Why are mountains pointy?

This question was sent in by Paul ia the Naked Scientist forum. Chris Smith put this to geology expert, Owen Weller, from the University of Cambridge.

Owen - Once you’ve had some kind of rock uplift the shape or morphology of a mountain is simply controlled by the principal style of erosion at the particular locality. So to generate pointy peaks or, as they’re more formally known, pyramidal peaks, you have the process whereby you have three or more glaciers which are diverging from a central point under the influence of gravity and this leave behind the pyramid. This has given us such iconic peaks as the Matterhorn, Mont Blanc, Mount Everest and, conceptually, this is how most people view mountains, sort of in their mind's eye they see these pyramids.

However, I should point out that not all mountains are pointy.

Chris - I was just saying Lee’s adopted homeland, of course it’s South Africa. If you go to Capetown you see a very famous flat top mountain. It’s called Table Mountain for a very obvious reason.

Owen - Absolutely.

Chris - How did that happen?

Owen - It’s wonderfully named. Table Mountain is composed of flat lying sedimentary rocks, and it’s actually capped by a particularly resistant type of sedimentary rock called a Quartzite. So what happens here is that you get erosion that preferentially weather out underneath the top lying Quartzite, which leads to failure at the edges, so you get steep sided cliffs and a flat lying top.

Chris - Talking of pointy things and permeable in structure because this is your bag isn’t it Meghan. How did they actually come up with the shape of the pyramids? What was their inspiration - do we know?

Meghan - The idea is in one particular creation method in ancient Egypt that Egypt evolved from this mound of Earth that came up from this watery void of chaos called Nun, and this original mound of earth is thought to have been shaped similar to a pyramid. So the idea is that pyramids are a way of recreating that original sort of mound of creation.