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The world my setting takes place on is an artificial shell world set into the FAR future (somewhere between several hundred million and nearly a billion years) after a cataclysmic event wiped out it's advanced civilization, leaving the human and non-human survivors to evolve into various sapient and non-sapient creatures.

The planet in question (lets call it Terra Ultima) is around ten times the size of earth while still having the same gravity, has oxygen levels around 30%, and is spilt into seven main layers built around a small black hole with entrances to each layer built at the poles and smaller ones all across the world.

The main problem I have is that I want at least the surface of my shell world to have geological activity such as moving plate tectonics and volcanos which are caused by the earth's mantle.

My question is how could I have volcanos and shifting plate tectonics on a hollow world?

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    $\begingroup$ Use whatever handwavium lets your shell world not crush itself. $\endgroup$
    – John
    Commented Jun 6, 2023 at 0:27
  • $\begingroup$ This reminds me of a certain setting with four-eyed aliens. $\endgroup$
    – SavioKingdom
    Commented Jun 7, 2023 at 19:37
  • $\begingroup$ Do your inner shells need to be habitable, because any heat source that makes part of your outer shell hot enough to be molten will radiate enough heat into your inner shell(s) to eventually become at least as hot. I think I have figured out a solution for a tectonically active inner shell, but a technically active outer shell with habitable inner shells becomes a lot more complicated. $\endgroup$
    – Nosajimiki
    Commented Jun 8, 2023 at 15:16
  • $\begingroup$ @Nosajimiki ideally I want the inner shells to also be habitable(but I'm fine with the bottom most ones being hotter) which obviously presents a problem in regards to heat. $\endgroup$
    – icewar1908
    Commented Jun 8, 2023 at 20:14

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You have a black hole captured at the center of a world and you're wondering how to have a sub-mantle magma layer?

The Piezo Effect

Piezoelectricity ... is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure and latent heat. It is derived from Ancient Greek πιέζω (piézō) 'to squeeze or press', and ἤλεκτρον (ḗlektron) 'amber' (an ancient source of electric current).

The piezoelectric effect results from the linear electromechanical interaction between the mechanical and electrical states in crystalline materials with no inversion symmetry. The piezoelectric effect is a reversible process: materials exhibiting the piezoelectric effect also exhibit the reverse piezoelectric effect, the internal generation of a mechanical strain resulting from an applied electric field. For example, lead zirconate titanate crystals will generate measurable piezoelectricity when their static structure is deformed by about 0.1% of the original dimension. Conversely, those same crystals will change about 0.1% of their static dimension when an external electric field is applied. The inverse piezoelectric effect is used in the production of ultrasound waves. (Source)

The crushing gravity of the black hole acts to draw your unobtainium shell toward it. This incredible force is converted, through the piezo effect, to electrical energy. That energy is dumped into the sub-mantle layers to heat thriving tectonics (and granting you a rationalized magnetosphere... ♪You're Welcome!♪).

Note: You're question is tagged . The piezo effect can't do what I'm suggesting it can do. There's no scientific solution to what you're asking for, especially since you're not asking a question. Therefore, I'm answering from the perspective of .

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  • $\begingroup$ "You have a black hole captured at the center of a world and you're wondering how to have a sub-mantle magma layer?" <- LOL $\endgroup$
    – Nosajimiki
    Commented Jun 6, 2023 at 23:51
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The forces necessary to maintain any mass describable as a world in a hollow shell are enormous. Indeed, there are currently none known. It was logically deduced that the rings of Saturn had to be many particles rather than a solid ring before they could be seen to be so.

Plate tectonics and volcanoes are a consequence of such forces working unevenly, when the sphere is perturbed, or affected by gravity from outside the system.

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Volcanos and plate tectonics are engineered features

Your problem is you're looking at them as geological features. But your world is entirely engineered; "plate tectonics" and volcanos would exist if and only if it were engineered to exist, and would simply be mimicking the natural counterpart and be implemented through other technological means (much as your world's "Sun" would have to be.)

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  • $\begingroup$ Terry Pratchett's early novel Strata contains an engineered flat world, with earthquake and volcano machines. $\endgroup$
    – John Dallman
    Commented Jun 8, 2023 at 15:50
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The Outer, Earth Like Shell

If you make the outer shell thick enough to create magma using the same method as our own world, then the outer shell will be so thick and fluid that the local gravity with overwhelm the gravity of the black hole causing it come apart and form planets. So, you need it to be too thin for gravity to be the source of its magma layer. That said, there is a second source of heat that contributes to our plant's molten core, and than is nuclear decay. If your shell contains a significant amount of fissile material, then you can melt a silicate layer even if the shell is relatively thin... but in doing so, you must ensure that the inner part of your shell does not melt because if it did, then gravity would pull it apart.

