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Super-habitable planets and volcanism?
In this article: 
There's an interesting discussion on super-habitable planets. But something caught my eye:

"Once you get about twice the mass of the Earth, plate tectonics start to shut down, so try to stay under that amount."

...why? Wouldn't plate tectonics be more vigorous on larger terrestrial planets because of greater internal heat?
Mike Miller, Materials Engineer

"Everbody's always in favor of saving Hitler's brain, but when you put it in the body of a great white shark, oh, suddenly you've gone too far." -- Professor Farnsworth, Futurama
I think they are talking things like bigger gravity. This gravity outcompetes the force that push out.
So apparently, according to this source, the increased gravity strengthens the crust more than thermal convection can rip it apart. In a more recent paper, it was found that the thermal convection in superearths might not even be that much stronger, switching to being conductive above 2.5 earth masses. Hope this helps.
The planet Seattle is currently the largest SuperGaian that has a complex biosphere in OA. With a mass of 7.5 Earths this is on the large size for a planet with plate tectonics, but it is much less dense than a planet with an Earth-like composition, so the iron core must be a much smaller fraction of the whole. Perhaps this factor has allowed the tectonic activity to persist.

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