MohoroviÄić discontinuity
The Mohorovičić discontinuity [ mɔhɔrɔʋitʃitɕ ], usually referred to simply as the Moho, is an interface within the shell construction of the earth; they separates the crust from the mantle. On one hand, this transition is a discontinuity of the speed of seismic waves decisive, on the other hand, it means a change in the mineralogical composition of the rock. Not always fall between these two limits together, in some cases, is not the transition leaps and bounds, but continuously over a larger area.
Among the oceans of the depth range of the Mohorovičić discontinuity is quite low ( ~ 10 km or less) beneath the continents, however, is relatively high ( ~ 50 km and more ).
Discovery
The discontinuity was discovered in 1910 by the Croatian geophysicist Andrija S. Mohorovičić, as he analyzed seismograms of the earthquake of Pokupsko near the Croatian capital Zagreb on 8 October 1909. Mohorovičić was noticed that some P and S earthquake waves (longitudinal or transverse waves ) was significantly later arrived as expected, and he suspected that they had been bent at a border in about 54 km depth. Later studies confirmed the existence of such a layer boundary at a depth of 30-50 km under continents and in about 5-7 km under oceans.
Below this limit, the physical thicker mantle, which is characterized by a higher seismic propagation rate begins.
Geosciences background
The Moho discontinuity is characterized by an abrupt increase in seismic velocity and hence the acoustic impedance. Seismic waves are therefore reflected at this boundary layer partially. The reflections can be observed in a seismogram, which allows detection of the density jump. Caused the difference in speed by the differences in the density of crustal and mantle rocks.
The mantle is composed of different minerals, all of which have a high density of 3.2 g / cm ³ and are collectively referred to as peridotite. The crust, however, consists in oceanic areas mainly of basalt and related rocks ( ρ ≈ 2.9 g / cm ³), on the continents from the even lighter granite and granodiorite ( ρ = 2.7 g / cm ³ ) and gneiss, and in deeper areas of amphibolite or gabbro. Since the peridotite the jacket has a significantly higher density, the seismic waves propagate therein away faster than in the crust, which is influenced in accordance with the Snell's law of refraction, the beam path of the wave. Since the velocity contrast at the Moho discontinuity is usually quite pronounced, and also involves a mostly low depth interval, it is one of the most prominent discontinuities of the earth.
Mathematical argument
Imagine that in the underground as the epicenter of a designated location ( eg in Turkey ) at the point A longitudinal seismic waves arise (P- waves), which are measured by a Erbebenwarte at location B (as in Potsdam). The recorded signal here then does not correspond to the geometrically shortest path between A and B, but must be weighted by the speed (more precisely, with the reciprocal of velocity) is equivalent in optics Fermat's principle of the so-called " shortest effective path length ".
Accordingly, under certain circumstances, an apparent detour that leads across the Moho - singularity and the adjacent parts of the mantle and will not appear optimal from the length of her, in truth - ie after weighting - optimal in the sense of the arrival time indicated by dots terms of the strength of the quake and the respective measuring apparatus depend (see also calculus of variations ).
The depth variation of the interface
Strictly speaking, to distinguish between two different definitions of the Moho surface must: The petrological Moho relates solely to the change in the rock (see figure), while the seismic Moho by the speed increase of P waves from 6-7 to about 8 km / s is defined. Because the rate is essentially determined by the density (and the elasticity) of the rock is determined to fall the depths Moho by two definitions together normally. In some cases, however, deviations can occur for example if the transition is not leaps and bounds or by entry of additional chemical components of the mineral structure and thus the respective modulus of elasticity of the rock is changed.
Under the oceans the Moho surface is only 5-7 km deep on average, under continental areas is their depth of 30-50 km. Near the coast the crust already ends in 20-30 km depth, in Saxony, for example, it is 32 km, but under the mighty mountain ranges (such as the Andes and the Himalayas ) they can also 70-80 km down range. The cause of the varying depth of the Mohorovičić discontinuity is the diverse origin of the earth's crust: The oceanic crust formed by melting of ascending mantle material at mid-ocean ridges, continental crust, however, even today with magmatism on the continental margins ( subduction or collision zones). The depth of the Moho discontinuity correlates approximately with the height of the topography, because the lighter crust like a floating iceberg, the deeper in the upper mantle " dips ", the higher its towering mountains. ( See also the mountain root and principle of isostasy. )