Yeah, much of the area is volcanic, which is how it is speculated that the island was created in the first place. I believe the steam wall, much like the geyser vents which can be found nearby, are most likely a reflection of ocean water reacting with the super-heated volcanic material causing it to convert to steam. The velocity of the steam moving upwards is most likely the result of a lot of air trying to move through a confined space, the venturi effect. It is highly likely that the central part of the island (what looks like a salt flats) was created through this process by the minerals left behind by the ocean water as it turns to steam. Over time the land builds up on the salt and other minerals and eventually pushes upwards and out.
I know what’s behind it, but I won’t tell because it would contain spoilers. You can actually pass through it… It’s related to the current events happening on Southsun.
