Conrad discontinuity

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The Conrad discontinuity corresponds to the sub-horizontal plane that takes place in the continental crust in which the seismic wave velocity increases in discontinuous mode. This plane is observed in various continental regions in a depth of 15 to 20 km, however not found in oceanic region. The Conrad discontinuity is not so expressive as the Mohorovičić discontinuity, and absent in some continental regions. The discontinuity is considered to be the border between the upper continental crust and the lower one. Up to the middle 20th Century, it is interpreted that in continental regions, the upper crust is constituted by felsic rocks such as granite, so-called SiAl, and lower one, by mafic rocks like basalt, SiMa. Therefore, the seismologists of that time considered that the Conrad discontinuity should correspond to the sharp contact between the chemically distinct two layers, that is, SiAl and SiMa. However, form the decade of 1960, a strong doubt to this opinion has emerged among the geologists. The exact geological significance of the Conrad discontinuity is still not clarified. The partial melting zones scattered in the continental crust is a candidate.