I was not aware that Earth was the only planet to have confirmed tectonics. That seems really weird. I wonder what the percentage winds up being between planets with and without tectonics.
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...
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.
Water may be one of the bigger requirements.
But yeah the factoid is 7x the water of the oceans.
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.
Well that would be great evidence that you don't need plate tectonics to sustain 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.