted by the hotspot volcanism, a slightly different chemical composition show than those that occur at mid-ocean ridges. In the past decades, theoretical modeling and seismological studies of hotspot areas have contributed to a better understanding of mantle plumes. The concept is now generally accepted.
Physical background
Birth and Evolution of Plumes
Plumes are ascending currents hot material from the deep mantle, which move in the form of a narrow column to the surface. Through it material is transported from the depths to the surface, while elsewhere material is transported by subduction into the deep. Thus Plumes contribute to compensate for the mass balance and therefore represent an important part of the mantle dar.
Mantle plumes occur according to present knowledge of instability of a thermal boundary layer. Such provides, inter alia, the so-called D "layer is in 2900 km depth, a transition zone between the liquid outer core and the lowermost mantle. This boundary layer has been discussed for many years as the source region of all observed mantle plumes. Recent studies, however, suggest that at least part of today postulated Plumes in or just below the mantle transition zone ( 410 km to 660 km depth ) arise. , This zone, which forms the transition from the lower to the upper mantle is defined on phase transformations of the mineral olivine. the endothermic character of the 660 - km discontinuity, ie the lower boundary layer of the transition zone, hinders the rise of Plumematerials and could act as a barrier below which dams up the material and thus produces a further thermal boundary layer. mantle of smaller diameter would therefore not be able to penetrate into the upper mantle, while Plumes of large diameter have sufficient buoyancy to continue their ascent.
Once a plume has passed through the viscoplastic mantle, the material meets the upper area on the stronger lithosphere, where it spreads like a mushroom. In the uppermost part of the mantle of the plume exceeds the solidus of the mantle, ie its temperature is above the temperature begins to melt under the prevailing pressure in the mantle rock. The farther the plume rises, the further below the melting away as a result of decompression. The melts separate from the parent rock and flowing through existing fractures and a network of the rock pores formed by the melting upwards, as they have a lower density than the Gesteinsresiduum and are also extruded through mechanical stresses in the host rock and the tamper head. When they finally reach the upper limit of the melting zone in the mantle, they can pass through Dykes (volcanic or magmatic transitions ) to weakness zones of the Earth's crust up to the surface and thus are sources of hotspot volcanism.
Research
Due to the great depth of the source region of mantle plumes direct observation escapes. Their formation and their upgrades can therefore only be indirectly investigated and researched. Important tools that have led to today's picture of the mantle, are numerical modeling and laboratory experiments. Modeling calculated from known or derived physical parameters of the material such as the density or viscosity of the fluid dynamic laws in connection with the temporal evolution of an ascending plume and its effect on the surrounding rock., In laboratory experiments, however, the development of ascending plumes in a strongly reduced scale is investigated. To this end, the situation inside the earth is simulated by viscoplastic fluids are heated with comparable viscosities from below, which leads to the formation of instabilities and Aufströmen. , Results of both methods provide clues for interpreting real seismic observations, which are attributed to effects of plumes.
Shape and secondary effects
The combination of such studies is to infer today that the narrow tube of a plume usually has a diameter of a few tens to a few hundred kilometers, while the Plumekopf can spread over much larger areas. From the results of the research will be further deduced that the temperature of the Aufstroms 100 ° C higher to 300 ° C than that of the surrounding material., With the occurrence of mantle plumes a number of observable geophysical effects are linked, provide the scientific findings and to identify of plumes and their superficial appearance, the hotspots contribute.
