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Isostasy - The Theory Of Isostasy

earth crust density hayford

Scientists and mathematicians began to speculate on the thickness of the earth's crust and distribution of land masses in the mid 1800s. Sir George Biddell Airy (1801-1892) 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 the 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 (1809-1871) also 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 (1841-1912), an American seismologist and geologist, also studied the tendency of the earth's crustal layers to seek equilibrium. He is credited with naming this phenomenon "isostasy."

A third hypothesis developed by Finnish scientist Weikko Aleksanteri Heiskanen (1895-1971) is a compromise between the Airy and Pratt models. But it is the Hayford-Bowie concept that has been most widely accepted. John Fillmore Hayford (1868-1925) and John William Bowie (1872-1940) were American geodesists who studied gravitational anomalies (irregularities) and first began surveying gravity in the oceans. Geodesists, or specialists in geodesy, are mathematicians who study the size, shape, and measurement of the earth and of Earth forces, like 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 is an oblate sphere (a bowed sphere) rather than a true sphere.


Resources

Books

Marshall, Clare P., and Rhodes W. Fairbridge. Encyclopedia of Geochemistry. Boston: Kluwer Academic Publishers, 1999.

Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. 4th ed. John Wiley & Sons, 2000.

Woodhead, James A. Geology. Boston: Salem Press, 1999.


Gillian S. Holmes

KEY TERMS

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Batholith

—A huge mass of igneous rock that is intruded (forced by pressure) into the earth's crust but may not reach the surface.

Convection current

—Massive currents within the semi-molten mantle of the earth that move due to differences in temperature.

Density

—The amount of mass of a substance per unit volume.

Depth of compensation

—The line at which the earth's land masses above the line are balanced by those below.

Geodesy

—The mathematics of measurements of the earth including its size, shape, and location of points on its surface.

Geosyncline

—A massive downward bend in the earth's crust; the opposite of an anticline, which is a huge upward flex in the earth's surface.

Gravity

—The force of attraction of Earth's mass for objects near it.

Hydrologic cycle

—The continuous, interlinked circulation of water among its various compartments in the environment.

Magma

—Molten rock within the earth. When magma reaches the surface, it cools and forms igneous rock.

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Superb source of basic knowledge.

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