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Formation of Mountains

Canadian Rockies

Q: What is the general accepted theory/fact regarding the formation of the mountains on our planet?

From Mr Chris Lohler (June 2010)

Reply by Dr Ted Nield

Mountains on the continents are created in two circumstances.

First you must remember that continental rocks are less dense and so cannot ever be dragged down into the planet’s interior. They ride on the surface of the tectonic plates, like scum on the surface of a bucket of water.

The classic mountain-building event takes place when two pieces of continent collide with one another head on, as happened 30-50 million years ago between India and Asia. Because, at the junction between the two colliding plates, the continental rock cannot sink into the planet (like the bed of the ocean can) it just stays at surface and gets squeezed together. For this reason it gets thicker and thicker – and so becomes mountainous. The bit that sticks up above the planet is mirrored by a “root” that sticks downward into the Earth. Mountains are higher than the average continental elevation for the same reason that big icebergs are taller than small icebergs. They are supported from below by a large “floating” mass, or “root” which plunges deep into the dense rocks of the Mantle.

Mountains can also form at the leading edges of continents. Thus from the Andes in South America to the Rockies in North America, you have two continental leading edges under which the floor of the Pacific has been consumed for hundreds of millions of years. As the Pacific floor plunges down beneath the Americas, small fragments of continental rocks, called Terranes, occasionally reach collision point with the continent as they care carried towards it, and get scraped off against it, adding to its thickness. This is rather like the rubbish that collects at the ends of an escalator, as the treads disappear down into the bowels of the machine.