The most striking immediately visible phenomenon is the formation of a linear chain of volcanic islands and seamounts, which ultimately led to the development of Plumemodells in oceanic areas. On continents corresponding volcanic chains may arise. According to today's view also flood basalt regions ( Large Igneous Provinces ) are seen as a sign of Plumeaktivität: Achieved the low- Plumekopf the lithosphere, may lead to large-scale volcanic activity, in much larger quantities magma be promoted in a comparable time than conventional volcanism. In later stages, however, the Plumeschlauch leaves the comparatively small-scale distinct volcanic chain., The association of flood basalt regions with the impingement of the plume head has consequences for the theory of so-called Superplumes, which are shown in the following section. This unusually large scale, but short-lived Plumeereignisse have been postulated to explain the Exzistenz the enormously powerful flood basalt provinces such as the Deccan Traps in western India. With the concept of the impinging plume head, however, is already a sufficient explanation possible. Consequently, the Deccan Traps basalts are now brought to the Réunion hotspot connection, even if this interpretation is not undisputed. More flood basalt regions such as the Paraná basalts in Brazil ( the Trindade hotspot ), the Siberian Trapp in northern Russia or the Emeishan Traps in China. With the last two no mantle plume associated, due to their advanced age (Permian ), however, is unlikely that the causative Plumes still exist today.
Effects that are indirectly detectable by seismic methods, the presence of a hot Aufstroms also inside the earth: Thus, the increased temperature - as explained in the previous section - a decrease in seismic velocities, for the variation of the depth of the 410 - km -, the 660 -km discontinuities in the mantle transition zone, as well as the lithosphere- asthenosphere boundary ( right).
Superplumes
After the late 1980s and early 1990s, published theory by Robert Sheridan ( Rutgers University) and Roger Larson ( University of Rhode Iceland ) Superplume large-scale activities have taken place in the Cretaceous period. The center of activity was according to this theory under the Western Pacific. The affected area has a diameter of several thousand kilometers, ten times the affected by current models by Plumes surfaces. For this reason, the phenomenon of Larson Superplume was called.
Sheridan and Larson developed their idea of Superplume activity 120 million years ago based on the following evidence:
- Enhanced eustatic sea-level changes
- Enhanced production of oceanic crust
- Enhanced spreading rates of the Ocean Floor
- Absence of polarity reversals of the geomagnetic field during the relevant time
- Rise in global temperatures
- Increased deposition of black shales
- Enhanced oil formation
- Increase in Karbonatkompensationstiefe
As still visible remains of this event led Larson to the so-called South Pacific Superwell to, an extensive area abnormally thin oceanic crust and higher heat flux in the South Pacific.
Other activities of Superplumes have been postulated for the Jura, the transition from the Carboniferous to Permian and Proterozoic and Archean for. Some theories also lead volcanic phenomena on other celestial bodies back on the activity of Superplumes, such as the formation of the Tharsis volcanoes on Mars.
Current state of research
The theory of Superplumes is not yet generally recognized in the art and will remain working area of current research. In recent years, the term has been used for lack of a clear definition in different word meanings, which created additional irritation. Superplumes were, for example, postulated as an explanation for the break-up of former major continents such as Pangaea. After the occurrence of massive flood basalt provinces can also be described by simple Plumes now, the name Superplume in the recent literature is mainly used only for two regions, which are characterized by particular currently extended plume signatures and related geoid elevations. One is the above-described South Pacific Superwell, which is characterized by an increased heat flow and four hot spots on the surface. Another Superplume is presumed under the African continent, which presents itself as an enormously large-scale low- velocity structure beneath the southern part of Africa. These seismically derived structure protrudes from the core - mantle boundary about 1200 km vertically up and could achieve a similar horizontal extent. ,.
Recent seismological studies show frequent but also structures within the hypothetical Superplumes that were previously not resolvable by the ever improving technical measuring instruments. The formation of a Superplumes from an instability of the D "layer appears from fluid- dynamic point questionable. Contrast, it is assumed that neighboring Manteldiapire by triggered by the updraft circulation currents tend to move toward each other. Conceivable, therefore, that Superplumes actually rather an accumulation of normal mantle are. In a previous numerical modeling has been shown, however, that a relatively cooler plate material, which has passed through subduction to the core - mantle boundary, could produce a much stronger instability. This model would be suitable, a large-scale, catastrophic event Superplume to describe.