Isostasy is essentially the principle of hydrostatic equilibrium applied to the Earth. In its simplest form, it considers rigid blocks of the Earth (usually taken to be the crust) to be buoyantly supported in an underlying fluid medium (usually taken as the mantle) and free to move vertically. These blocks will then move until their weight is exactly balanced by their buoyancy, at which point they are said to be ‘in isostatic equilibrium’.
The concept was originated by French surveyors in the eighteenth century working around the Andes, who noted that the observed gravitational attraction of the mountains was less than that predicted. They inferred the presence of a low-density ‘root’ that balances the excess weight of the mountain range and also reduces its gravitational attraction.
The theory was further developed by the English geodesists Pratt and Airy in the nineteenth century, who have given their names to two forms of the theory.
Pratt hypothesized that elevation is inversely proportional to density. Therefore, the higher the mountain the lower is its density (i.e., light rocks “float” higher).
Airy hypothesized that mountains have “roots” which extend down into the mantle. Therefore, elevation is proportional to the depth of the underlying “root”.
Many regional-scale geological processes tend to disturb isostatic equilibrium. For example, erosion makes blocks thinner or reduces their weight, so that a mountain being eroded will tend to rise to maintain equilibrium. In contrast, the deposition of sediments represents an added load, so that sedimentary basins tend to sink. Extension of the crust to form rift valleys thins it, while compression in mountain-building thickens it. Loading and unloading by glaciers also affects isostatic equilibrium. In all these cases, the lithosphere will move in order to maintain or regain isostatic equilibrium, and the ductile asthenosphere will flow in response.
Isostatic forces are thus of major importance in controlling the topography of the Earth’s surface.