D ″ layer

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The D ″ layer (pronounced: "D-two-line layer") forms the lowest part of the lower earth mantle ; it thus represents the transition zone between this and the (outer) core of the earth .

nature

According to the current state of knowledge, the thickness of this layer is around 200 to 300 km and shows strong fluctuations, as was investigated using seismological methods . While the lower coat is largely homogeneous according to popular model ideas , the D ″ layer shows strong heterogeneities . These heterogeneities, known as large low-velocity provinces (LLVP), consist of material that has a lower density than the underlying core material but a higher density than the overlying cladding material. In analogy to the behavior of the continents towards the upper mantle, these areas are therefore also called C-continents . They migrate over time and have an impact on jacket convection and the formation of hotspots . The physical and mineralogical character of the D ″ layer is largely not understood, but there are various hypotheses that partially complement each other:

  • One possibility is that there are metallic and non-metallic silicates that result from reactions of the iron from the liquid core with the perovskite of the lower mantle.
  • Another hypothesis states that the remains of subducted lithospheric plates are dammed up and deposited at this boundary layer .
  • In 2004 a phase transformation of the mineral perovskite into a modification with a CaIrO 3 structure was discovered. This has since been referred to as post-perovskite. The transformation takes place at jacket temperatures from a pressure of approx. 120 GPa, which corresponds very well to the depth of the D ″ layer and is therefore considered to explain complex velocity structures at the core-jacket boundary.

The D ″ layer is characterized by a decrease in seismic velocities, which indicates an exponential decrease in temperature towards the outside. The D ″ layer is therefore also referred to as the thermal boundary layer. Such a thermal boundary layer is a prerequisite for the development of instabilities, from which upflows of hot material can arise, the so-called mantle plumes .

Naming

The designation D ″ layer goes back to an earlier nomenclature of the earth structure according to Keith Edward Bullen , which used the first letters of the alphabet for the earth layers. Thereafter, the letter D designated the entire lower mantle of the earth. After the transition zone was discovered, this layer was divided into D ′ and D ″.

Individual evidence

  1. P. Keary & FJ Vine: Global Tectonics , 2nd ed., Blackwell Science, Oxford 1990, ISBN 0-86542-924-3
  2. M. Sylvander, B. & A. Ponce Souriau: Seismic velocities at the core-mantle boundary inferred from P waves diffracted at the core , 1997, Phys. Earth planet. Int., Vol. 101, pp. 189-202
  3. ^ Leszek Czechowski: The Origin of Hotspots and the D ”Layer. In: H. Montag, C. Reigber (Ed.): Geodesy and Physics of the Earth. International Association of Geodesy Symposia, vol 112.Springer , Berlin / Heidelberg 1993, doi: 10.1007 / 978-3-642-78149-0_91
  4. ^ Trond H. Torsvik, Mark A. Smethurst, Kevin Burke, Bernhard Steinberger: Large igneous provinces generated from the margins of the large low-velocity provinces in the deep mantle. Geophysical Journal International. Vol. 167, No. 3, 2006, pp. 1447–1460, doi: 10.1111 / j.1365-246X.2006.03158.x (Open Access)
  5. T. Kito, F. Krüger & H. Negishi: Seismic heterogeneous structure in the lowermost mantle beneath the southwestern Pacific , 2004, J. Geophys. Res., Vol 109, B09304, doi : 10.1029 / 2003JB002677 .
  6. ^ X. Ding & DV Helmberger: Modeling D "structure beneath Central America with broadband seismic data , 1997, Phys. Eart Planet. Int., Vol. 101, pp. 245-270.
  7. S.-H. Shim, TS Duffy, Raymond Jeanloz & G. Shen: Stability and crystal structure of MgSiO 3 perovskite to the core-mantle boundary , 2004, Geophy. Res. Lett., Vol 31, L10603, doi : 10.1029 / 2004GL019639 .
  8. Klaus Strobach: Our Planet Earth: Origin and Dynamics , Gebr. Borntraeger, Berlin, Stuttgart 1991, ISBN 3-443-01028-8
  9. ^ A b Frank D. Stacey: Physics of the Earth , 3rd ed., Brookfield Press, Brisbane 1992, ISBN 0-646-09091-7