Hotspot (geology)

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Selected examples of suspected hotspots.
Assumed depth of the source region of the mantle diapirs:
_ Lower mantle, _ Jacket transition zone, _ Upper mantle

As Hotspots [ hɔt.spɔ͡ts ] (engl .: hot spots') are with Manteldiapiren related properties, limited locally, relatively stationary, particularly hot areas in the Asthenosphäre denoted, resulting in the overlying crust by volcanic activity or at least a express increased heat flux density . Hotspot volcanism usually occurs at a greater distance from the plate edges. This is also referred to as intraplate volcanism . The possibility of the existence of hotspots as a cause of intraplate volcanism was first in 1963 by geologist John Tuzo Wilson considered.

Geodynamics

Hotspot

The earth's mantle in the area of ​​a hotspot is particularly hot, because there mantle material rises from the deep interior of the earth (possibly from the core-mantle boundary ). The approximately 150 km wide zones of the ascent are also known as mantle diapirs (English: mantle plumes ). The higher temperatures cause increased melting in the upper jacket. Due to their low density, the melts can rise to the surface and cause mostly basaltic volcanism there. The geochemical signature of the OIB ( ocean island basalt ) formed in this way often differs significantly from the so-called MORB ( midocean ridge basalt ).

Since the lithospheric plates slide steadily over the largely constant hotspot while the hot mantle material "welds" through the plate, several volcanic buildings are gradually created , each of which is supplied with melt as long as it lies above the mantle anomaly. This is how volcanic chains are formed like the Hawaiian Islands . It should be noted that the crust thickness of the ocean basins is only 6 km on average, while it is on average around 30 km thick under continents. The continental crust is therefore harder to penetrate, which is why intense hotspot volcanism occurs mainly in oceanic areas.

Hotspots and plate movements

Hotspot volcanoes on the oceanic crust are a data source for determining “current” (post- Miocene ) or past plate movements. From the apparent migration of a hotspot and the determination of the age of the basalts of the volcanic islands created by it, one can reconstruct the direction and speed of the movement of the corresponding lithospheric plate. In the case of the Hawaiian Archipelago, a. Basalts from Mauna Kea ( Hawaii main island , "Big Island") to 0.20–0.25 million years, from Haleakalā to 0.95–1.0 million years and those of Waiʻanae on Oʻahu to 3.05– dated 3.1 million years, which, including the youngest member of the island chain, which still lies below sea Lō'ihi Seamount , an apparent migration of the hotspot of about 10 cm / year (100km / million years) in a direction of movement of about 300 ° (ONO). In comparison with data from other hotspots (the so-called hotspot reference frame ), depending on the calculation model, this results in a "current" mean speed of the Pacific plate between 8.3 cm / year (83 km / million years) and 10.5 cm / Year (105 km / million years).

If one takes the basic assumption that hotspots are stationary over long geological periods of time , it is also possible to reconstruct movements that lie further in the past through corresponding studies of even older volcanic structures that go back to the Hawaii hotspot ( Hawaii-Emperor chain ) . The sharp "kink" in the Hawaii-Emperor chain was explained by the fact that the sense of movement of the Pacific plate changed dramatically in the course of the Eocene (approx. 43 mya ).

Both laboratory tests and detailed studies of the relative movements of hotspot island chains show, however, that hotspots can also move by 1–2 mm / year. The example of the "kink" in the Hawaii-Emperor chain is the subject of a scientific controversy: Such an abrupt change in movement of the Pacific plate should have been accompanied by significant tectonic events in the Pacific region around 43 million years ago. However, there are no visible signs of this, so that a relatively strong natural movement of the plume must also be taken into account. Paleomagnetic and gravimetric surveys of the ocean floors support this assumption. Taking into account the rheology of the earth's mantle, it is more likely that "kinks" in hotspot island chains are due to changes in mantle currents caused by mantle convection. A comparison of paleotectonic reconstructions confirms this.

