Mobilism

a theory that posits large (up to several thousand km) horizontal displacements of the continental blocks of the earth’s crust (the lithosphere) relative to one another and to the poles in the course of geological time. Mobilism is opposed to fixism, the hypothesis that denies such displacement and assigns the chief role in the development of the earth’s crust to vertical movements. The idea that continents are mobile arose in the 19th century, but the scientific hypothesis of mobilism was first formulated in 1912 by the German geophysicist A. Wegener, who called it the theory of continental drift.

The modern version of mobilism, known as new global tectonics, or plate tectonics, is largely based on studies of the relief of the ocean floor, magnetic fields of the oceans, and paleomagnetism= According to these theories, there is a slow displacement (averaging 1–5 cm per year) of the monolithic plates, including not only the continental blocks but also the vast areas of oceanic crust adjacent to them and the top layer of the mantle. The plates move away from the midocean ridges toward the young folded belts (the Andes and Himalayas) and the island arcs. There, at the contact of the two mobile plates, the leading edge of one of them plunges to depths of up to 700 km under the other along inclined faults that have high levels of seismic activity. Folds and overthrusts form in the continental crust of the other plate through compression. Rifts form at the back of the moving blocks, along the axis of the midocean ridges. The rise of matter from the upper mantle into the “opening” formed when the plates move apart and the subsequent extrusion of basalt lavas form a new layer of the crust in the rift zone, thereby expanding the ocean floor.

Paleotectonic reconstructions have been proposed on the basis of the similarity in the geological structure of the disconnected parts of the Paleozoic continents of Gondwanaland and Laurasia and the matching contours of their continental slopes. Gondwanaland encompassed South America, Africa, the Indian subcontinent, Australia, and Antarctica, and Laurasia comprised North America, Europe, and the northern half of Asia. These reconstructions are reinforced by paleoclimatic and paleomagnetic findings showing that the different parts of Gondwanaland were much closer to the south pole at the end of the Paleozoic than they are today and that North America lay next to Europe. The movements that occurred during the Mesozoic and Cenozoic resulted in the almost complete disappearance of the geosynclinal ocean Tethys and the formation of the Indian and Atlantic oceans. Continental mobility is generally thought to be caused by convection currents of mantle material.