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I am working on a worldbuiding project. In it, an advanced alien civilization traveling the galaxy find a habitable planet with conditions similar to Earth before the development of multicellular life. The aliens construct a monument on the planet, a heptagram-shaped pyramid (777 km width from tip to base, 77 km tall, area of 181,000 square km and volume of 423,000 cubic km), at the center of the largest ocean. Afterwards they return to their journey across the stars, and in the following millennia, life begins to develop on the planet (or a handwaved earth-like biosphere with humanoids develops from a colony, stargates, etc.)

Is there any material that could be used for the construction of this that would be resistant to weathering from the water and air, doesn't easily absorb water, and wouldn't be reactive with the surrounding environment? The pyramid would be completely solid and homogenous, apart from a small capstone made of a separate to-be-determined material at the top.

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  • $\begingroup$ Welcome to worlduilding. Please take a good read of our help center and then rework your post to fit our standards: one question per post, worldbuilding related and answerable with facts rather than opinion without needing to write a whole book. $\endgroup$ – L.Dutch - Reinstate Monica♦ Nov 2 '20 at 6:06
  • $\begingroup$ Rather than split this into 6 questions (which would be a pain for both asker and answers), I'd suggest pick the question your least sure of and ask that, and with your other 5, reword them as assumptions. "I assume this would affect the climate and result in wind cycling around it and rainfall on one side, dry on the other, but nothing else.". If people see issues with your assumptions they'll correct or build on them, but otherwise they'll focus on your main unknown. $\endgroup$ – Ash Nov 2 '20 at 6:49
  • $\begingroup$ Hi, Sorry about reposting! I misunderstood the banner at the top of the screen and thought I had to repost the question; I have deleted the repost. I have edited this post down to one question, and would like to know whether it is acceptable to be reopened. Also, I am sorry about any trouble I have caused to the moderators or other members of this site through my mistake, I am new and didn't fully understand the rules of posting. I hope you are well, and thank you both for your suggestions, and hope you have a nice day! $\endgroup$ – radarbeast Nov 2 '20 at 7:03
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77km tall, on an Earth-like planet, is simply not achievable.

A pure Granite block of sufficient size might be able to hold its own weight, as you have the structure having a very wide base.

But the crust under it would simply sag down, triggering all sorts of tectonic havoc around and under the block. Even if the block survived that, the end result would not be much more than 15-20km above sealevel.

Consider going the Full Monty, and making your pyramid out of one big monolithic Diamond tower instead? It would have similar density to the Granite, but the much greater compressive strength would allow you to make a structure that looks like a tack. i.e. wide, light base hidden below the surface, and a single thin spire pointing up in the middle, with a total mass less than 1% of the granite mountain.

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  • $\begingroup$ I don't know the Full Monty reference (got a strange search result and don't know if it is what you are referencing) , but the diamond idea is cool! After looking into it, though, I think I will keep the wide base of granite with a cap at around 20 km; pretty similair to Mons Olympus after looking it up, and although it isn't exactly what I had in mind, I think I will still be satisfied with that result! Out of curiosity, though, could you elaborate on the tectonic havoc? Would it just cause molten material to break through the mantle where it broke? what would happen? $\endgroup$ – radarbeast Nov 2 '20 at 8:28
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    $\begingroup$ Why would the crust sink under granite but not under diamond? $\endgroup$ – L.Dutch - Reinstate Monica♦ Nov 2 '20 at 10:18
  • $\begingroup$ Diamond is carbon. It burns. Diamond piramid is like pile of coal. It would not survive oxigenation event. $\endgroup$ – ksbes Nov 2 '20 at 12:46
  • $\begingroup$ @ksbes Yes diamond can burn. But in air, a slab of diamond on fire self-extinguishes. You need pure oxygen AND 900C, or normal air and 1200C to ignite it in the first place. And the energy released from combustion is less than the energy needed to heat up normal air that hot, to sustain the fire. I.e. A Diamond slab burns about as well as a steel anvil does. $\endgroup$ – user79911 Nov 3 '20 at 19:10
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For an Earth like planet there is no material which can make a structure 77 km tall. The maximum height different on Earth is short of 20 km, from the top of Mount Everest to the Challenger's Deep in the Marianne trench. Even Mons Olympus on Mars is just 26 km.

Anything taller than that would simply collapse under its own weight. This is also the reason why planets are spheroids: once it is big enough, a body will be shaped by hydrostatic forces into something as close as possible to a sphere.

That consideration apart, granite is a pretty sturdy material, as you can see from peaks like El Capitan.

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