Asked • 04/03/19

How much of a mountain is below the surface?

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?

Michael D.

tutor
To come up with an exact equation is difficult. However, I think you misinterpreted this quote from the article that "The subduction of continental crust to this depth has never been reported in the Himalayas and is also extremely rare in the rest of world." The Himalayan orogeny-type subduction is not common on Earth today, true (though there are hundreds of subduction zone on Earth). However, there have been many orogenies in the past that have done the exact same thing. The general rule of thumb is that in large collision events like the Himalayan Orogeny, Grenville Orogeny, Taconic Orogeny, etc. the mountains initially form with a very large portion of their volume below the Earth's surface as plutonic and metamorphic rock. An example of this is the Adirondack Mountains here in New York. The mountains were forced to raise due to the density of the material their "roots" are composed of, and this rising to the surface over millions of years is abundantly evident in the faulting that you can find there. I recommend you read about the Carthage-Colton Mylonite Zone (CCMZ) in the Adirondacks to get a better understanding of this process.
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05/27/19

Michael D.

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Orogeny: Mountain building event. (For future readers)
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05/27/19

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Inactive Tutor answered • 06/22/26

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The visible mountain is typically only a small fraction of the total structure. In many major mountain ranges, 80–90% of the mountain system's vertical extent may lie below the surface as a crustal root.

As a geophysics example, the Himalayas are roughly 9 km high above sea level but are supported by crust that extends tens of kilometers downward, making the hidden part several times larger than the visible mountain.

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