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In my world, human civilization would live on an Alderson Disk.

An Alderson disk (named after Dan Alderson, its originator) is a hypothetical artificial astronomical megastructure, like Larry Niven's Ringworld and the Dyson sphere. The disk is a giant platter with a thickness of several thousand miles. The Sun rests in the hole at the center of the disk. The outer perimeter of an Alderson disk would be roughly equivalent to the orbit of Mars or Jupiter. According to the proposal, a sufficiently large disk would have a larger mass than its Sun. (Source and additional info)

In short, it is like a giant DVD with sun as it’s center. The outer perimeter of it will be equal to the orbit of the Jupiter. It is a giant megastructure that can be home for quadrillion of peoples and aliens.

The disk will be constructed using asteroids and will therefore be rich in metals and nutrients.

It seems to me that an Alderson Disk will have no volcanism or tectonic movement, therefore no mountain building and, above all, no rock cycle.

The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

The question: Can an Alderson Disk have either a natural or artificial rock cycle?

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    $\begingroup$ Welcome to worldbuilding. As you can read in our help center, we prefer to answer 1 question per post, while you are asking 3 at the moment. $\endgroup$
    – L.Dutch ♦
    Commented Feb 20, 2023 at 18:33
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    $\begingroup$ Welcome to Worldbuilding. Please make your question self-contained. What is an Alderson Disc and what are the important features? $\endgroup$
    – Daron
    Commented Feb 20, 2023 at 21:20
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    $\begingroup$ Of course it can have an artificial rock cycle, if you want it to. I don't see why they would do it, but the incredibly rich and powerful inhabitants can always build gigantic machines to dig up the rocks, heat them up, subject them to massive pressure, oxidize them, hydrate them, and so on. On the other hand, the entire disc is an artificial construction, so that there can be nothing natural about a rock cycle. $\endgroup$
    – AlexP
    Commented Feb 21, 2023 at 1:22
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    $\begingroup$ @HarryMu: It cannot be a natural object. If it were a natural object, gravity would quickly re-arrange into a sphere. Something un-natural is keeping it from collapsing under its own gravity and something must be actively keeping the sun in the center of the disc. $\endgroup$
    – AlexP
    Commented Feb 21, 2023 at 2:10
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    $\begingroup$ The disk requires active upkeep at all times--if the systems fail the disk is lost as the strength required to keep it in existence far exceeds the strongest materials. Thus you have active systems that can do whatever you want with the rock cycle. Civilization might have fallen but the systems maintaining the disk have not. $\endgroup$
    – Loren Pechtel
    Commented Feb 21, 2023 at 2:34

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"It seems to me that an Alderson Disk will have no volcanism or tectonic movement"

Well... why does it seem that way to you?

An Alderson disk is not a natural object, and it certainly could be constructed as a rigid, solid object with no internal dynamics... but in order to have Earthlike gravity, it would have to be several thousand miles thick, and could easily support internal fluid layers just like a ball-shaped planet. Convection in the interior would thus produce plate motion and volcanism just like it does on Earth.

That would complicate the delicate balancing act necessary to keep the whole structure from collapsing, though, so there's certainly good reason to intentionally build it as a cold, solid structure. In that case, the rock cycle would have to be handled artificially, much like it is on Niven's Ringworld--some mechanism dredges sediment from seabeds and redeposits it at the tops of mountains, with the weight of new sediment at the top crushing lower levels back into rock. And you would probably want such a system as part of the larger mass redistribution system that keeps the disk in balance anyway.

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  • $\begingroup$ How much weight will it need to crush the lower levels into rock? $\endgroup$
    – Cubelite
    Commented Feb 21, 2023 at 7:12
  • $\begingroup$ @Cubelite: It is not clear what you mean by "lower levels" of the rock. The main gravitational force will be towards the center of the disk, parallel with the surface. The actual lower levels of the rock are situated towards the center of the disc, being crushed by the rock situated outwards from the center. $\endgroup$
    – AlexP
    Commented Feb 21, 2023 at 9:22
  • $\begingroup$ The gravitational force will be just like earth, pulling you down to the middle of the disk. It could be done by aritfical gravity making. I would like to know how much weight of new sediment at the top is need to crush lower levels back into rock. $\endgroup$
    – Cubelite
    Commented Feb 21, 2023 at 11:07
  • $\begingroup$ @AlexP Not when you include the centrifugal component. Over most of an Alderson disk, the surface can be well-approximated as an infinite plane, and gravity points mostly perpendicular to the surface. The remaining components can be cancelled by spin if the disk has an appropriate radial mass distribution. It's not a stable equilibrium... but that's why you have machinery to maintain it. $\endgroup$
    – Logan R. Kearsley
    Commented Feb 21, 2023 at 18:39
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    $\begingroup$ @Cubelite Depends on the type of sediment, but generally... not much. Tens to hundreds of tons of pressure may sound like a lot, but dirt is heavy, and just a few meters of it will start to compact. If your disk has mountains and seas like Earth, it will form sedimentary rocks like Earth. $\endgroup$
    – Logan R. Kearsley
    Commented Feb 21, 2023 at 18:45
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Logan R. Kearsley has a perfectly good solution but it ignores certain atmosphere-lithosphere interactions that seem small but add up rapidly over geologically time. I think that to be really stable heat processes that break carbon dioxide free of eroded sediments can't be ignored, the good news is that an Alderson Disc has access to vast amounts of heat and energy. Dredges pulling sediment from the sea floor deliver it to pipes that pump it to the inner face where solar furnaces melt it into magma before it is released to the surface along a chain of artificial volcanic mountains. The volcanoes release CO2 , water vapour, and other gases into the atmosphere while the lava flows add weight pushing the land at the inner edge down and out under the force of gravity.

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