Plume (geology)

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World map with marking of the places under which hotspots , and thus mantle plumes, are suspected. However, not all of these hotspots are currently expressed as intense volcanism.

Mantle plume (short also plume , from English / French for "bushy feather" or "plume of smoke") is a geoscientific technical term for an upflow of hot rock material from the deeper mantle of the earth . Mantle plumes have a slender, tube-like shape in the depths and, when they reach the rigid lithosphere, widen in the shape of a helmet or mushroom. In the German-speaking world, the term Manteldiapir (or Diapir for short ) is also used. Mantle plumes are responsible for a special form of volcanism that is not bound to plate boundaries and is known as hotspot volcanism.

History of the Plume Concept

The Hawaii-Emperor chain , which was completely submarine in its older section , led to the development of the Plumemodel

The model conception of the mantle plumes originated in the 1960s and 70s. Volcanism is a geoscientific phenomenon that occurs predominantly at active plate margins, i.e. at subduction and rift or oceanic spreading zones . The formation of the magma takes place in the asthenosphere , a zone of the upper mantle, and can be fully explained physically by plate tectonics . However, the so-called intraplate volcanoes , which occur at any location regardless of plate boundaries and could not be explained by the concept of plate tectonics , remained incomprehensible .

Most active intraplate volcanoes can be observed in oceanic areas, often in connection with linear island and seamount chains. One of the best-known examples of this is the Hawaiian Archipelago , whose islands are the youngest link in the Hawaiian-Emperor-Island-Seamount chain that extends to the far northwest of the Pacific Basin . Dating of the age of the lava rocks of the Hawaiian Islands showed that the islands continuously get older with increasing distance from the active center of volcanism today. In 1963, John Tuzo Wilson derived a connection between volcanism and the drift of the plates from this observation and concluded that the source region of the magma must lie much deeper in the interior of the earth than in ordinary volcanoes. The deep source supplies the active volcano, which is carried away by the plate movement with the lithospheric plate on which it is located, until it can no longer be fed by the stationary deep magma source. Instead, a new volcano is created, which in turn extinguishes after a while, even if it has moved too far from the source region. Over geological time, an island chain is created that traces the direction of movement of the plate.

The concept was expanded and improved in 1971 by the geophysicist W. Jason Morgan . Morgan postulated that the deep magma sources he hot spots ( "hot spots") calls associated with upflowing Plumes related to expression of convection processes are in the lower mantle. With this assumption, he was able to explain another observation at the same time, namely that the basalts that are extracted by hotspot volcanism show a somewhat different chemical composition than those that arise on mid-ocean ridges . The concept gained general acceptance in the 1980s and 1990s and has been continuously further developed on the basis of findings from laboratory tests, computer simulations and seismological studies. However, the real existence of hotspots and plumes has been increasingly doubted since the middle of the first decade of the 21st century.

Physical background

Formation and development of plumes

Mantle plume, schematic

Plumes are ascending currents of hot material from the deep mantle of the earth, which move to the surface of the earth in the form of a narrow column. Through them, material is transported from the depths to the surface of the earth, while in other places material is transported into the depths through subduction . Plumes thus help to balance the mass balance and therefore represent an important part of the mantle convection .

According to current knowledge, mantle plumes arise from instabilities in a thermal boundary layer. One such layer is the so-called D "layer at a depth of around 2900 km, a transition zone between the liquid outer core and the lowest mantle . This boundary layer has been discussed for many years as the source region of all mantle plumes observed. However, recent studies suggest that At least some of the plumes postulated today arise in or directly below the mantle transition zone (410 km to 660 km depth). This zone, which forms the transition from the lower to the upper mantle, is defined by phase transformations of the mineral olivine . The endothermic character of the 660- km discontinuity , i.e. the lower boundary layer of the transition zone, hinders the ascent of the plume material and could act as a barrier below which the material builds up and thus creates a further thermal boundary layer. Thus, mantle plumes of smaller diameter would not be able to enter the upper mantle penetrate, while plumes with a large diameter eaders would have enough buoyancy to continue their ascent.

