Oscillatory Movements of the Earth's Crust
Oscillatory Movements of the Earth’s Crust
slow crustal uplifts and subsidences occurring everywhere and occurring continuously. Owing to them, the earth’s crust is never immobile; throughout its history it has always been divided into rising and subsiding regions. Oscillatory crustal movements have occurred throughout all previous geological periods, and they are continuing at present. They determine the location of and change in the outline of the land and the sea on the earth’s surface, and they play an important role in the formation and development of the earth’s relief.
Different methods are used to study the oscillatory crustal movements of past geological periods, the Anthropogenic period, and the modern era. Geodetic methods are used for identifying movements that have occurred in historic times and are continuing. These are based on extended observations of the sea level or on repeated accurate levelings. These observations indicate that the usual rate of modern oscillatory crustal movements can be measured in millimeters (up to 2–3 cm) per year. The oscillatory crustal movements that began in the Neocene and created the present landforms are termed recent and are studied chiefly by geomorphological methods. The oscillatory crustal movements of earlier geological periods are recorded in the composition, stratification, and thickness of deposits.
The basic patterns related to oscillatory crustal movements were worked out by A. P. Karpinskii. His conclusions were developed in A. D. Arkhangel’skii’s works. Subsequently, the problem of oscillatory crustal movements was further developed by such scientists as M. M. Tetiaev, G. F. Mirchink, N. M. Strakhov, V. V. Belousov, A. B. Ronov, and V. E. Khain.
Abroad, oscillatory crustal movements were identified at the end of the 19th century by the American geologist G. Gilbert, who called them epeirogenic movements. In the 20th century the French geologist G.-E. Haug and the German geologists H. Stille and S. Bubnoff (or Bubnov) also studied these movements. Research has established two varieties of oscillatory crustal movements—general oscillatory and undulating. General oscillatory movements are expressed in a synchronal uplift or subsidence of vast areas encompassing an entire continent or a significant part of it. General oscillatory movements cause transgression and regression, change the outlines of the land and the sea, produce vertical changes in the composition of marine sediments, and stratify marine sediments; owing to these movements marine and river terraces form. General oscillations consist of movements of many orders superimposed on each other. The largest general oscillations have a period of 200–300 million years. They form the basis of tectonic cycles, which are manifested primarily in the repetition of major transgressions and regressions of the ocean. Particular transgressions and regressions with shorter periods occur against the background of the general movements. The shortest cycles of transgressions and regressions can be measured in thousands or even hundreds of years. The shorter the period of the cycle the more local its manifestation. The average rate of the general oscillations measured over an extended geological period is usually expressed in hundredths and tenths of a millimeter per year. Individual brief oscillations of the higher orders occur much more rapidly, at a rate close to that of the modern oscillatory movements of the earth’s crust.
The undulating oscillatory movements of the earth’s crust are superimposed on the general oscillations and are expressed in the protracted subdivision of any large surface area into uplift and subsidence zones. These movements become fixed in the relief of the earth’s surface and in the distribution of facies and the thickness of sedimentary deposits. Their amplitude can reach 15–20 km.
As undulating oscillatory movements of the earth’s crust develop, they exhibit different regimes, the principal types being geosynclinal and platform. In geosynclinal zones the waves of undulating oscillatory movements of the earth’s crust contrast sharply and reach a great amplitude: narrow (several dozen kilometers) uplift and subsidence zones are very close to each other and are often separated by deep faults. On platforms oscillatory crustal movements are characterized by a small amplitude (up to several kilometers) and extremely slight contrast: there are wide (hundreds and thousands of kilometers) areas, circular in plan, of slow uplifting and subsiding of the crust, and they give way to each other smoothly and gradually.
Since over the geological history of the continents as a whole the geosynclinal regime has gradually given way to the platform regime, the oscillatory crustal movements from the later periods are in sum less intensive than the same movements in the earlier periods. In areas of tectonic activization (for example, in the Tien-Shan), however, oscillatory crustal movements are again becoming extremely intense, although previously a quiescent platform regime had already been established there for an extended period.
On the surface of islands and the shelf floor of seas there are indications of ancient, recent, and modern oscillatory movements of the earth’s crust. Very little is known about oscillatory crustal movements on the floor of deep oceans.
It is thought that there is a link between oscillatory crustal movements and changes in the density of material in the upper mantle and deep in the earth’s crust and the movement of this material (tectonic hypotheses).
The study of oscillatory crustal movements is of great practical interest, since it helps to establish the distribution patterns of those formations of sedimentary rock in the earth’s crust that are associated with mineral deposits (including oil, gas, coal, and sedimentary ores of Fe, Mn, phosphorites, and bauxites).
REFERENCESKarpinskii, A. P. “Obshchii kharakter kolebanii zemnoi kory v predelakh Evropeiskoi Rossii.” In Sobr. soch., vol. 2. Moscow-Leningrad, 1939.
Strakhov, N. M. Osnovy istoricheskoi geologii, vols. 1–2. Moscow-Leningrad, 1948.
Ronov, A. B. “Istoriia osadkonakopleniia i kolebatel’nykh dvizhenii Evropeiskoi chasti SSSR (po dannym ob”emnogo metoda).” Tr. Geofizicheskogo in-ta AN SSSR, 1949, vol. 3 (130).
Ronov, A. B. “Nekotorye obshchie zakonomernosti razvitiia kolebatel’nykh dvizhenii materikov (po dannym ob”emnogo metoda).” In Problemy tektoniki. Moscow, 1961.
Belousov, V. V. Osnovnye voprosy geotektoniki, 2nd ed. Moscow, 1962.
Khain, V. E. Obshchaia geotektonika. Moscow, 1964.
V. V. BELOUSOV