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Transcript
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