Eustacy

(also eustatism or eustatic change), a slow worldwide change in the level of the world ocean and the seas linked with it. Eustatic movements were first identified by E. Suess in 1888. A distinction is made between movements of the shoreline as a result of the formation of marine basins, when real changes in the level of the ocean occur, and movements of the shoreline as a result of tectonic processes, which cause apparent changes in the level of the ocean. Such changes, which result in local transgressions and regressions and are caused by different tectonic forces, were termed deleveling changes, whereas broad transgressions and regressions caused by fluctuations in the level of the actual water shell were called hydrokinematic changes by F. Iu. Levin-son-Lessing (1893). A. P. Pavlov called (1896) negative movements of the shoreline geocratic movements and advances by the sea hydrocratic movements.

One of the hypothetical factors responsible for eustacy is the change in the total volume of water in the ocean in the course of the geological history of the earth, a change occurring as a result of the evolution of the continents. During the initial stages of the development of the earth’s crust, juvenile waters constituted the single most important factor in eustacy. At later stages, the importance of this factor lessened. The volume of water stabilized in the Proterozoic, according to A. P. Vinogradov, and has not changed significantly in the hydrosphere since the Paleozoic. Somewhat important are the processes of sediment accumulation and volcanic flows on the ocean floor (sedimento-eustacy) and the resulting rise in the level of the world ocean.

Beginning with the Paleozoic, the tectonic factor acquired great importance (tectono-eustacy), causing changes in the volume of marine and ocean basins and alteration of the relief and structure of the ocean floor and adjacent continents. It appears that the main changes in the level of the world ocean are linked to the development of the system of midoceanic ridges and with the phenomenon of sea-floor spreading.

Along with tectono-eustacy, the climatic factor acquired considerable importance in recent geological times, in the form of glacio-eustacy. During the glacial epochs, when water was concentrated on the continents in the form of ice sheets, the level of the world ocean dropped by about 110–140 m; after thawing, glacier waters again flowed into the world ocean, raising its level by approximately one-third of the original level. The decrease in temperature and the associated change in salinity affected water density, and consequently the level of the world ocean in the high latitudes differed several meters from the level in the equatorial regions. The formation of the lowest terrace, 3–5 m, is linked to these factors. Planetary factors, such as changes in the earth’s speed of rotation and the shifting of the poles, have also somewhat affected the mechanism of eustacy.

The study of the processes of eustacy is very important for historical geology and for gaining an understanding of the formation of the shelf zones, with which the formation of various minerals is linked.