So to do this, your outter part of the shell would mostly be Silicates which melt at 600-1200°C. Next you need a thin layer of something too hot to melt like Osmium or Tungsten that melts at about 3400°C to keep your fuel from mixing with your silicates... otherwise, your volcanoes will be extraordinarily radioactive. Next you have your fuel layer, probably some mixture of molten salts and a fissile fuel source like uranium, plutonium, thorium, etc. The salts will keep the fissile fuel from going critical, while the the fuel will create the heat that conducts through the outer tungsten layer to melt the silicates. So this layer will need to be somewhere between 1200-3400°C... probably closer to 1200 is best. In this way, your shell could conceivably be only 10-20km thick (plate tectonics will make sure that this number is variable depending on where you stand). Not nearly the thousands of km required if relying on gravity pressure to heat your mantle; so, local gravity will not cause the shell to break up. Lastly, you need an inner tungsten layer to keep your molten radioactive salt bath from spilling out onto the inner shell. This layer will also glow red-hot providing 24 hours of artificial light for the layer below.

enter image description here

The Middle Firmament Like Shell

With all that light radiating off of the bottom of the outer shell, the 2nd shell needs a lot of help to be a habitable place. An entire sky, glowing at 1200°C will make the surface temperature of this shell very hot. The main purpose of this shell will be to reflect heat back into the outer shell reducing fuel consumption and blocking the heat from reaching the inner shell. Since the outer shell can absorb more heat via conduction + radiation where as this only gets through radiation and maybe a negligible amount of convection, it should heat slower than the mantle, but it will not actually be able to passively stay cooler because trapped between 2 shells, there is nowhere for the heat to go. So, this layer will need some kind of cooling system that exchanges heat back up through the outer shell where the heat can radiate off into space.

To do this, you need vents in the outer shell to create a convection cooling system. The Middle Shell will be covered in oceans that will boil from the intense heat, that steam will form cloud that will move up to the surface vents releasing clouds that will radiate the heat off into space. Those clouds then rain down to fill the top layer's oceans that then pour cooled off water in great waterfalls back down onto the second layer.

While the surface of this ocean will be boiling hot, the deeper you go, the more it cools off; so, you could still have a massive aquatic ecosystem here. The other interesting feature this shell has is that it will act as a filter, allowing some of the radiated heat from the 1st layer to reach the 3rd layer. So with great windows, this layer will act like the Greek idea of the firmament: a sky with windows that lets in just enough light through holes that create the "sun" and "stars".

enter image description here

The "Dark" Inner Shell

While the middle shell will let though enough heat to make the 3rd shell comfortable to live on, it will all be infra-red light. Not a good place for Earth like life to thrive, but any organism that evolved to live around a dim red or brown dwarf star will feal quite at home here.

Why Build such a wacky world?

While each ecosystem is unique, they could each support life the evolved in very different ways. This "world" could be the capitol planet of a massive 3-species alliance where no species can live on the same kind of world, but this lets them all live in very close proximity to one another.

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  • $\begingroup$ +1 for such an in depth answer but for further clarification all or at least of the shells of my world were originally built to support human/post human life on a massive capitol world $\endgroup$
    – icewar1908
    Commented Jun 11, 2023 at 2:04
  • $\begingroup$ @icewar1908 Yes, the outer shell could be very Earth like. The inner shells could survivable if done right, but would not get the sunlight needed for agriculture. So, you could have an agrarian outer shell and urbanized inner shells if this is purely a human construct. $\endgroup$
    – Nosajimiki
    Commented Jun 12, 2023 at 14:44
  • $\begingroup$ fair enough my main idea was for the lower layers to use low tech(relatively speaking) artificial lighting which could be powered through various ways. $\endgroup$
    – icewar1908
    Commented Jun 13, 2023 at 14:32
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Lava Sandwich

There is a layer of lava in between the floor of the outermost layer and the ceiling of the second outermost layer. This causes tectonics and volcanoes. Layer thickness not to scale.

enter image description here

What moves the lava around? Whatever moves the lava around on real planets. Tidal forces.

What keeps the lava hot? Whatever keeps real lava hot. Nuclear forces.

What stops the lava falling out? Whatever handwavium stops the megastructure collapsing in the first place

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