Upheaval and Subsidence

Upheaval and Subsidence are terms applied to movements of the earth's crust that result in more or less permanent changes of level. Such movements are believed to be due to the sinking in of the crust upon the cooling and contracting nucleus. The crust under such conditions is necessarily subjected to great crush and strain, from which it gets relief, it is thought, by wrinkling—the wrinkles running in linear directions—or by bulging up over much broader areas. Hence two kinds of movement are recognised—(a) linear or axial, to which mountains of elevation owe their origin (see MOUNTAINS), and (b) regional, affecting broad areas, over which the crust seems to rise or, as the case may be, to sink without much disturbance or tilting of strata, although these may often be more or less fractured and dislocated. Such earth-movements are believed to take place very slowly and gradually as a rule. These are the generally received views; but of late years doubt has been expressed as to whether regional elevation of the crust is possible. The only movements of elevation of which we have obvious evidence are those that give rise to mountain-chains. These movements are tangential—the crust is squeezed and puckered up in rapid folds—but a vertical uprise of a continental area seems to Professor Suess and others impossible. Probably many of the supposed evidences of regional elevation really point to sinking of the crust under the great oceanic basins. These basins, there is reason to think, are pre-eminently subsiding areas, and if this be so the mere withdrawal of the sea from the continental areas must produce an apparent elevation of the land. Some now begin to suspect, however, that changes of sea-level may also be due to other causes. Thus, Professor Suess—believing that in equatorial regions the sea is upon the whole gaining on the land, while in other latitudes the reverse appears to be the case—points out that this is in harmony with his view of a periodical flux and reflux of the ocean between the equator and the poles. Dr Schmick also thinks that the apparent elevations and depressions of continental areas are the result of secular movements of the hydrosphere, but the sea according to him attains a high level in each hemisphere alternately—the waters being at present heaped up in the southern hemisphere. Others again, as Dr Hilber, have suggested that sinking of the sea-level may be due in part at least to absorption. This recalls the view of Celsius (q.v.), who attributed the retreat of the sea from the coasts of Sweden to gradual desiccation. At present none of the hypotheses that would attribute change of sea-level to secular movements of the hydrosphere has found favour with British geologists, who continue to maintain that all such changes are the result of upheaval and depression of the lithosphere caused by subterranean action. It seems probable that this view will eventually be modified.