Back-arc basins are perfectly possible in the mid-ocean, as with the Mariana islands, but these do not produce Japan-sized islands if the overriding crust is not continental. That's the catch. There's only two clear examples of it on earth: Japan, which has formed a back-arc basin in the Asian continental crust, and Indonesia which is mainly continental crust too, but there it happens not to be quite thick enough to be consistently above sea level, so you get lots of large islands instead of a single landmass. All other back-arc basins in the world have overriding oceanic crust, and none of them feature large islands.Basilius wrote:Radius: first off, an hour ago I'd say that I'd hate any major revisions of our maps. My idea behind the claim that we need more islands was basically to provoke a discussion which would clarify e. g. how big the island could realistically be which we already have north of the Tt. Arc in our most recent tectonic map, and whether there might be smaller islands around it. Also, I agree that groups of smallish islands would do the job as well, maybe even better since they'd provoke faster progress in navigation.
But your observation about our islands being already too large by whole order of magnitude is... ehm... embarrassing... May be we should discuss *this* first, before we make any decisions on migrations &like.
I am a real hardcore nube in geotectonics, so please bear with me. The example of Japan seems to illustrate that relatively large islands can look like part or continuation of subduction-zone island chains (the Ryukyus and the Kuriles in this case), and Wikipedia says this is due to back-arc basin type of formations. Is anything of that sort imaginable for Sumarušuxi and Ttiruku?
So adjacent continental crust is crucial to getting those large islands to happen at subduction zones, and in Ttiruku there clearly isn't any that's close enough. If there were, it would be evidenced by (at minimum) a number of equally large islands set back a ways from the front of the Ttiruku arc - like Indonesia, which has Borneo and Sulawesi and the Malay peninsula behind the islands of the arc front.
So to bring the Ttiruku islands more in line with Earthlike reality, there are only a few possibilities:
1. Scale down the islands' sizes by a factor of ten or so, and deal with the difficulties this presents for our migration and cultural scenarios.
2. Draw in a bunch of new large islands behind the front arc. And it will take lots, or extremely large ones, because it's an awfully long island chain - fully the equal of Indonesia's.
3. Reduce most of the islands' sizes but leave one large one that we deem important, and chalk it up to a hotspot or an ancient continent fragment. That would be completely plausible for one or even two islands if they're near each other, an example being New Zealand, just not for the 4+ we currently have there.
There is also
4. Declare that Akana's geotectonics are different enough from Earth's to support what we've already got, even though we have no idea what specific difference this would be, let alone what its other implications would be.
I don't really like any of those. That is why I still recommend:
5. Continue to ignore the error on the grounds that what we've got is not completely impossible, it's just way out at the far end of the probability curve. There's no doubt that many features of Akana conlangs are in the same position, and some of this just has to be accepted because we can't know everything about everything, and the most important thing is that everyone enjoy their participation. So I've mostly held my silence about island sizes for years, and brought it up today only to keep us from worsening it.