After a plume has crossed the viscoplastic mantle of the earth, the material hits the solid lithosphere in the upper area , below which it spreads mushroom-shaped in all directions. The hot plume material heats the sublithospheric mantle so much that the solidus curve of the mantle rock is exceeded, i.e. H. its temperature rises above the temperature at which parts of the mantle rock begin to melt under the prevailing pressure. The further the plume rises, the more material melts as a result of the decreasing pressure. The melt (magma) flows upwards through existing fissures and a network of rock pores formed by melting in the mother rock, because it has a lower density than the rock residue and is also pressed out by mechanical stresses in the mother rock and the load pressure. Following the pressure and density gradient, it migrates in the cleft through the lithosphere to the earth's crust , where it collects in a magma chamber . If the pressure in the magma chamber increases to a sufficient level, the melt can finally penetrate to the surface of the earth and cause intense hotspot volcanism there.

exploration

Due to the great depth, the source region of the mantle plumes cannot be directly observed. Its origin and its rise can therefore only be investigated and researched indirectly. Important tools that have led to today's picture of the mantle plumes are numerical modeling and laboratory experiments. Modeling calculates from known or derived physical parameters of the material such as B. the density or the viscosity in connection with fluid dynamic laws the temporal development of a rising plume as well as its effect on the surrounding rock. In laboratory tests, on the other hand, the development of ascending plumes is investigated on a greatly reduced scale. For this purpose, the situation in the interior of the earth is simulated by heating viscous plastic liquids with comparable viscosities from below, which leads to the development of instabilities and currents. The results of both methods provide clues for the interpretation of real seismological observations that are attributed to the effects of plumes.

Suitable seismological investigation methods for the thermally induced effects are, for example, seismic tomography and receiver functions . Tomography is able to detect the reduction in the propagation speed of seismic waves in the earth's interior caused by the hot upcurrent and to show their rough three-dimensional structure. The receiver-function method, on the other hand, is used to map depth changes in seismic boundary layers , which are also caused by the greatly increased temperature.

Shape and secondary effects

From the combination of such investigations, it is now concluded that the narrow tube of a plume generally has a diameter of a few tens to a few hundred kilometers, while the plume head can spread over much larger areas. From the results of the research it is further deduced that the temperature of the upstream is 100 ° C to 300 ° C higher than that of the surrounding material. The occurrence of mantle plumes is linked to a number of observable geophysical effects that provide scientific knowledge and contribute to the identification of plumes and their superficial appearance, the hotspots.

The flood basalts on the east coast of Greenland at Scoresbysund are associated with the opening of the Atlantic and are probably related to the head of today's Icelandic plume

The most conspicuous directly visible phenomenon in oceanic areas is the formation of a linear chain of volcanic islands and seamounts , which ultimately led to the development of the plume model. Corresponding volcanic chains can arise on continents. According to today's view, flood basalt regions ( Large Igneous Provinces ) are also seen as a sign of plume activity: If the low-viscosity plume head reaches the lithosphere, large-scale volcanic activity can occur, in which significantly larger amounts of magma are extracted in a comparable time than with conventional volcanism. In the later stage, however, the plume tube leaves behind the relatively small-scale volcanic chain. The association of flood basalt regions with the impact of the plume head also has consequences for the theory of the so-called superplumes . These unusually large-scale but short-lived plume events were postulated to explain the existence of the enormously powerful flood basalt provinces such as the Dekkan-Trapp in the Indian suburbs. With the concept of the impinging plume head, however, a sufficient explanation is already possible. Logically, the Dekkan-Trapp- Basalt are today associated with the Réunion hotspot, even if this interpretation is not undisputed. Other flood basalt regions are z. B. the Paraná basalts in Brazil (to the Trindade hotspot ), the Siberian Trapp in northern Russia or the Emeishan Trapp in China . No mantle plume is associated with the latter two, but due to their advanced age ( Permian ) it is unlikely that the causative plumes still exist today.

Effect of the thermal influence on the lithosphere-asthenosphere boundary under Hawaii through the interaction with the hot upflow of the mantle plume: As time goes on (older islands, behind) the lithosphere is thinned out more and more.

Another measurable effect of plumes is the formation of a topographic threshold in the vicinity of the recent hotspot volcanism, as well as a regional elevation of the geoid . Such a phenomenon was investigated using the example of the Hawaii hotspot (indicated by the lighter shades of blue in the illustration of the Hawaii Emperor chain above). Originally, the geoid high was explained as an ascent caused by simple thermal expansion, but heat flow measurements and the geologically short period of ascent show that this explanation alone is not sufficient. An additional effect is created by the upflow of the plume itself, which leads to dynamic uplift.

The presence of a hot upstream also has effects inside the earth that can be indirectly detected using seismological methods: As explained in the previous section, the increased temperature leads to a decrease in seismic velocities and a change in the depth of the 410 km , the 660 km discontinuities of the mantle transition zone, as well as the lithosphere - asthenosphere boundary (figure on the right).

