Gutenberg discontinuity

This article deals with the Gutenberg discontinuity in the upper mantle. For the Wiechert-Gutenberg discontinuity see core-mantle boundary .

The Gutenberg discontinuity (named after Beno Gutenberg , one of the most important German seismologists and geophysicists) represents the lower edge of the lithosphere and thus the solid outer area of ​​the earth , which has broken up into various tectonic plates. Below the lithosphere is the asthenosphere , which consists of softer, flowable rock material because it has partially melted .

The Gutenberg discontinuity thus represents the boundary between these two layers of the earth, which can be proven by seismological studies, since the seismic speeds in the asthenosphere decrease significantly. Strictly speaking, the term Gutenberg discontinuity (often also referred to as G-discontinuity for short ) is only used in oceanic areas, while among the continents one often speaks only of the lithosphere-asthenosphere boundary . The reason is that the speed contrast in oceanic areas is usually sharper. The decrease in seismic speed is faster here than under continents, which means that the signal of the G discontinuity is more pronounced in the seismogram .

Physically, the decrease in speed is caused by melting processes, so that ultimately the temperature prevailing in the upper mantle is the defining factor for the depth of the discontinuity. If the temperatures are higher, as is the case in oceanic regions, the melting processes start at a shallower depth than in continental areas, for example. These are older and have therefore cooled down further. Under continents the average depth of the boundary layer is 110 km, under very old areas, the so-called cratons , the depth can increase to 150 km.

Oceanic areas, on the other hand, are much younger and hotter. The average depth of the Gutenberg discontinuity here is 60 to 90 km. It is flattest on the mid-ocean ridges , where oceanic lithosphere is newly formed from rising magma . The further the newly created plate moves away from its place of origin, the more it cools down and increases in thickness accordingly. The depth of the G-discontinuity allows conclusions to be drawn about the age of an oceanic lithospheric plate at a certain location.