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So I have an iron planet, and its name is Randall.

Since Randall is an iron planet, he is basically the core of a planet with no crust/mantle, and very few silicates. Now, I, the creator outside of the universe from which Randall resides in, wants Randall to have plate tectonics, for at least a couple gigayears (billion years). However, iron planets cool pretty quickly, and iron is, relative to silicate, much more durable and has a much higher melting point, so I can't figure out how an iron planet could keep said plate tectonics for long. How could Randall have these plate tectonics for a couple of billion years?

Note: I realize there are other "forms" of tectonics, like pipe tectonics, but the main feature I am looking for is that material deposited on the surface can eventually find its way into the "mantle" and out a volcano, allowing for a "rock cycle"-like cycle, so if you can find an alternative to plate tectonics that can achieve that and work on an iron planet, then that would be fine as well.

Note 2: My definition of an iron planet is that the planet is mostly made of iron all the way up to the surface, but plenty of the pie chart is composed of other metals, like nickel, zinc, etc.

TLDR: How can an iron planet remain geologically active for at least a couple of billion years?

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    $\begingroup$ How pure do you want the iron to be? The heat for geological activity originates mostly in radioactive decay, so if you have sufficient radioactive isotopes (a fraction of a percent should be more than enough), this should work. $\endgroup$
    – Matthias
    Mar 23, 2023 at 6:49
  • $\begingroup$ Well my definition of an Iron Planet is a that the planet is mostly made of iron all the way up to the surface, but plenty of the pie chart is composed of other metals, so it is plausible that there is a bunch of radioactive material lying around in the innards. $\endgroup$
    – KaffeeByte
    Mar 23, 2023 at 11:10
  • $\begingroup$ @Matthias you should put that in an answer $\endgroup$ Mar 23, 2023 at 11:55
  • $\begingroup$ Related: A planet made of iron $\endgroup$ Mar 23, 2023 at 16:22
  • $\begingroup$ My knowledge on tectonics isn't extensive, but isn't anything planet sized basically a liquid? It might look and feel solid to us, but over geological timescales it will move more or less like a liquid. Even iron planets. I think that is enough for billions of years already, but adding things like moons or other massive objects can help these processes to keep going, thanks to their gravity. $\endgroup$
    – Trioxidane
    Mar 24, 2023 at 14:53

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Short answer: Any amount of heat is possible with enough radioactivity.

Geological activity is driven by heat which is mostly produced through radioactive isotopes. Uranium, thorium and radioactive potassium are the main sources of the heat. They take very long to decay and are partly produced by other decaying isotopes, which is why they are still around billions of years after formation of the earth. Their concentrations is in the low ppm (parts per million) range. Your planet is probably smaller than the earth and conducts heat much faster, but even if your planet has isotope concentrations ten times higher than on earth it would still contain around 0.01% radioactive isotopes. They will run out eventually, but they will keep going for a few billion years.

So whatever amount of heat your planet needs to stay geologically active can be created without exceeding a plausible concentration of isotopes. The mountain ranges on the surface might look a bit weird, since iron has a higher elasticity than silicates.

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You could generate plate tectonics by giving the planet a crust of iron oxide. Randall's nickname would be Rusty.

In order to generate plate tectonics, all you need is for the base material (in Earth's case, basalt) to be chemically transformed into a lighter weight material (in Earth's case, granite). When the heavier rock gets subducted, the lighter rock will stay near the surface and pile up into continents.

The problem you would have is that rust is a LOT easier to form than granite. Granite doesn't form until it's under 15MPa and more than 200 degrees C. If you had a breathable atmosphere, iron ore would cover the entire surface. I don't know if you'd be able to differentiate between continents and oceans, and that's what comes to mind when you talk about continents.

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Imagine an all-iron planet forming as a molten ball containing a small but significant fraction of radioactive elements like uranium and orbiting relatively close to a very large hot star. Given the correct circumstances such a planet should be able to a solid crust of iron radiating heat and insulating the interior. The interior would contain a vast reservoir of heat which could be topped up with heat from radioactive decay as well as tidal forces.

If this planet had a very significant magnetic field and rotated (both likely) then it should also generate the complex fluid dynamics that drive the currents deep inside Earth ultimately driving plate tectonics. So plate tectonics of some form should be possible.

That said those plate tectonics would not necessarily be like ours as plates on Earth tend to float on the basis of variable material content causing difference in density, rather than on Randell where it would be the same material at a different temperature causing a difference in density.

One possibility would be that colliding plates would simply merge and become thicker forcing the lower parts down into the hotter lower regions where they would eventually melt and the upper parts up into mountain ranges (rather than one plate diving under another).

Another possibility is that the crust would contain a disproportionate amount of the lights elements that exist such as zinc and virtually all of the small amount of silicates and so would be light enough to mimic some vaguely Earth like tectonic plates.

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extreme tidal forces. make it a moon, and throw it around a planet a bit larger than Jupiter. give it him the Io treatment. that will teach him

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Not a scientist, but I suspect your iron planet with plate tectonics is going to be absolutely too hostile to be inhabited or even visited.

If there was crust thin enough to easily break up into pieces, then the molten iron is right beneath your feet, relatively speaking. Vacuum of the space is an excellent insulator, and the surface cannot cool down fast enough to provide habitable temperatures, probably way above boiling pount of water.

If the surface was cool enough for humans, on the orher hand, it would be solid, and if there still was plate tectonics, the "earthquakes" on the planet would be something never experienced here: the whole planet would vibrate like an enormous churchbell whenever the crust cracked, shattering everything, possibly throwing smaller objects around with such force that nothing manmade could survive.

It would be an interesting planet though, but only to be observed far away.

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