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The diagram above shows two columns of rock, a crust of density 2.75 g/cc (brown) over mantle lithosphere of density 3.25 g/cc (purple) over asthensophere of density 3.20 g/cc (red). If you grab the handles at the base of the crust and mantle lithosphere, you can see how changing the thicknesses of these two parts of the lithosphere changes the isostatic elevation of the crust. Note that the topography is shown with an exaggeration of 3 relative to the rest of the plot.
The condition of isostasy is that the pressure in the asthenosphere is everywhere the same for a given depth. The pressure is calculated to be the weight of all the overlying material. The two curves on the right show the weight vs. depth for the two columns, red for the right column, green for the left column, and only the green line when the two overlap. Note that the weights at and below the bottom of the lithosphere are the same (the weight curve below the deepest lithosphere is always a single green line)--this is the condition necessary for isostasy.
Note too that above that depth, the two lines are not usually the same. If the rocks become weak enough, then the difference in weight between two adjacent rock columns can lead to flow from one to another.
C. H. Jones | CIRES | Dept. of Geological Sciences | Univ. of Colorado at Boulder
Last modified at Tuesday, April 25, 2000 2:59 PM