When the top of a mountain is eroded, it must re-adjust its mountain roots until it reaches isostatic equilibrium. For this question, use perust = 2.75g/cm and pmantle = 3.3g/cm. Perust = 2.75 g/cm Pmantle = 3.3 g/cm a. A mountain range originally had an average elevation of 5 km. It covers an area of 10000 km2 (assume this area did not change over time). How much mass must have been eroded away if its current average elevation is 3km? If we are assuming a constant erosion rate of 0.2 mm/yr, how long will it take to erode away the entire mountain range from present day? (Hint: You are not simply removing the 3 km of load. You also have to worry about the root which will keep coming up as you continue eroding from the top.) b. If two mountains of the same height are eroded at the same constant rate, but they have different densities, would you expect them to be eroded away after the same amount of time? If you answered "no", explain why, and which one would erode away earlier (the denser one or the less dense one)?