Why are planets round and mountains pointy?

What processes make planets spherical and mountains pointy?
10 March 2023


Someone climbing the Matterhorn.



Have a go at this question from James who, who says his seven year old son Hamrith wants to know why mountains are pointy, and if so, why are planets round then?


Will - Okay, well there's different scales, right? Planets form through planetesimals, over vast periods of time.

Chris - What's one of those?

Will - So a baby planet. So if we were in our solar system and we have asteroids colliding and accreting and forming progressively larger and larger bodies, they essentially start having their own sort of gravitational field, which kicks in a feedback loop and attracts more space to breed to them. So they get bigger and bigger and bigger until ultimately you've cleared up all the muck in your general area and you've got your planet.
Chris - So that explains why you get a ball because under gravity, everything's pulling towards the center of the ball and it's the ball shape is the closest everything can reasonably get precisely without being too far from anything else. So that's why they're round from a distance, but close up mountains pointy. So what makes mountains go pointy?

Will - And plate tectonics will have a big part of that. So it's not just a perfect beautiful sphere, but it's a fractured sphere and our plates are constantly moving along it over very small periods of time. We famously talk about the Atlantic sort of rifting and North America getting further away from us today at about the rate our fingernails are growing, but geological time is long. We're talking hundreds of millions of years, billions of years. So these plates do collide in force uplift, and with that uplift comes essentially, you take your ball and you get large amounts of it and project it upwards. And when you are topographically higher, then you're more vulnerable to erosion. But that erosion doesn't necessarily hit all the same spots equally.
Chris - So in summary then gravity makes planets and it makes them round because it pulls all the material together a bit, like a raindrop, which is all the water trying to get as close together as possible. That gives you the round shape from a distance. But when you zoom in a bit closer, you find that local factors have pushed things up to make pointier surfaces. And that's why mountains are pushed up and they're pointy. But eventually they'll wear down and go flat if nothing's making more mountains.

Will - Absolutely. And these are continuous processes that will happen time and time again.


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