Mobilism

The Mobilism ( lat. Mobilis = mobile) refers to a mindset of Geotektonik and global geodynamics that of Alfred Wegener was established in 1912 and 1929. She sees not only vertical but also noticeable horizontal movements as the cause of the structure-forming processes in the lithosphere .

Other geologists at the turn of the century were among the pioneers of mobilism, which is now considered to be proven by evidence of continental drift . a. Otto Ampferer .

The continental drift according to Alfred Wegener

In his thesis of continental drift, Wegener assumed a formerly closed, but half thinner sialic earth crust , of which currently only remains in the form of the continents are present. For mechanical reasons, he rejected the shrinkage of the earth on which the fixism is based as the cause of mountain formation , as did the formation of the oceans through the disappearance of former land bridges .

In his opinion, the specifically lighter continents (average rock density of the upper crust of the earth about 2.7 g / cm³) "swim" in accordance with the law of swimming equilibrium ( isostasy ) in the somewhat denser, viscous underlay of the earth's mantle , more precisely in the asthenosphere . The engine for the geologically and biologically verifiable continental shifts, which can simultaneously cause both compression and strain structures (folds or fractures), is seen by him mainly in a west drift caused by earth rotation and tidal friction as well as in the " pole flight of the continents".

While, according to the displacement hypothesis, there is a passive “drifting” of the continents on the simatic base, Otto Ampferer (1875–1947) named in his undercurrent hypothesis for all movements of the earth's crust - based on the far-reaching thrusts in the folds - mass displacements under the solid earth sphere as the cause . For a long time mobilism could not prevail against the fixistic ideas. New investigation methods, especially the exploration of the seabed that began in the 1950s , then produced a wealth of arguments in favor of mobilism and continental drift . Today almost all geoscientists recognize the mobilistic conception, and with cosmic methods such as GPS and VLBI the annual movement rates (2–20 cm) are clearly proven.

Evidence of Paleomagnetism

The first reason for Wegener's conjecture was found in paleomagnetic findings , which reflect a shift in the continents in the geological past on the polar migration curves. Close relationships between the flora and fauna on both sides of the Atlantic and the Indic confirmed the hypothesis of continental migration. In the middle of the 20th century, the mid-ocean ridges were discovered at the bottom of the oceans - a system of connected mountain ranges with central trenches that spanned the entire globe and is characterized by increased magnetism, volcanism and frequent earthquakes .

The American geologist Harry Hammond Hess (1906–1969) conceived the model concept of seafloor spreading in 1960 as an explanation for the formation of table mountains in the deep sea, the so-called guyots . In 1963, Frederick John Vine (* 1939) and Drummond Hoyle Matthews (1931–1997) gave it a physical underpinning from the field of paleomagnetism. Strip-shaped magnetic anomalies had been discovered in the basaltic ocean floor (MORB), which were arranged symmetrically to the mid-ocean ridges and interrupted by transform disturbances . They document the regularly occurring pole shifts of the earth's magnetic field , which became apparent in the oceanic crust .

The model of plate tectonics

The hypothesis of ocean floor spreading based on the assumption of mantle convection led at the end of the 1960s - supported by newly compiled, detailed earthquake maps - to a new global tectonics, which under the name plate tectonics is the dominant model of thought in geosciences to this day. The authors Bryan Isacks , Jack Oliver and Lynn Ray Sykes (1968) initially derived the model from geophysical facts only, but it soon found decisive support through an extensive program of deep drilling . The drill cores showed, as evidenced by fossils , a clear increase in the age of the rock on both sides of the mid-Atlantic ridge system. In addition, a striking correspondence with the absolute age determinations of the basaltic oceanic crust drilled under the thin ocean floor sediments could be established. The observation of an increasing thickness of the sediment cover of the ocean floor with increasing distance from the mid-ocean ridges also supports the theory.

The theory of plate tectonics assumes that the lithosphere , consisting of the continental and oceanic crust and the uppermost mantle, is composed of a series of giant plates 70 to 100 km thick, which move relative to one another on the order of centimeters per year . The movement should not be passive - as in Wegener's analogy to drifting icebergs - but rather be caused by convective currents in the lower part of the upper mantle. With all the arguments in favor of it, plate tectonics (beyond the proven kinematics ) remains an unproven hypothesis because it cannot explain numerous phenomena and the forces on which they are based consistently. One of the main accusations raised against its advocates is the insufficient consideration of the geological facts systematically explored on the continents for the last 150 years and the over-generalization of inconclusive geophysical findings from the ocean area. Furthermore, the unclear mechanisms of plate movements as well as subduction and convection are criticized. Nevertheless, the theory is the best-founded, albeit not completely secured, model of global tectonics, which has already proven itself many times in its practical application in the successful prospecting of deposits.

Expansion theory

Unlike the plate tectonists, according to whom the earth's radius has not changed noticeably in the last 300 million years, the exponents of expansion theory assume a constant increase in the earth's volume (according to Egyed's textbook, it is in the millimeter range). The cause would either be a slow decrease in the gravitational constant (assumption by Paul Dirac 1938) or the heat production in the earth's interior, which should exceed the cooling-related loss. This could be caused by radioactive processes ( John Joly (1857–1933), Ref. (1) published 1925), or by mineralogical phase changes ( OC Hilgenberg , publ. 1933). According to the hypothesis, the oceans only formed at the beginning of the Cambrian , and the increase in radius since then gives László Egyed (publ. 1957) 500 km. The changes in density in the Earth's interior and the parallel phase transitions are associated with Dirac's hypothesis of the decrease in gravity, as a result of which Pascual Jordan (1902–1980) postulated an expansion applicable to all planets in 1964 .

As easily as today's continents can be traced back to a closed shell of a previously smaller earth according to the expansion concept ( Samuel Warren Carey (1911–2002), Lit. (2) published 1976), the scientific proof of the processes involved is just as difficult. The expansion theory is rejected by most geoscientists today.

literature

John Joly : Radioactivity and geology. An account of the influence of radioactive energy on terrestrial history. Constable, London 1909.
S. Warren Carey : The expanding earth (= Developments in Geotectonics. 10). Elsevier, Amsterdam et al. 1976, ISBN 0-444-41485-1 .