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It would have to be much hotter to sustain any kind of magnetosphere yes, but assuming that it could, with the remaining 30% to play around with for other kinds of elements, and the 70% heavy metal being anything iron or heavier, could it support a crust and plate tectonics?

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    $\begingroup$ Please pick one between the two mutually exclusive tags you have selected: science based and science fiction. $\endgroup$
    – L.Dutch
    Mar 19 at 4:53

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Yes

While the heavy metals in general need to get a bit hotter, most of the heat in our mantle comes from the nuclear decay of heavy metals. If your planet has more heavy metals, then it stands to reason that there is more uranium and such decaying inside the planet heating things up allowing for much larger outer iron core.

The crust itself would not be all that different than other rocky planets. Even at 30% of the planet's mass, there is still plenty of mass for silicates, oxides, etc. for a normal crust to form, and even for there to be an outer mantle of molten silicates hundreds of km thick. With a very similar top composition to that of Earth, the mantle could still do all the things our mantle does, you'd just hit the transition zone between molten silicates and molten iron at a much shallower depth.

This shallower transition zone and hotter core will impact how aggressively volcanic your world is, but your planet's age and size can also be used to modulate this; so, it is possible to design a world with very Earth like tectonic activity despite the different overall composition just by making it older and/or smaller.

It's hard to say if this world would have a stronger/weaker magnetic field than Earth because you have not specified the proportions of heavy metals, or if the solid core is the same size as Earth's or not. Depending on what heavy elements you are adding, you could still have a solid inner core, but it might consist mostly of heavier than iron metals displacing the iron to the liquid outer core which would ruin the magnetic field... or you could add just iron which would increase the size of the solid iron core making the magnetic field much stronger. So again, there is a lot of room to modulate this to meet the demands of your setting.

interior of a heavy metal world compared to earth

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  • $\begingroup$ Iron and nickel are heavy metals too, as is copper and tin, for example. No need to change anything about the core other than its total volume and mass. The much lower depth of the mantle, on the other hand, will probably have significant impact on plate tectonics - less opportunity for slab pull, different temperature and density gradients... The convection patterns will probably be very different than Earth's. $\endgroup$
    – Luaan
    Mar 20 at 14:00
  • $\begingroup$ @Luaan The OP was not clear about his ratios of heavy metals; so, I was trying not to say anything in absolutes, but I just came off as unclear myself. My intended meaning was that you can use the variables the OP did not define to modulate plate tectonics to be Earth like if you need to. I've updated my answer to try to be more clear about it. $\endgroup$
    – Nosajimiki
    Mar 20 at 14:57
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Yes

Let us begin with the planet Earth. 30% iron, maybe another 8% other metals. So let's say 38% metal.

Now we add metal. Let's assume iron or nickle. If we add metal equal to the entire mass of planet Earth to a planet like Earth we would have about 69% metal in the core.

But everything else is unchanged.

Would there be consequences? Probably — but we can't really prove it. As I mentioned to an earlier question of yours, we have but one data point: Earth. Worse, there are a lot of variables that could contribute to active plate tectonics, like the gravity of the sun and other planets in the system mixing up the core of our test planet and how fast it's rotation is. But, if we just assume everything is Earth-ish, we end up with a planet that's twice the mass of Earth so, if I remember correctly, double the gravity. That's going to have the effect of compressing the crust and making tectonics more difficult.

But not impossible.

Is your goal believable? I believe it is.

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    $\begingroup$ Twice the mass does not mean double the gravity. Try designing a high metal content planet with a specific density, radius, and mass, and then use omnicalculator.com/physics/acceleration-due-to-gravity and this omnicalculator.com/physics/escape-velocity to calculate the surface gravity and the escape velocity. $\endgroup$ Mar 19 at 18:26
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    $\begingroup$ This answer seems a little meandering. Aside from incorrect assumptions (see comment above, only having one data point, everything being unchanged), I think it's lacking a definitive point of proof to reinforce the answer beyond the ever-popular "you're the author, make it happen however you want" mantra. Which may be appropriate considering the science-fiction tag, but I don't think this answer actually helps figure anything out about the problem. $\endgroup$ Mar 19 at 19:19
  • $\begingroup$ @M.A.Golding Your first link proves exactly what I said, 2X the mass, 1X the radius, 2X the gravity. I mentioned nothing about escape velocity and neither did the OP. You are in error. $\endgroup$
    – JBH
    Mar 20 at 3:25
  • $\begingroup$ @FlightDeck0112 You're in error, too. Having just proven that the link MA provided demonstrated 2X the mass, 1X the radius = 2X the gravity, I contend my answer is neither meandering nor makes incorrect assumpitons, but you're welcome to prove me wrong with actual detail. In fact, if you're sure that I said the author could do what they want (where in my answer did I say that?), why don't you step up and provide an answer yourself. Every point of view is valuable to the OP. $\endgroup$
    – JBH
    Mar 20 at 3:30
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    $\begingroup$ @JBH (Sorry if you got an extra notification, accidentally hit enter as I was typing.) Your comment added the clarification that you meant retaining the original radius of Earth, which was not clear in your answer and I think a reasonable interpretation is that you meant just adding the metal content without replacing any other material in the mantle or crust. Also, I recognize I didn't provide an answer myself - I considered writing one, but I'm quite happy with the one @ Nosajimiki provided above so didn't see the need. I hope you have a good day, my friend. $\endgroup$ Mar 20 at 12:41

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