Does it not possibly refer to the 海底火山? (As opposed to the other 噴火 which they called しるし - if the context is that there's more than one). I think it's something they're looking at, not something actually mentioned in the bit of text you've given.
lit. "on this island, the thing/case that you'd read 灼 as あらたか, would it not be something like this?"
Where are the speakers while having this conversation, and have the terms あらたか・しるし (with or without 灼 ) been brought up before?