Loihi is currently 900 m below sea level, but geologists believe that it will reach a height of 4,000 m above sea level in the next millions of years . It would then be exactly as big as the Mauna Kea volcano today. This thesis follows Wilson's view that the growth of all hotspot islands will always be the same. The different ages of the Hawaiian Islands can be seen among other things. a. in the different weathering stages, whereby the youngest island of Hawaii is still growing due to regular volcanic eruptions and the northwestern neighboring islands are already shrinking again due to erosion and subsidence .

Well-known hotspots

Since the identification of smaller plumes is very difficult, the exact number remains unclear. Various numbers of catalogs of the hotspots observed worldwide have been published in the specialist literature. Around 50 of these have so far been clearly verified as mantle plumes or classified as very likely candidates by means of seismological investigations. The best-known examples of hotspot volcanism are the Hawaiian Islands and Iceland (there in combination with the volcanism of a mid-ocean ridge) the Eifel in Germany, the Galápagos Islands , which politically belong to Ecuador, and the Yellowstone National Park in Wyoming.

See also

literature

  • Joachim RR Ritter, Ulrich R. Christensen: (Ed.): Mantel Plumes. A multidisciplinary approach. Springer, Berlin 2007, Berlin, ISBN 978-3-540-68045-1 .

Web links

Commons : Hotspot  - collection of images, videos and audio files

Individual evidence

  1. ↑ Vincent Courtillot , Anne Davaille, Jean Besse, Joann Stock: Three distinct types of hotspots in the Earth's mantle. In: Earth and Planetary Science Letters. Vol. 205, No. 3/4, 2003, pp. 295-308, doi : 10.1016 / S0012-821X (02) 01048-8 .
  2. ↑ Gillian R. Foulger: Plates vs. plumes. A geological controversy. Wiley-Blackwell, Chichester, et al. a. 2010, ISBN 978-1-4443-3679-5 .
  3. ^ Hans Murawski , Wilhelm Meyer : Geological dictionary. 12th, revised and expanded edition. Spectrum - Akademischer Verlag, Heidelberg 2010, ISBN 978-3-8274-1810-4 , p. 74.
  4. ↑ John Tuzo Wilson : A possible Origin of the Hawaiian Islands . In: Canadian Journal of Physics . tape 63 , no. 6 , 1963, pp. 863-870 , doi : 10.1139 / p63-094 .
  5. ↑ a b Alice E. Gripp, Richard G. Gordon: Young tracks of hotspots and current plate velocities . In: Geophysical Journal International . tape 150 , no. 2 , 2002, p. 321-361 , doi : 10.1046 / j.1365-246X.2002.01627.x .
  6. ↑ Shimin Wang, Ren Wang: Current plate velocities relative to hotspots: implications for hotspot motion, mantle viscosity and global reference frame . In: Earth and Planetary Science Letters . tape 189 , no. 3/4 , 2009 S. 133-140 , doi : 10.1016 / S0012-821X (01) 00351-X .
  7. ↑ Anne Davaille, Fabien Girard, Michael Le Bars: How to anchor hotspots in a convecting mantle? In: Earth and Planetary Science Letters. Vol. 203, No. 2, 2002, pp. 621-634, doi : 10.1016 / S0012-821X (02) 00897-X .
  8. ^ A b Ian O. Norton: Plate motions in the North Pacific: The 43 Ma nonevent. In: Tectonics. Vol. 14, No. 5, 1995, pp. 1080-1094, doi: 10.1029 / 95TC01256 .
  9. ↑ John A. Tarduno, Rory D. Cottrell: Paleomagnetic evidence for motion of the Hawaiian hotspot during formation of the Emperor seamounts. In: Earth and Planetary Science Letters. Vol. 153, No. 3, 1997, pp 171-180. Doi: 10.1016 / S0012-821X (97) 00169-6 .
  10. ↑ John Tarduno, Hans-Peter Bunge, Norm Sleep, Ulrich Hansen: The Bent Hawaiian-Emperor hotspot track: Inheriting the Mantle wind. In: Science . Vol. 324, No. 5923, 2009, pp. 50-53, doi: 10.1126 / science.1161256 .