Superplumes

According to a theory published in the late 1980s and early 1990s by Robert Sheridan ( Rutgers University ) and Roger Larson ( University of Rhode Island ), large-scale superplume activities took place in the Cretaceous Period . According to this theory, the center of activities was under the western Pacific. The affected area has a diameter of several thousand kilometers, ten times the area affected by plumes according to current models. It is for this reason that Larson called the phenomenon Superplume .

Sheridan and Larson developed their concept of superplume activity 120 million years ago based on the following evidence:

As a remnant of this event that is still visible today, Larson cited the so-called South Pacific Superswell , an extensive area of ​​abnormally thin ocean crust and increased heat flow in the South Pacific.

Further activities of superplumes were postulated for the Jurassic , the transition from the Carboniferous to the Permian as well as for the Proterozoic and Archean . Some theories also attribute volcanic phenomena on other celestial bodies to 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 specialist circles and remains the field of current research. In the past few years the term has been used in various meanings due to the lack of a clear definition, which created additional confusion. Superplumes, for example, were postulated as an explanation for the breakup of earlier major continents such as B. Pangea . Since the occurrence of massive flood basalt provinces can now also be described by simple plumes, the term superplume is mainly used in recent literature only for two regions that are currently characterized by particularly extensive plume signatures and associated geoid uplifts. One of them is the South Pacific Superswell described above, which is characterized by an increased heat flow and four hotspots on the surface. Another superplume is suspected to be under the African continent , which presents itself as an enormously large-scale, low-speed structure under the southern part of Africa. This seismologically derived structure rises vertically approx. 1200 km starting from the core-mantle boundary and could reach a similar horizontal extent.

More recent seismological studies, however, often show structures within the hypothetical superplumes, which are only now being able to be resolved by the constantly improving technical measuring devices. The formation of a superplume from an instability of the D "layer seems questionable from a fluid dynamic point of view. On the other hand, it is assumed that neighboring mantle diapirs tend to move towards each other due to the circulation currents triggered by the upstream. It is therefore conceivable that superplumes actually tend to be an accumulation of normal mantle plumes In previous numerical modeling, however, it was shown that a plate of relatively cooler material that has subduced to the core-mantle boundary could create significantly greater instability. This model would be suitable for a large-scale, catastrophic superplume event to describe.

See also

literature

  • Joachim RR Ritter, Ulrich R. Christensen (Eds.): Mantle Plumes - A Multidisciplinary Approach. Springer Verlag, Berlin 2007, ISBN 978-3-540-68045-1 . (English)
  • Kent C. Condie: Mantle Plumes and Their Record in Earth History. Cambridge University Press, Cambridge 2001, ISBN 0-521-01472-7 . (English)

