History of Earth's crust and isostasy

Isostatic equilibrium occurs in many parts of the Earth that differ in history and current crustal structure, indicating that major changes,in crustal composition and thickness during development have tended to preserve the state of equilibrium. A state of equilibrium disturbed by tectonic processes is soon restored. Processes that disturb the equilibrium development of the crust are called causative; those that arise as a consequence and tend to restore equilibrium are compensative. There is evidence for these two processes, which are responsible for displacements of opposite sign in the base and surface of the consolidated crust when equilibrium is regained. These processes indicate changes in different parts of the crust and enable us to say that the crust shows two types of development. The structure of the crust corresponds to the Airy-Plan general model, and this structure may be described by isostatic equations when the two types of processes are in balance. The same equations can be used when the state is not in equilibrium if there is introduced a correction to be deduced from gravity anomalies and data on the deep structure of the crust. These equations can be used to describe changes in the crust, and isostatic equations enable one to combine ideas with new data to describe changes in thickness and composition in some layers of the crust during development as well as some aspects of block movement. Examples of the use of these isostatic equations are given.—C. E. Sears