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a single system of major tectonic crustal fractures, or rifts, on the earth’s surface. The rifts belong to the mid-oceanic ridge, which stretches for more than 60,000 km from the central regions of the Arctic Ocean along the axis of the Atlantic Ocean and on through the Indian Ocean to the eastern part of the Pacific. The rifts of this ridge are longitudinal depressions having steep walls. The great length of the rift zone shows that extension has been just as important as compression in the development of the structure of the earth’s crust.
The rifts of the mid-oceanic ridge continue onto the continents; these rifts include the belt of rifts in Arabia and Africa and those of the Great Basin in the Cordilleras of North America. These branches of the rift zone are young, dating from the Neogene and Anthropogene (Quaternary) periods. The age of the system of rifts on the ocean floor can only be approximated; in the opinion of some researchers, these rifts are from the Mesozoic and Cenozoic eras. The rift zone exhibits a high level of seismicity throughout and is associated with volcanic activity. The rift zone, geosynclines, and cratons are the chief structural elements of the earth’s crust.
The discovery of the rift zone in the 1950’s was one of the main factors in the development of new tectonic hypotheses of mobilism; the new global tectonics, for example, proposed that the system of ocean rifts corresponds to the boundaries along which the continents first separated from each other. The rift zone, however, can also be viewed as a new formation on the ocean floor related to the rising of magma from the upper mantle; as a result of this process, the earth’s crust was lifted and only to a small degree extended.
V. V. BELOUSOV