Individual evidence

  1. ^ Frank Press, Raymond Siever: General geology. Spektrum Akademischer Verlag, Heidelberg 1995, ISBN 3-86025-390-5 .
  2. ^ JT Wilson: Evidence from islands on the spreading of ocean floors. In: Nature . Vol. 197, 1963, pp. 536-538.
  3. ^ WJ Morgan: Convection plumes in the lower mantle. In: Nature . Vol. 230, 1971, pp. 42-43.
  4. See corresponding brief overviews in:
    • Yaoling Niu, Marjorie Wilson, Emma R. Humphreys, Michael J. O'Hara: The Origin of Intra-plate Ocean Island Basalts (OIB): the Lid Effect and its Geodynamic Implications. In: Journal of Petrology. Vol. 52, No. 7-8, pp. 1443-1468, doi : 10.1093 / petrology / egr030 .
    • Vincent E Neall, Steven A Trewick: The age and origin of the Pacific islands: a geological overview. In: Philosophical Transactions of the Royal Society B: Biological Sciences. Vol. 363, No. 1508, pp. 3293-3308, doi : 10.1098 / rstb.2008.0119 .
  5. ^ D. Bercovici, A. Kelly: The non-linear initiation of diapirs and plume heads. In: Physics of the Earth and Planetary Interiors. Vol. 101, 1997, pp. 119-130.
  6. a b L. Cserepes, DA Yuen: On the possibility of a second kind of mantle plumes. In: Earth and Planetary Science Letters. Vol. 183, 2000, pp. 61-71.
  7. ^ A b c R. Montelli et al.: A catalog of deep mantle plumes: New results from finite-frequency tomography. In: Geochemistry, Geophysics, Geosystems. Vol. 7, 2006, ISSN  1525-2027 .
  8. ^ G. Marquart, H. Schmeling: Interaction of small mantle plumes with the spinel-perovskite phase boundary: implications for chemical mixing. In: Earth and Platentary Science Letters. Vol. 177, 2000, pp. 241-254.
  9. a b J. Korenaga: Firm mantle plumes and the nature of the core-mantle boundary region. In: Earth and Planetary Science Letters. Vol. 232, 2005, pp. 29-37.
  10. T. Nakakuki, DA Yuen, S. Honda: The interaction of plumes with the transition zone under continents and oceans. In: Earth and Planetary Science Letters. Vol. 146, 1997, pp. 379-391.
  11. N. Ribe, UR Christensen: Three-dimensional modeling of plume-lithosphere interaction. In: Journal of Geophysical Research. Vol. 99, 1994, pp. 669-682.
  12. RC Kerr, C. Mériaux: Structure and dynamics of sheared mantle plumes. In: Geochemistry, Geophysics, Geosystems. Vol. 5, 2004, ISSN  1525-2027 .
  13. a b A. Davaille, J. Vatteville: On the transient nature of mantle plumes. In: Geophysical Research Letters. Vol. 32, 2005, doi : 10.1029 / 2005GL023029 .
  14. D. Zhao: Global tomographic images of mantle plumes and subducting slabs: insight into deep Earth dynamics. In: Physics of the Earth and Planetary Interiors. Vol. 146, 2004, pp. 3-34.
  15. ^ I. Wölbern et al.: Deep origin of the Hawaiian tilted plume conduit derived from receiver functions. In: Geophysical Journal International. Vol. 166, 2006, pp. 767-781.
  16. LP Vinnik, V. Farra, R. child: Deep structure of the Afro-Arabian hotspot by S receiver functions. In: Geophysical Research Letters. Vol. 31, 2004, doi : 10.1029 / 2004GL019574 .
  17. ^ Li et al.: Seismic observation of narrow plumes in the oceanic upper mantle. In: Geophysical Research Letters. Vol. 30, 2003, doi : 10.1029 / 2002GL015411 .
  18. T. Dahl-Jensen et al: Depth to Moho in Greenland: receiver-function analysis suggests two Proterozoic blocks in Greenland. In: Earth and Planetary Science Letters. Vol. 205, 2003, pp. 379-393.
  19. a b c B. Steinberger: Plumes in a convecting mantle: Models and observations for individual hotspots. In: Journal of Geophysical Research. Vol. 105, 2000, pp. 11127-11152.
  20. AM Jellinek, M. Manga: Links between long-lived hot spots, mantle plumes, D ", and plate tectonics. In: Reviews of Geophysics. Vol. 42, 2004, doi : 10.1029 / 2003RG000144 .
  21. The Deccan beyond the plume hypothesis (English)
  22. L. Cserepes, UR Christensen, NM Ribe: Geoid height versus topography for a plume model of the Hawaiian Swell. In: Earth and Planetary Science Letters. Vol. 178, 2000, pp. 29-38.
  23. J. van Hunen, S. Zhong: New insight in the Hawaiian plume swell dynamics from scaling laws. In: Geophysical Research Letters. Vol. 30, 2003, doi : 10.1029 / 2003GL017646 .
  24. ^ P. Wessel: Observational constraints on models of the Hawaiian hot spot swell. In: Journal of Geophysical Research. Vol. 98, 1993, pp. 16095-16104.
  25. Kent Ratajeski: The Cretaceous Superplume. Contribution to All Things Cretaceous: A Digital Resource Collection for Teaching and Learning from the Science Education Resource Center (SERC), Carleton College, Northfield, Minnesota.
  26. ^ KC Condie: Supercontinents and superplume events: distinguishing signals in the geologic record. In: Physics of the Earth and Planetary Interiors. Vol. 146, 2004, pp. 319-332.
  27. ^ F. Niu et al.: Mantle transition-zone structure beneath the South Pacific Superswell and evidence for a mantle plume underlying the Society hotspot. In: Earth and Planetary Science Letters. Volume 198, 2002, pp. 371-380.
  28. ^ S. Ni, DV Helmberger: Further constraints on the African superplume structure. In: Physics of the Earth and Planetary Interiors. Vol. 140, 2003, pp. 243-251.
  29. ^ S. Ni et al .: Sharp sides to the African Superplume. In: Science. Vol. 296, 2002, pp. 1850-1852.
  30. ^ G. Schubert and others: Superplumes or plume clusters? In: Physics of the Earth and Planetary Interiors. Vol. 146, 2004, pp. 147-162.
  31. E. Tan, M. Gurnis, L. Han: Slabs in the lower mantle and Their modulation of plume formation. In: Geochemistry, Geophysics, Geosystems. Vol. 3, 1067, 2002, doi : 10.1029 / 2001GC000238 .

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