The transition from the earth's mantle to the lower core of the earth is referred to as the core-mantle boundary (rarely referred to as Wiechert-Gutenberg discontinuity ; not to be confused with the Gutenberg discontinuity of the upper mantle) . Since the speed of the seismic waves drops drastically on it (see second figure), it can be recognized and mapped using seismological methods. According to the reference earth models commonly used today , it is located at an average depth of 2,889 km (according to IASP91 ) or 2,891 km (according to PREM ).
This very distinctive seismic discontinuity of the earth's body was named after Emil Wiechert and Beno Gutenberg , two of the most important German seismologists and geophysicists . At the end of the 19th century, Wiechert and Svante Arrhenius concluded from the dimensions of the earth, its gravity and the gravitational effects of the crustal rocks that an iron core existed . Its radius was determined in 1913 by Gutenberg from seismological measurements. Its calculation is still considered exact today.
The existence of a heavy core had previously been postulated because of the earth's mean density (5.52 g / cm³), which is twice that of granite . Wiechert took this discovery as an opportunity to theoretically and practically calculate two-shell equilibrium models of the earth's interior (earth core + mantle) ( Wiechert model ). As Karl Ledersteger was able to show around 1965, this division of the inner earth figure lies between an earth ellipsoid with constant density ( Maclaurin ellipsoid ) and the Wiechert model, but closer to the latter.
The boundary layer between the earth's mantle and the outer core of the earth is the most pronounced of all the earth's discontinuities. The speed of propagation of the P wave here abruptly decreases from almost 14 km / s to around 8 km / s, while S waves do not propagate at all in the outer core of the earth. This leads to the assumption that the outer core must be liquid.
Above the Wiechert-Gutenberg discontinuity is the D ″ layer , which was discovered much later and, by definition, forms the lower end of the earth's mantle. This transition zone is between 200 and 300 km thick and is characterized by a strong temperature gradient and a heterogeneous structure.
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