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Comment by throwaway5752 | Hacker News Reader

If you have a rotating spherical body with a (sufficiently) heterogeneous fluid composition, it seems like you'll get plate tectonics. When you realize it's just a fluid with convection currents and the surface of the fluid "freezing", you realize it should be relatively common for across planets for a portion of their existence.

ps - if any actual physicists out there are reading, I've been curious about the case of Mars and if tectonics there could have been halted by whatever caused the hemispheric dichotomony for a long time, in case there's any good reading/research on that out there to pass along...

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"Plate tectonics" means something a bit more specific than "convection currents with the surface of the fluid freezing". In particular, it implies 1) relatively large stable plates, 2) organized regions where crust is created/destroyed, and 3) large horizontal movement of those plates relative to each other.

Prior to ~2 billion years ago, Earth didn't have plate tectonics. You had large vertical motion (not horizontal), and creation+destruction of crust, but no organized centers where this was occurring. In other words, you didn't have large plates and what plates there were did not move long distances horizontally.

As another example, Venus has tectonic activity (it's periodically resurfaced and has volcanoes), but it doesn't have plate tectonics. You see regions of compression and extension, but no subduction zones and spreading ridges.

It's likely that what we call plate tectonics requires abundant water, at least on "rocky" planets. Hydrous phases of silicate minerals are required for the very weak faults and localized weak mantle required to have subduction. Without subduction, you don't have plate tectonics.

> Prior to ~2 billion years ago, Earth didn't have plate tectonics. You had large vertical motion, and creation+destruction of crust, but no organized centers where this was occurring. In other words, you didn't have plates.

Well that would be great evidence that you don't need plate tectonics to sustain life. ;-)

Yeah, the exact start of plate tectonics on Earth is very hard to pin down, but it's almost definitely after the start of life.

On the other hand, even when Earth didn't have plate tectonics, it probably never had a stagnant lid, which is what this article is modeling.

This is only tangentially-relevant, but I recently read that the earth's mantle is mostly solid! (It does still flow, but as a solid.)

The state of the rock beneath the crust is subject to two competing influences:

* High temperature [promotes liquid state]

* High pressure [promotes solid state]

and apparently the latter influence (mostly) wins.

Of course, when such rock reaches the surface, number 1 wins so we see liquid rock.

Also, the deep earth is filled with lots of water, well mixed that is. Evidently this lowers the melting point and promotes plasticity, from what I've read.

Water may be one of the bigger requirements.

Not exactly water, hydrated minerals. So stuff (mostly olivine) with OH groups stuck to them.

But yeah the factoid is 7x the water of the oceans.

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