What is the highest possible mountain on Earth?

Hannah - So, how do mountains form and what's to stop them reaching further up into the sky?

Nicky - My name is Nicky White and I'm a Geologist who works at the Department of Earth Sciences in the University of Cambridge. It's well-known that the Himalayas are amongst the highest mountains on Earth with Mount Everest peaking at just over 8 kilometres and an important question is, why on Earth do you not get mountains that are higher than that? The main reason is that there are two things fighting against each other when you make mountains. First of all, you have the plate forces. So, for the Himalayas, India is colliding with European, the Eurasian plate and forcing the crust to become thicker which pushes the Himalayas up in the air. But something else is going on that's often not talked much about which is that mountains eventually tend to fall down, just through their own weight. And the important concept here is something called viscosity which if you like in the runniness of the rock that's inside the mountains. A useful analogy is something like syrup. So, if you imagined that you created the Himalayas out of syrup, I think it would be easy to see that as soon as you start pushing the mountain up in the air, it also starts to fall down because the syrup will flow out sideways. It's just the same in the Earth. So rocks are not quite as runny as syrup, but they still have a certain ability to flow. So it means when you build a very large mountain chain, as soon as you start to reach a certain height, it will begin to fall down under its own weight. So, the Himalayan chain is actually beginning to squirt out sideways, mostly towards China which means we're probably at the limit of the maximum possible elevation achievable on Earth. On other planets such as Venus and Mars, the viscosity of the rocks may be different which is why you can achieve higher mountains.

Hannah - Thanks, Nicky. So, mountains fall down due to their own weight and the height at which they start to fall is due to how runny the material is that they're made up from. There's actually a number called the Argand number which is essentially the weight of the mountain divided by its runniness and this helps us to know how high a mountain can grow. Everest is just about at its max.