Why aren’t there mountains that are 10 or 15 miles high on Earth?

Question

Mt Everest is just under 5.5miles high. Olympus Mons on Mars is 16 miles high. Why aren’t there much larger mountains on Earth? What’s the highest a mountain can go on Earth?

In: 4987

Answer 1

**TL;DR:** *Gravity both pulls the breaking-off bits of structure of the mountain down, and pulls the entire mountain down into the crust and inner parts of the planet it’s on. Mars has less gravity, so less pulling in both cases, so its mountains are higher.*

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Take a whole bunch of something small and loose and powdery and dry. Let’s use a truckful of sand grains that has been blown around in the desert for centuries and so they’re worn down and no sharp edges.

Now take a little and pile it up. Add some more. It slides down the slopes to form an angle. Keep going, and watch what happens as the sand continues to pile up.

**No matter the size of the pile,** unless you have something like a big wind blowing or moisture that’s sticking the sand together, the sand always seems to form the same angle to the ground. This is the angle of repose, or angle of rest.

And it’s affected by how coarse and rough-edged the material is, how big the particles are… and more than anything else…

…how much gravity there is.

High gravity means a short, shallow hill. Low gravity means you can pile that sand up a lot higher before it starts to slip down. The angle changes with the gravity.

Now for mountains.

Rocks on mountains will fall off if frost or wind or a tremor or daily changes in temperature or a higher rock falling on it or whatever cause them to break off… and then that rock will fall if they exceed the angle of repose. The more gravity there is, the more they break off and erode the mountain.

But also, mountains are HEAVY. VERY heavy. They press down a LOT on what might be molten or plasticky-hot rocks deep within the planet… and cause that part of the planet to sink over time. Mars is smaller and its innards are not as plasticky as Earth’s are, and all the rocks in its mountains weighs a great deal less due to less gravity, so it can support a much bigger mountain before that mountain starts to push down into it.