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I'm working on a concept for a terraformed world with artificial mountain ranges/plateaus that stretch higher than a natural formation possibly can. I picture them like extremely tall table-top mountains where people and animals can live comfortably on top. The water content of the planet is similar to Earth's but perhaps with much shallower seas. The planet will also have a much thicker atmosphere, though how much thicker may depend on how high a mountain can get.

Here's the rub: the mountaintops and the surface should be at extremely different atmospheric pressures. For the effect I'm interested in, where traveling on the surface is an extremely taxing endeavor, the mountains will need to be 30-50 km tall. As far as I know, because of gravity and the strength of different types of stone, Earth's mountains can only be about 15 km at their tallest (Everest is about 8 km).

The terraforming of the world will be performed by giant machines, possibly everything from bulldozers to giant extruders that pull up mantle material and print it onto the surface of the tectonic plates. So fewer glaciers and erosion, more giant machines.

I've only come up with two possible ways that my artificial mountains could achieve such towering heights with the same planetary gravity: A specifically designed type of stone that doesn't occur naturally (and therefore has the strength to hold together inside such a tall mountain). Or, to somehow surround a metallic core (such as the giant machines needed to build mountains) with more typical types of stone.

Assuming that power and materials are effectively unlimited, how could these mountains be created?

If you have any other suggestions, I'd love to hear them too.

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    $\begingroup$ It's not just the strength of the stone of the mountains, but the strength of the underlying rock, going all the way down to the mantle. You might look at how continental crust (which is lighter than the underlying oceanic crust) is thicker under mountain ranges. $\endgroup$
    – jamesqf
    Jul 19, 2020 at 5:04
  • $\begingroup$ Where do you get the material to build those artificial mountains? I think you didn't do the calculation regarding how much building material is needed. $\endgroup$
    – Gimelist
    Jul 19, 2020 at 6:36
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    $\begingroup$ (a) You've asked 3 very different questions (what formations hint... how created... what formations on ultra tall...). Please ask only one. (b) Please take a paragraph to explain "terraforming" in this context. Natural terraforming (erosion) would have different answers than Magrathean terraforming (big bulldozers). Finally, (c) what's your point of reference? Earth's Challenger Deep is 11km below sea level. Everest is 8.8km above. That's ~20km right there and "sea level" is defined by water volume and temperature (i.e., it changes), so the Deep isn't easily dismissed (desert planet...). $\endgroup$
    – JBH
    Jul 19, 2020 at 7:54
  • $\begingroup$ I've narrowed down my question as you suggested and offered a little more context on the planet and terraforming process. Thanks for the tip. $\endgroup$
    – DehLeprechaun
    Jul 20, 2020 at 13:19

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Your mountains are enormous diamonds that have been lowered into place.

The terraformers wanted huge mountains. They retrieved great single diamonds from their birthplaces in gas giants, and then lowered them into place. The diamonds would sink down until they had displaced their weight of crust and mantle - made of materials weightier than diamond. Much of the giant diamond continues to protrude from the surface.

I did the math here:

Can a 25 mile high volcano form naturally on a desiccated desert earth planet?

Diamond: density of 3.5 and compressive strength of 60 GPA; maximum mountain height is 196 km

These diamonds would have been formed in the atmosphere of a ancient gas giant and then incorporated in the crust of this Earthlike world. I envision this huge, partly fused mountain of diamond extending farther down below the surface than it does above. Despite the huge mass balanced on one point, it does not sink further down because the bottom of the mountain is floating in denser, partly metallic molten materials. The diamond mountain is essentially an iceberg in the crust. Diamond is one of the best thermal conductors there is. With its big bottom side down into the mantle, the entire thing would be very hot.

So what would you see that would be weird? Hot air and thermals rushing up past the great diamond at all time and condensing into swirling clouds high above as the air cooled. Gleaming cliffs of diamond where life could not get a purchase. Very cool effects when the sunset hit it right.

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