1700 km discontinuity

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The 1700 km discontinuity is a scientifically disputed layer boundary in the lower mantle . Its existence was already deduced from seismological studies at the beginning of the 20th century . This is a seismological boundary layer that is created by a change in seismic velocities, i.e. H. the speed of propagation of seismic waves .

Since this structure has been observed in investigations of records from various worldwide distributed measuring stations, it was interpreted as existing globally. It was named after the average depth of its occurrence at around 1700 km. However, since the discontinuity has so far only been weakly proven in very few studies, it is controversial. A study of relatively high frequency seismic waves from 2003 found no evidence of the 1700 km discontinuity, so an abrupt increase in speed with high impedance at this depth is unlikely. The possibility of a speed gradient , i.e. a slow increase over several tens of kilometers, cannot be ruled out. Although such a gradient could be observed at low frequencies , it would be invisible to high-frequency seismic signals.

One can only speculate about the cause of an increase in speed at this depth, since so far only a few laboratory experiments have been carried out under realistic temperature and pressure conditions. A phase transformation of the wustite  (FeO), one of the two main components - in the form of the mixed phase magnesiowustite (Fe, Mg) O - of the lower mantle was suggested as a possible explanation . According to this idea goes something wustite the observed depth of the NaCl - crystal structure in the NiAs about structure. The associated increase in density could explain the observed speed contrast.

Individual evidence

  1. ↑ a b c Lev Vinnik, Mamoru Kato, Hitoshi Kawakatsu: Search for seismic discontinuities in the lower mantle . In: Geophysical Journal International . tape 147 , no. 1 , 2001, p. 41-56 , doi : 10.1046 / j.1365-246X.2001.00516.x .
  2. ↑ JC Castle, RD van der Hilst: Searching for seismic scattering off mantle interfaces between 800 km and 2000 km depth . In: Journal of Geophysical Research . tape 108 , 2003, p. 2095 , doi : 10.1029 / 2001JB000286 .
  3. ↑ HK Mao, J. Shu, Y. Fei, J. Hu, RJ Hemley: The wüstite enigma . In: Physics of the Earth and Planetary Interiors . tape 96 , 1996, pp. 135-145 .