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

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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

Gravity, basically. The gravity on Earth is strong than on mars. So Martian mountains can grow much taller.

The taller a mountain gets the heavier it gets. And when a mountain gets heavier and heavier two things will happen.

1. it can collapse under it’s own weight and crumple away.
2. it will start to sink back down into the Earth.

The force of Earth’s gravity we have end up with a theoretical max high of around 10 miles. But based on the way mountains form there’s basically no way that could happen.

Fun fact that’s probably a coincidence gravity on Mars is about 38% as strong as it is on Earth. Take Mt Everest’s height of 5.5 miles and divide by .38 and you get 14.5 miles. Pretty close to the size of Olympus Mons all things considered.

**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.

Mars lacks mobile tectonic plates
The extraordinary size of Olympus Mons is likely because Mars lacks mobile tectonic plates. Unlike on Earth, the crust of Mars remains fixed over a stationary hotspot, and a volcano can continue to discharge lava until it reaches an enormous height.

If you took all the water away it would make a significant difference. There isn’t any surface water on mars. The Pacific is 6.7 miles plus deep so add that to Everest it’s pretty high.

Mountains have roots and there is a maximum depth a root can grow. To sidestep for a moment – take an icecube in a glass of water – it floats, but most of the ice is below the surface of the water itself. Some of the water below the ice is displaced to support the ice standing above the surface. Geologists call this principle isostasy.

Something similar happens in the Earth – except with solid rock. The Earth’s Crust is less dense than the underlying Mantle. As a mountain grows on the surface, the Crust below the mountain thickens into a root – the less dense Crust pushes aside some of the Mantle. This means the thickest Crust on the Earth – about 70km is under the Himalayas which stand about 8km tall, when the typical value for the Crust is just 30km.

However, you can’t keep thickening the Crust, as the root develops, the minerals in the rocks at the bottom of the root are under immense pressure and temperatures and begin to change their composition into denser minerals and create a rock called eclogite. Eclogite is denser than the Upper Mantle and the whole root can detach and sink deep into the Earth.

Without a root, the mountains can’t stand tall, so they rapidly collapse (in geological terms) and the area might actually end up being split apart. Something like this might have happened under Tibet which if it wasn’t right next to the Himalayas would be thought of as very high but is relatively flat and has volcanoes – which the Himalayas lack.

The other thing that limits the ability of mountains to grow much higher on Earth is that the higher the mountain, the greater the erosive force of rivers and glaciers to demolish them simply because there is further for water or ice to descend to the sea. So not only are the Himalayas the tallest mountains on Earth, they have the greatest levels of erosion and the rivers draining the mountains carry unbelievable amounts of sediment to the oceans – which is good news for that whole civilisation thing.