ISOSTASY

theoretical balance of all large portions of the Earth's crust as though they were floating on a denser underlying layer, about 110 km (70 miles) below the surface. Imaginary columns of equal cross-sectional area that rise from this layer to the surface are assumed to have equal weights everywhere on Earth, even though their constituents and the elevations of their upper surfaces are significantly different. This means that an excess of mass seen as material above sea level, as in a mountain system, is due to a deficit of mass, or low-density roots, below sea level. In the theory of isostasy, a mass above sea level is supported below sea level, and thus there is a certain depth at which the total weight per unit area is equal all around the Earth; this is known as the depth of compensation. The depth of compensation was taken to be 112.7 km (70 miles) according to the Hayford-Bowie concept (named for John Fillmore Hayford [18681925] and William Bowie [18721940]). The Airy hypothesis says that the Earth's crust is a more rigid shell floating on a more liquid substratum of greater density. Sir George Biddell Airy (180192) assumed that the crust has a uniform density throughout. The thickness of the crustal layer is not uniform, however, and so this theory supposes that the thicker parts of the crust sink deeper into the substratum, while the thinner parts are buoyed up by it. According to this hypothesis, mountains have roots below the surface that are much larger than their surface expression. This is analogous to an iceberg floating on water, in which the greater part of the iceberg is underwater. The Pratt hypothesis (of John Henry Pratt ) supposes that the Earth's crust has a uniform thickness below sea level with its base everywhere supporting an equal weight per unit area at a depth of compensation. In essence, this says that areas of the Earth of lesser density, such as mountain ranges, project higher above sea level than do those of greater density. The explanation for this was that the mountains resulted from the upward expansion of locally heated crustal material, which had a larger volume but a lower density after it had cooled. The Heiskanen hypothesis (of Weikko Aleksanteri Heiskanen [18951971]) is an intermediate or compromise hypothesis between Airy's and Pratt's. This hypothesis says that approximately two-thirds of the topography is compensated by the root formation (Airy model) and one-third by the Earth's crust above the boundary between the crust and substratum (Pratt model).