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Isostasy
Isostasy
Isostasy (Greek ísos "equal", stásis "standstill") is a term used in geology to refer to
the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such
that the tectonic plates "float" at an elevation which depends on their thickness and density.
One interesting property of the continental and oceanic crust is that these tectonic plates have
the ability to rise and sink. This phenomenon, known as isostasy, occurs because the crust
floats on top of the mantle like ice cubes in water. When the Earth's crust gains weight due to
mountain building or glaciation, it deforms and sinks deeper into the mantle. If the weight is
removed, the crust becomes more buoyant and floats higher in the mantle.
This process explains recent changes in the height of sea-level in coastal areas of eastern and
northern Canada and Scandinavia. Some locations in these regions of the world have seen
sea-level fall by as much as one meter over the last one hundred years. This fall is caused by
isostatic rebound. Both of these areas where covered by massive glacial ice sheets about
10,000 years ago. The weight of the ice sheets pushed the crust deeper into the mantle, these
areas are slowly increasing in height to some new equilibrium level (Fig. 1.9.).
Fig. 1.9. The addition of glacial ice on the Earth's surface causes the crust to deform andsink (a).When the
ice melts, isostatic rebound occurs and the crust rises to its former position before glaciation (b and c). A similar
process occurs with mountain building and mountain erosion
Airy's Theory
Lecture Delivered by: Mr. Manzoor Jahangir
Isostasy
He presumes that the crustal blocks are of equal density and unequal thickness. As such the
blocks constituting the mountains are thicker than those on which the plains lie and therefore
they stand higher up as is the case with the masses of ice floating in water.
Fig. 1.10: Illustration of Air’s view
Floating ice is eight-ninths submerged and the higher the ice rises above the water level the
deeper is the submerged portion. Thus the roots of the mountains sink in the basaltic
substratum to depths proportional to the heights above.
This has greater support from recent geophysical data. For example Mt. Everest in the
Himalayas rises to a height of about 9 Kms. whereas right beneath it the crust is about 80
Kms thick. Thus Airy suggested that blocks of the lithosphere had a constant density of 2.7
gm. per cubic centimetre and floated in the asthenosphere of density 3.3 gm per cubic
centimetre. (Fig. 1.10.)
Pratt's Theory
According to this theory, there is a difference in the density of rocks in the crust and at the
heights of the crustal blocks are determined by their densities. As such blocks made up of
lighter material are at higher elevation than those consisting of denser material. Lighter
material, has therefore, been assumed to lie under mountains and heavier material under
ocean and there also exists a boundary, between the upper blocks and the lower dense rocks,
at a uniform depth known as the level of compensation. Thus, the rocks constituting the
elevated masses and depressed areas exert equal pressure at the level of compensation.
Lecture Delivered by: Mr. Manzoor Jahangir