How much of a mountain is below the surface?

Question

Is there some kind of math rule for how much does a mountain extend (depth) below the surface?I found an article like this http://www.livescience.com/6595-depth-himalayan-mountain-roots-revealed.html says that subduction is rare, does this mean mountains don't usually extend below surface?

Answer 1

First to clarify, the article says "subduction of continental crust to this depth ... is also extremely rare", so it is not that subduction is rare, but the depth of it here is.

Beyond that this is mostly the physics of Archimedes principle- the mass of the object (a mountain) will displace (move out of the way) an equal mass of the material it is floating in (mantle material). The actual calculation is going to be complicated by the irregular shape and the fact that crust is only semi-continuous.