Contraction Hypothesis

in geotectonics, a wide-spread 19th-century hypothesis (the French geologist L. Elie de Beaumont and the Austrian geologist E. Suess) that regarded the folding of layers of rock as the result of a gradual decrease in the radius of the earth and the surface of the earth’s crust. Prolonged cooling of the earth was considered to be the reason for the reduction in its dimensions, since the contraction hypothesis proceeded from the now rejected hypothesis of the formation of the planets from molten masses that became covered with a hard crust as they cooled.

With the discovery of the decay of radioactive elements, accompanied by the liberation of heat, the physical basis of the contraction hypothesis was found to be unsubstantiated. At the same time, the inability of the contraction hypothesis to satisfactorily explain tectonic processes and, in particular, magmatic activity was revealed. Nevertheless, a number of scientists (the Austrian L. Kober and the German H. Stille) continued to develop the contraction hypothesis even in the 20th century. Even today the stressed condition of rock in the interior of the earth and changes in the earth’s speed of rotation are viewed as evidence of a reduction in its size. However, geologic data show that, along with contraction in the lithosphere, expansion— which leads to the formation of rift zones—is widespread. There-fore, concepts of the pulsating volume of the earth (the pulsational hypothesis) and the expanding earth, as well as of the coexistence in the lithosphere of zones of expansion and contraction that compensate one another (new global tectonics), have been proposed in contrast to the contraction hypothesis.