How do I calculate isostacy for a continent or landmass? A continent behaves like an icecube in water: it erodes. How do I calculate the rate? 

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Your analogy is not truly accurate.  An ice cube will erode until it has completely disappeared.  A continental landmass erodes but it is also being regenerated as well.  As long as these systems are in relative equilibrium, the landmass will not ultimately disappear.  Isostacy is the measure of the buoyancy of the tectonic plates of the lithosphere floating on the astheosphere.  In areas where the landmass is eroding, the plate will rise with higher buoyancy.  In areas where the landmass is being added to (sediment deposition), the plate will sink lower under the added mass.  The classic model for measuring isostacy is the Airy-Heiskanen model.  It assumes that the density of the crust and mantle is uniform and constant.

I'm not sure if you got confused with terminology, but measuring the rate of erosion of a landmass is not the same as isostatic calculations.

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