Undercurrent theory

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A number of geotectonic and geophysical hypotheses that deal with the possibility of large-scale flow movements below the rigid earth crust are summarized as the undercurrent theory (also known as subfluence theory ) . The movements of viscoplastic, magmatic masses in the subsurface should be responsible for the large geotectonic structures on the earth's surface, such as fold mountains , deep sea channels and regional crevice systems . From such ideas , the model of mantle convection developed around the 1960s, within the framework of plate tectonics .

Undercurrents in the context of the geosyncline theory

The concept of undercurrents goes back to the Austrian geologist Otto Ampferer . As early as 1906 in the Alps , he developed the hypothesis that the upper crust with its sedimentary cover was only a relatively thin, flexible layer that floats on molten rock (similar to the skin on boiled milk). The overlying crust is dragged along by downward-directed magma flows, which leads to intensive narrowing and thrusting of the rock layers in the folds of the mountains. However, this hypothesis initially received little attention from experts, as it was unable to explain the exact process of mountain formation in the Alps satisfactorily. In addition, Ampferer made no statements about the causes of the mass movements.

The Graz geophysicist Robert Schwinner (1878–1953), on the other hand, developed ideas of thermally induced convection currents in the lower crust (or in the upper mantle ), which are generated by the heat gradient between the earth's core and earth's crust , since 1919 . In 1935, Schwinner spoke of a "friction coupling" between the magma flowing at a depth of up to 400 km and the overlying earth's crust. He suspected that the fold mountains formed where cooling magmas sink under the "cooling floors" of the oceans , and crustal rocks drag with them into the depths ( cyclones ), while heated magmas rise below the continents, which emerge in volcanically active regions and at the Surface lead to clear signs of strain and fracture in the crust ( anticyclones ).

Up until the 1960s, attempts were made to integrate the concept of igneous undercurrents into the classical geotectonic model of geosyncline theory . Ernst Kraus (1889–1970) distinguished between convection currents in the lower crust ( hyporheon ) and a second, deeper system of large-scale convection cells ( bathyrheon ). The formation of large tectonic structures, such as island arches or the conspicuously curved shapes of many fold mountains (for example the Carpathian Arch ), he attributed to the summation or differentiation of the same or opposite flow movements in the hyporheon and bathyrheon. Basically, he postulated mountain formation in narrow, elongated subsidence flows ( geosynclines ) under the influence of sinking convection flows.

The volcanologist Alfred Rittmann made rising, viscous convection currents responsible for the elevation of the continental crust (" sial "). The rubble resulting from the exposure of the continental shields then collects in the deep sea in the border area between the continental and oceanic crust (“ Sima ”). In contrast to the classical geosyncline theory, Rittmann did not primarily blame the additional weight of the accumulated sediments for the further sinking of the geosyncline, but rather mass flows that were initially sinking towards the continents. In a self-reinforcing process, the continental crust is thereby lifted even further, etc. By moving such cooler crust material into the deep sea channel, the thermal equilibrium in the magma zone would be disturbed and, in addition, compensatory currents directed towards the ocean, which now combined with those directed towards the continent lower. Ultimately, this leads to the formation of a "sial bulge" in the depths, and to a swallowing ( subduction ) of both oceanic and continental crust, to the folding of the accumulated sediments, to the transformation and melting of the rocks, which in turn form magma Participate in convection currents. The disturbance of the isostatic equilibrium caused by the relatively light sial bulge that has been pulled down is finally eliminated by the lifting of the entire folded complex, the actual mountain formation .

Another proponent of this theory was the geologist Hans Georg Wunderlich , who described the convection current as the "weather inside the earth".

Undercurrents in the context of plate tectonics

Since the paradigm shift in the geosciences away from geosynclinal theory towards plate tectonics, numerous suggestions have been made as to how the concept of igneous undercurrents could be integrated into the new geotectonic model. In addition to the methods of geophysics ( seismics ) and experimental petrology , computer simulations were increasingly used .

