Q: I am a high school teacher and I have been teaching about Wegener - his theory of continental drift and the opposition he faced. A pupil asked, 'If the Himalayas were formed from India colliding with Asia, is the surface area of the mountains the same as the surface area of the continents before they collided?'
I could think of arguments for both yes and no, but not a definitive answer.
From Mr Simon Rockett (June 2011)
Reply by Dr Ted Nield
This is a very interesting question. During continental collision, the key issue is "crustal shortening" - there has to be shortening during tectonism, because all that thickening of the crust, which goes to build the mountains both upward and downward (into the "roots) has to come from somewhere. However, the total degree of shortening may not quite be as great as we might imagine necessary, because a lot of the matter that ends up building the mountain range will consist of continental slope and even ocean basin facies - even in some cases, obducted ocean floor material, that forms the well-known ophiolite suites of pillow lavas, dykes and cherts. Overall though, one can say that crustal thickening presumes a certain amount of crustal shortening.