In analyzing certain geological features, it is often appropriate to assume that

Question

In analyzing certain geological features, it is often appropriate to assume that the pressure at some horizontal level of compensation, deep inside Earth, is the same over a large region and is equal to the pressure due to the gravitational force on the overlying material. Thus, the pressure on the level of compensation is given by the fluid pressure formula. This model requires, for one thing, that mountains have roots of continental rock extending into the denser mantle (the figure below). Consider a mountain of height H =6.5 km on a continent of thickness T = 32km. The continental rock has a density of 2.9 g/cm^3, and beneath this rock the mantle has a density of 3.3 g/cm^3. Calculate the depth D of the root. Number Units

Explanation / Answer

The preessure at b is

rho c ( 3.2 * 10^4 m) + rho m y

the pressure at a is

rho c ( 3.85 * 10^4 m + d) + rho m ( y-d)

equate above two equations

rho c ( 3.85 * 10^4 m + d) + rho m ( y-d)=rho c ( 3.2 * 10^4 m) + rho m y

rhoc ( 6.5 * 10^3 m+ d) = rho m d

d = rho c ( 6.5 * 10^3 m)/ rho m - rho c

=2900 ( 6.5 * 10^3)/400

=47125 m

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