Initially, models were favored in which ascending currents were below the mid-ocean ridges and continental rift systems , and descending currents below the subduction zones . On the other hand, the shape of the undercurrents was increasingly viewed as cylindrical or conical, that is, in the shape of a closed body of revolution, which cannot easily be reconciled geometrically with the first-mentioned model.

There was also long disagreement about the energy source (s) that drive the undercurrents. The generation of heat through radioactive decay or through physico-chemical phase transitions in the crystal structure of certain minerals has been suggested . Other authors considered a supply of heat from below superfluous; cooling from above alone was sufficient to set the convection rollers in motion, or a purely mechanical process was suggested. In the 1970s, it was also discussed at length whether convection currents only form in the upper mantle or in the entire mantle. Models with one, two or three superimposed flow stories have been proposed. Keith Runcorn suspected that the number and shape of the mass flows had changed in the course of the Earth's history, and that the downward transport of heavy (iron and nickel-rich) minerals by convection currents contributed to the enlargement of the Earth's core . Thomas Nelson and Peter Temple, however, suspected a single main stream, with poles of rotation near the earth's poles, which moved in an easterly direction through the entire Upper Mantle.

Nazario Pavoni (1969) reckoned with two opposite, main centers of ascending currents, under Africa and under the Pacific Ocean. This model was described in 1987 by Gerhard Bischoff for everyone to understand. The stretching on the mid-ocean ridges therefore only takes place on the moving mantle material. No material from the asthenosphere rises below the mid-ocean ridge, but only flows below the sea floor. This should explain why the African plate is almost exclusively surrounded by mid-ocean ridges, and the resulting new oceanic crust is nowhere subducted, while the Pacific plate is almost completely surrounded by subduction zones (the " Pacific fire belt ") and does not contain any continental crust .

literature

  • Undercurrent hypotheses (convection currents). In: Rudolf Hohl (ed.): The history of the development of the earth. With an ABC of geology. 6th edition. Verlag für Kunst und Wissenschaft, Leipzig 1985, ISBN 3-7684-6526-8 , pp. 246–249.

Individual evidence

  1. ↑ Christoph Hauser: Otto Ampferer and Alfred Wegener - two pioneers on the way to the theory of plate tectonics, Federal Geological Institute 2005
  2. ^ Erich Thenius: The Austrian Geologist Otto Ampferer as founder of the sea-floor spreading concept
  3. ↑ Wolf-Christian Dullo, Fritz A. Pfaffl: The theory of undercurrent from the Austrian alpine geologist Otto Ampferer (1875-1947): first conceptual ideas on the way to plate tectonics. doi : 10.1139 / cjes-2018-0157
  4. ↑ Hans Georg Wunderlich: The new image of the earth. Fascinating discoveries of modern geology . Hoffman and Campe, 1975, ISBN 3-455-08993-3 .
  5. ↑ The earth as a puzzle . In: The time . No. 31/1975.
  6. ↑ Thomas H. Nelson, Peter G. Temple: Mainstream Mantle Convection; A Geologic Analysis of Plate Motion. In: American Association of Petroleum Geologists Bulletin. Vol 56, No. 2, 1972, Col. 226-246.
  7. ^ Nazario Pavoni: Zones of lateral horizontal displacement in the earth's crust and conclusions on global tectonics that can be derived from them. In: Geologische Rundschau. Vol. 59, No. 1, 1969, pp. 56-77. ( doi: 10.1007 / BF01824942 )
  8. ^ Nazario Pavoni: The pacific-antipacific bipolarity in the structure of the earth and its geodynamic interpretation. In: Geologische Rundschau. 74, 1985, pp. 251-266.
  9. ^ Gerhard Bischoff: An extended, global model of plate tectonics. Spectrum of Science, March 1987; also published in: Peter Giese (Ed.): Understandable Research - Geodynamics and Plate Tectonics . Spectrum Academic Publishing House, 1995.