Contraction theory

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The contraction theory (also cooling or shrinking theory ) is an explanatory model of the earth's development and mountain formation , which prevailed in the geosciences from around 1860 to 1950. The theory was successively developed by various geologists - often in conflict with other views such as expansion , oscillation or undercurrent - in order to explain the large-scale processes in the earth's crust and in the uppermost part of the earth's mantle .

Until around 1960, contraction theory was the classic view of geotectonics on which most geology textbooks were based (see also fixism ), although there have been doubts since the turn of the century. It has only been definitively refuted since the paradigm shift to plate tectonics , which began in 1912 with Alfred Wegener's hypothesis of continental drift, but did not gain acceptance until the 1960s. The exploration of ocean floors (sea floor spreading), isostasis and radioactivity in the earth's interior have also contributed to this upheaval in earth sciences .

Contraction of the earth by cooling

The central idea of ​​the contraction hypothesis was that the earth was hot and glowing liquid at the beginning and cools down over time. As a result, the earth's body slowly shrinks - but unevenly, because not all areas of the surface and the layers below cool down at the same rate. These differences create tensions on the earth's surface and in the earth's mantle, which lead to earthquakes , cracks, folds and subsidence in the earth's crust.

Many textbooks have compared the contraction of the earth to an apple , which wrinkles as it dries from the loss of water inside.

The shrinkage thesis was first expressed by Giordano Bruno , taken up by Leibniz around 1700 and geologically justified by Élie de Beaumont in 1830 . It is based on known facts:

  1. Mining: The temperature increases inwards everywhere
  2. Rocks: in the interior of the earth roughly correspond to those on the surface of the earth
  3. Laplace planetary theory: the earth as a cooling celestial body, whereby
  4. the earth's mantle has to cool down more slowly than the earth's crust.

The most important researchers on the topic were the Austrian Eduard Suess (1831–1914), the Swiss Albert Heim (1849–1937) and the German Hans Stille (1876–1966), see section Main exponents of the contraction theory .

Heim estimated the shrinkage of the earth based on the Alps at 0.3%, later geologists at half of it. But this is hardly sufficient for older mountains. Further objections concerned the large horizontal faults and the peripheral location of the fold mountains , while the discordances could be explained well by locally different cooling and there was no better theory overall.

When it was discovered around the turn of the century that radioactivity in the interior of the earth counteracts or even eliminates cooling, some geologists began to question the contraction theory and to develop " mobilism ". This notion sees the horizontal movements and foldings not as a result of vertical shifts, but as a result of horizontal forces such as continental drift .

Degassing contraction theory

The Entgasungskontraktionstheorie (Engl. Degassing theory ), is a particular variant of the contraction theory. The theory is based on the observation of that day many millions m³ gases from the earth's crust to escape. This loss of volume and energy is said to result in shrinkage and, as a result, folding, mountain formation and fracture formation. The term was introduced into scientific discourse in 1952 by Thomas FW Barth .

Main proponent of the contraction theory

Gottfried Wilhelm Leibniz (1646–1716) advocated the idea that the earth must have been smooth in the beginning, because God would never create anything without form and structure. Later researchers built on this idea and developed various theories.

In 1829 , Élie de Beaumont did not consider volcanism to be the strongest cause of "earth revolutions" and mountain formation, but the high temperatures in the interior of the earth. Because "the inequality of the cooling forces the shell to constant narrowing so that the contact with the shrinking inner mass is not lost."

In 1841 Beaumont applied this to the Upper Rhine Rift, but his tectonic sketch shows incorrect marginal warps. According to A. Andreae (1887), they would speak in favor of strain instead of contraction. However, in Beaumont's view it was argued that sinking clods and folds balance each other out.

James Dwight Dana first presented the idea of ​​uneven contraction in 1873 because rocks have different thermal conductivity . This would also create mountains and earthquakes. Large-scale differences in cooling could explain the sinking of oceanic basins between the continent blocks.

Albert Heim (1849–1937) tried to calculate the reduction in the circumference of the earth by fictitiously smoothing all mountains. The basis of his considerations was that the existing area must have been just as large as that before the earth's contraction process. His results said that the circumference of the earth

  • without the Jura mountains it would be 5000–5300 meters taller,
  • without the Alps it would even be 120,000 meters taller.
  • According to this, the formation of the Alps would have reduced the circumference of the earth by only 0.3 percent, which made the often discussed question of causative forces less acute.

"If we estimate the folding of the other mountains cut by the Central-Alpine-Meridian in their convergence, we find that the reduction in circumference due to the entire mountain formation has so far been less than 1%."

Heim said that the earth had cooled down by about 500 ° C since its formation, but did not believe in further horizontal shifts and subsequent mountain formation. Here Otto Ampferer opposed him energetically. Heim explained the fact that earthquakes would continue to exist with changes in load, weathering and erosion.

Eduard Suess ' (1831–1914) idea was that when displaced by the high stresses, crevices arise, whereby the size of the crevices that arise during an earthquake is out of proportion to the extent of the tremor. There are two types of shifts:

  • vertical (sinking)
  • horizontal (sliding and folding)

The vertical displacements create the horizontal tensions, as the earth's interior becomes increasingly narrow. The volcanism is small when shifted horizontally and large when shifted vertically.

"It is the collapse of the earth that we witness."

If the horizontal tensions were to be perfectly balanced, the earth's crust would be a free, independent vault, that is, regular, smooth and completely covered with water. The interior of the earth would then contract independently of the surface. In summary, this means that the cooling of the earth's interior causes the earth's crust to collapse. This leads to mountain formation, which in turn leads to volcanism and earthquakes.

Hans Stille (1876–1966), an important pioneer of mobilism , explained the various phases of mountain formation through periodic shrinkage of the earth's body - see Stille cycle and cycle theory . As a proponent of the theory of contraction, however, he became an opponent of Wegener's continental drift (like Hans Cloos for other reasons ) .

See also

Web links

literature

  • George Gamow : The Life Story of the Earth. Past, present and future. F. Bruckmann, Munich 1941, Chapter VI.
  • Alan H. Anderson: The Drift of the Continents. Alfred Wegener's theory in the light of new research. FA Brockhaus, Wiesbaden 1974, ISBN 3-7653-0263-5 , pp. 16-27.

Individual evidence

  1. ^ André Cailleux : The Unknown Planet. Anatomy of the Earth (= Kindler's University Library. ). Kindler, Munich 1968, pp. 223-231.
  2. ^ Hans Murawski , Wilhelm Meyer : Geological dictionary. 11th, revised and expanded edition. Elsevier - Spektrum Akademischer Verlag, Munich 2004, ISBN 3-8274-1445-8 .
  3. ↑ Helmut Hölder : Brief history of geology and paleontology. A reader. Springer, Berlin et al. 1989, ISBN 3-540-50659-4 , p. 78 f.
  4. ↑ Helmut Hölder: Brief history of geology and paleontology. A reader. Springer, Berlin et al. 1989, ISBN 3-540-50659-4 , p. 81.