Reconstructing Eorzea's geological history.

[Xeia]

This topic is awesome, if a little confusing for me. >_>

• Thanalan Caves
  Thanalan is riddled by a network of tunnels that appear to have been naturally formed. In some locations, such as West Thanalan, these tunnels have opened up and connected with the surface. The presence of water suggests they are formed by subterranean water flows. (can someone find me a real-world comparison?)
  The mineral composition suggests rocks of felsic origin (mostly granites) that have undergone metamorphism through burial before erosion.
  The specific combination with amber (fossilised tree sap) suggests sedimentary deposits at least where this mineral is found (cave systems are easier formed in sandstones of eroded pegmatites rather than the original rock). In a previous age, parts of Thanalan would have had a very lush vegetation (wider historical coverage of the Twelveswood?).

Now, I admittedly no very little about geology, so I could be way off on this... I know you said the mineral composition makes it unlikely that it's sandstone, but it's possible the items recovered in mining can't be trusted, as Mjollnir pointed out with botany. As far as a real world example of that, I'm sure the sandstone caverns of Arizona (and surrounding areas) came to mind. They also often are sort of... "open-roofed" caverns, like the areas in Western Thanalan on the way to the ferry.

If you want something less... sandy... then I do have another suggestion but it comes with a heavy disclaimer of I probably have no idea what I'm talking about. Upstate New York's Howe Caverns. They're made of limestone, though I have no idea if that's ...felsic, but I'm guessing probably not, since it isn't granite. Now... I know Howe Caverns don't open up to the surface like you mentioned, but they do have flat ceilings with "pillars" reaching up to them. It looks a lot like the geography of Western Thanalan would if you just slapped a large flat rock on top of it. Conversely... I thought maybe Howe Cavern would look more like Thanalan if that ceiling got thin enough and collapsed, only leaving thin strips where the "support beams" are to hold it up -- like those floating paths we have in game. Though the color doesn't match.

Sorry for the long paragraph, especially if I'm talking nonsense >_<

Options for Eorzea include rapid geological progression during Umbral Eras, while Astral Eras return to "normal" (what we are familiar with).

If we take everything at face value, then this idea is supported pretty well by the game, I think. We've played the game during an astral era and everything was static (or, to be lore friendly, changing slowly enough that there's no observable differences day-to-day), while the Shroud of v1 looks drastically different from the Shroud in the alpha/beta gameplay trailers, and all that difference has occurred just over five in-game years. Concept art shows other dramatic changes besides what we've seen there.

Of course... a lot of these changes were actually caused by real world circumstances, so it's unclear how much of the 1.0 geography has actually changed as far as the Eorzeans are aware and how much we're supposed to consider retconned into its new 2.0 form.