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Isostasy

Isostasy

Isostasy (also spelled Isotacy) is a geophysical phenomenon describing the force of gravity acting on crustal materials of various densities (mass per unit volume) that affects the relative floatation of crustal plates. Isostasy specifically describes the naturally occurring balance of mass in Earth's crust .

Continental crust and oceanic crust exist on lithospheric plates buoyant upon a molten, highly viscous aethenosphere. Within Earth's crustal layers, balancing processes take place to account for differing densities and mass in crustal plates. For example, under mountain ranges, the crust slumps or bows deeper into the upper mantle than where the land mass is thinner across continental plains. Somewhat akin to how icebergs float in seawater, with more of the mass of larger icebergs below the water than smaller ones, this bowing results in a balance of buoyant forces termed isostasy.

Isostasy is not a process or a force. It is simply a natural adjustment or balance maintained by blocks of crust of different mass or density.

Within Earth's interior, thermal energy comes from radioactive energy that causes convection currents in the core and mantle. Opposing convection currents pull the crust down into geosynclines (huge structural depressions). The sediments that have collected (by the processes of deposition that are part of the hydrologic cycle ) are squeezed in the downfolds and fused into magma . The magma rises to the surface through volcanic activity or intrusions of masses of magma as batholiths (massive rock bodies). When the convection currents die out, the crust uplifts and these thickened deposits rise and become subject to erosion again. The crust is moved from one part of the surface to another through a set of very slow processes, including those in Earth's mantle (e.g., convection currents) and those on the surface (e.g. plate tectonics and erosion).

With isostasy, there is a line of equality at which the mass of land above sea level is supported below sea level. Therefore, within the crust, there is a depth where the total weight per unit area is the same all around the earth. This imaginary, mathematical line is called the "depth of compensation" and lies about 70 mi (112.7 km) below the earth's surface.

Isostasy describes vertical movement of land to maintain a balanced crust. It does not explain or include horizontal movements like the compression or folding of rock into mountain ranges.

Greenland is an example of isostasy in action. The Greenland land mass is mostly below sea level because of the weight of the ice cap that covers the island. If the ice cap melted, the water would run off and raise sea level. The land mass would also begin to rise, with its load removed, but it would rise more slowly than the sea level. Long after the ice melted, the land would eventually rise to a level where its surface is well above sea level; the isostatic balance would be reached again, but in a far different environment than the balance that exists with the ice cap weighing down the land.

Scientists and mathematicians began to speculate on the thickness of Earth's crust and distribution of landmasses in the mid-1800s. Sir George Biddell Airy (18011892) assumed that the density of the crust is the same throughout. Because the crust is not uniformly thick, however, the Airy hypothesis suggests that the thicker parts of the crust sink down into the mantle while the thinner parts float on it. The Airy hypothesis also describes Earth's crust as a rigid shell that floats on the mantle, which, although it is liquid, is more dense than the crust.

John Henry Pratt (18091871) proposed his own hypothesis stating that the mountain ranges (low density masses) extend higher above sea level than other masses of greater density. Pratt's hypothesis rests on his explanation that the low density of mountain ranges resulted from expansion of crust that was heated and kept its volume, but at a loss in density.

Clarence Edward Dutton (18411912), an American seismologist and geologist, also studied the tendency of Earth's crustal layers to seek equilibrium. He is credited with naming this phenomenon "isostasy."

A third hypothesis, eventually developed by Finnish scientist Weikko Aleksanteri Heiskanen (18951971) was a compromise between the Airy and Pratt models.

The model most accepted by modern geologists is the Hayford-Bowie concept. Advanced by American geodesists John Fillmore Hayford (18681925) and John William Bowie (18721940), geodesists, or specialists in geodesy, are mathematicians who study the size, shape, and measurement of Earth and of Earth forces (e.g., gravity). Hayford and Bowie were able to prove that the anomalies in gravity relate directly to topographic features. This essentially validated the idea of isostasy, and Hayford and Bowie further established the concept of the depth of isostatic compensation. Both gentlemen published books on isostasy and geodesy. Hayford was the first to estimate the depth of isostatic compensation and to establish that Earth has an oblate spherical shape (a bowed or ellipsoid sphere) rather than a true sphere.

See also Earth, interior structure

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isostasy

isostasy A model for the upper region of the Earth in which differences in elevation are compensated by either low-density roots or lower-density surface rocks. The rigidity of the tectonic plate allows some departure from this model. See ISOSTATIC ANOMALY; AIRY HYPOTHESIS; and PRATT HYPOTHESIS.

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isostasy

isostasy (īsŏs´təsē): see continent.

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