# Isostasy

Isostasy as a description of Earths balance

The theory of isostasy

Resources

Isostasy is the term describing the naturally occurring balance of masses within Earths crust that keeps the planets gravity in equilibrium. Isostasy is not a force or a process; it is the term for the phenomenon of adjustments Earth makes to stay balanced in mass and gravity.

## Isostasy as a description of Earth’s balance

Matter and energy exist in finite (specific) amounts and cannot be created or destroyed under normal circumstances. Rock is eroded from mountains and highlands, transported as sediment and deposited in valleys or stream channels, compacted under its own weight into rock, and lifted to form mountains again.

Deeper within Earth, balancing processes also take place as major shifts in the upper part of Earths crust change the planets gravitational balance. Under mountain ranges, the crust protrudes farther into the mantle than beneath other areas. The landmasses float on the crust and mantlelike icebergs float in seawater, with more of the mass of larger icebergs below the water than smaller ones. This balance of masses of Earths crust to maintain gravitational balance is called isostasy.

Isostasy is not a process or a force. It is a natural adjustment or balance maintained by blocks of crust of different thicknesses. In isostasy, there is a line of equality at which the mass of land above sea level is supported below sea level. So, within the crust, there is a depth where the total weight per unit area is the same all around Earth. This imaginary, mathematical line is called the depth of compensation and lies about 70 mi (112.7 km) below Earths 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 offers a good example of isostacy. 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 were to melt, the resulting water would 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.

## The theory of isostasy

Scientists and mathematicians began to speculate on the thickness of Earths 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 Earths 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. Pratts 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 Earths crustal layers to seek equilibrium. He is credited with naming the phenomenon of 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 scientists who study the size, shape, and measurement of 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 validated the idea of isostasy, and Hayford and Bowie further established the concept of the depth of isostatic compensation. Both scientists 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 spheroid rather than a true sphere.

## Resources

### BOOKS

Fowler, C.M.R. The Solid Earth: An Introduction to Global Geophysics. Cambridge, United Kingdon: Cambridge University Press, 2004.

### KEY TERMS

Batholith A huge mass of igneous rock that is intruded (forced by pressure) into Earths crust but may not reach the surface.

Convection current Massive currents within the semi-molten mantle of 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 Earths land masses above the line are balanced by those below.

Geodesy The mathematics of measurements of Earth including its size, shape, and location of points on its surface.

Geosyncline A massive downward bend in Earths crust; the opposite of an anticline, which is a huge upward flex in Earths surface.

Gravity The force of attraction of Earths mass for objects near it.

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

Magma Molten rock within Earth. When magma reaches the surface, it cools and forms igneous rock.

Hofman-Wellendorf, B., and H. Moritz. Physical Geodesy. Berlin: Springer, 2005.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Prentice Hall, 2004.

Gillian S. Holmes

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