Just past the entrance to the ruins is a circular lake pictured below. Upon closer investigation, it is quite possible that this lake is a caldera.
As you can see in the background, there are a few other "mountains"(as they are called in the game), insinuating that this is not just one single volcano, but part of a small volcanic arc, possibly caused by subduction, much like the Cascade Volcanoes. However, since its shape is never clearly seen, it is not clear exactly what type of volcano it is, but concerning the fact that it is either not active at all, or active very little, it is safe to say that it is a type of mafic volcano.
Adventuring inside of the ruins, one explores the innards of an in-active volcano. The fact that amount of magma increases as one travels farther down into the ruins is caused by the fact that one is venturing closer and closer to the magma chamber.
This is from the first, and highest floor in the ruins:
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Just two floors below, the magma increases dramatically:
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All images in this entry were taken by my from the game: Rune Factory Frontier. Marvelous Entertainment USA, Inc and XSeed Games.2007 Nintendo
I like your theory that this is actually a caldera. No we can't really see a whole lot from that image, and I'm not really familiar with the game to know if another shot would be helpful or not, but it does look like there could be a ring of mountains surrounding the lake. Also the little bits of land protruding just above the surface of the water in random spots kind of reminds me of this picture of a caldera from Mt. Pinatuba:
ReplyDeletehttp://www.smate.wwu.edu/teched/geology/GeoHaz/vo-Mt-Pinatubo/vo-Mt-Pinatubo-11.JPG
The water doesn't look particularly deep at this time in the picture, but the sections that stick out of the water, surrounded by a ring of mountains, kind of looks similar to what you're talking about here.
~ Colin Fry