Geosyncline
A large basin or a subsidence area in the earth's crust is called a geosyncline or geosyncline . The term is part of the geosyncline theory that is now considered obsolete. It was coined in 1873 by the American geologist James Dwight Dana , spelled Geosynclinale .
Geosyncline
The term geosyncline is derived from the Greek συγκλίνειν ( synklinein ), which can be translated as “ leaning towards each other”. This is based on the idea that the two sides of a geosyncline point towards each other in the shape of a trough or fold. In contrast to the syncline , which describes part of a geological fold , the geosyncline was the entire deposit space of the later folded sediments .
The geosyncline was assumed to be an elongated, stationary subsidence zone of the earth's crust over longer periods of time , which had at least the spatial extent of the mountains that later emerged from it, served as a sedimentary basin for a longer period of time and was later folded and lifted out .
In one word, the term 'geosyncline' encompasses many relationships that can be derived from geological structures and the rocks of mountains. For historical reasons and because of its brevity, it is still used in some cases today, although the geosynclinal theory behind it is considered outdated.
A distinction is made between several types of geosynclines:
-
Orthogeosyncline : Geosyncline with strong subsidence, usually several 100 km and over 1000 km long, is folded
into an orogen
- Eugeosyncline : core area of an orthogeosyncline, deeply marine, strong initial magmatism, first affected when folding, later in the interior of the resulting mountains (internids)
- Miogeosyncline : Outside area of a geosyncline, flat marine, magmatism hardly present or absent, attached to the orogen after the eugeosyncline, later on the front (more rarely also on the rear) of the resulting mountains (externids)
- Parageosynklinale : Geosynklinale with little depression and thin filling, mostly irregularly shaped
Other names of geosynclines were less common:
- Monogeosyncline: long and narrow sea space that sinks over a long period of time, separated from the ocean by a threshold
- Polygeosyncline: broad ocean space divided into several partial geosynclines, sinking over a long period of time, separated from the ocean by a threshold
- Mesogeosyncline: abyssal sea space between two continental blocks
- Parageosynkline: in contrast to the parageosynkline, a deep sea space on the edge of a continent, separated from the open ocean by chains of islands
Geoanticline
The counterpart to the Geosynklinale is the Geoantiklinale (also called Geantiklinale), which denotes wide-span, flat uplift areas. Since both were explained by bending of the earth's crust, they are mutually dependent. The geoanticlines are constantly being eroded by exogenous (external) forces such as erosion, but the uplift works so successfully against it that their height remains. The rubble is poured into the geosyncline. A structure like the geosyncline is not common here.
The Alpine geosyncline was the name given to the sea trough in which the material collected that was later unfolded to form the Alps .
The geosynclinal theory
The Geosynklinaltheorie was up to the paradigm shift of the geology in the 1960s, the significant tectonic model to explain the formation of mountains . In contrast to earlier theories of mountain formation, it was able to combine both the geological and geophysical knowledge of its time of origin without contradictions and to provide a chronological sequence of mountain formation, so that it was widely recognized.
On the basis of the geosyncline theory, Hans Stille built his theory of the Stille cycle around 1920 , which categorized various phases of mountain formation. It was not until the 1960s that the theory of plate tectonics prevailing today, thanks to new findings in the geology of the ocean floors, prevailed . It explains the mountain formation as a result of the collision of tectonic plates.
Basic assumptions
Geophysical Assumptions
The geosynclinal theory is based on basic geophysical assumptions that corresponded to the state of knowledge of the 1870s and are now considered obsolete:
- According to the theory of permanence, the position of the continents and thus also of the oceans is unchangeable. This was first challenged in 1915 by Alfred Wegener's theory of continental drift, but it was considered to be known as reliable until the 1960s.
- According to Beaumont's theory of contraction , the earth's crust should contract and wrinkle through slow cooling like the shriveling surface of a dried-up apple. This should create mountains and ocean basins. However, this theory had to be given up again after the discovery of radioactivity as an energy source within the earth.
Geological observations
The theory was also based on a number of geological observations that are still valid today:
- In mountains there are rocks that have been formed on the sea floor.
- Sediment stacks reach a thickness of several kilometers. This is more than the usual depth of seas near continents.
- The rocks are folded and pushed one on top of the other .
- Flysch and molasses were deposited after the marine sediments and were not fully included in the folding.
The geosyncline theory was able to explain these observations in accordance with the geophysical assumptions. The necessary drive for the formation of the geosyncline, the folding of the rocks and the uplift of the mountains was initially explained by the theory of contraction, after which it was eliminated by epirogenesis. These are long-term and large-scale deformations of the earth's crust, in which the storage conditions of the rocks are not significantly disturbed. The geosyncline theory remained internally consistent until new knowledge of the geology of the ocean floors was added.
Mountain formation according to the geosyncline theory
Formation of geosynclines
The formation of a geosyncline was seen as the first stage of mountain formation. Since the shifting of the continental plates and the possibility of new formation and subduction of the ocean floor were still unknown, the shortening of the earth's crust due to mountain formation was massively underestimated. The geosyncline was therefore assumed to be a relatively narrow, often elongated (up to well over 1000 km), deepening subsidence zone over long geological periods (more than 100 million years). The weathering and erosion debris from neighboring continents (the uplifted geoanticlines) was supposed to pile up in it through sedimentation . The inundation of the subsidence zone by the sea could explain the occurrence of ocean floor in mountains, its steady subsidence explains the thickness of the marine sediments. Penetrating volcanic rocks could also contribute to the filling of the geosynclines.
folding
The next stage of mountain formation was considered to be the folding of the sediments still deep down and the formation of thrusts . The folding was explained by the fact that material sliding from the geoanticlines into the geosynclines should cause compression, which could also explain the observed horizontal movements of clods at least to a limited extent. In addition, the folding was initially explained by the theory of the cooling of the earth. At the same time as the folding, flysch was poured into the geosyncline. At that time, the mountains should not rise out of the sea, or at most in the form of a few islands.
Uplift and erosion
The elevation of the mountains should only take place after the main phase of folding has been completed. The geosyncline thus became a highland that is subject to erosion. The rubble was deposited on the edge of the mountain as molasses , partly still folded and lifted up with it. This was later supplemented by silence to the effect that after the uplift was complete, the erosion continued until a craton was formed . From a geomorphological point of view, this created a hull surface .
literature
- James Dwight Dana: On some results on the Earth's contraction from cooling, including a discussion of the origin of mountains and the nature of the Earth's interior . American Journal of Science, Ser. 3, 5, pp. 423-443.
- Wolfgang Frisch , Martin Meschede: Plate tectonics - continent shifting and mountain formation. Darmstadt 2005 . ISBN 3-89678-525-7 .
See also
Individual evidence
- ^ Robert H. Dott: James Hall Jr. 1811-1898 . In: National Academies Press (Ed.): Biographical Memoirs . tape 87 . Washington, DC 2005, p. 13 f . ( Online [PDF; 2.9 MB ; accessed on June 20, 2010]). PDF; 2.9 MB ( Memento of the original from September 1, 2006 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Klaus Thalheim: Lecture material - Mineral raw materials: Mineral and deposit formation . Ed .: TU Dresden, Institute for Geotechnics, Professorship for Applied Geology. 2005, p. 38 ( tu-dresden.de [PDF; 3.1 MB ]).
- ^ Hans Murawski, Wilhelm Meyer: Geological dictionary . 11th edition. Elsevier / Spektrum, Heidelberg 2004, ISBN 3-8274-1445-8 , pp. 262 .
