Uggggh, I hate when people say "fractal" when they do not, in fact, mean anything remotely similar to a damned fractal.
You wouldn't happen to be my geometry teacher, would you?
I think he means it's causing a cascading effect of one ave-in causing another that ends up obliterating large sections.
However, as discussed previously, steam does NOT have a 'pressure' or otherwise cause the explosive collapses. It's simply a product of the instantly boiled water and floats around it's point of origin entirely benignly. It won't interact with anything besides causing creatures to register a 'caught in a burst of steam' announcement, without harming said creature. liquid water exposed to the surface in freezing biomes may be the culprit for the 'landmine' thread I linked, however:
As ice walls brush up against magma underneath it, it suddenly thaws into a 7/7 block of water, which either reacts directly with the magma, creating obsidian, or is displaced slightly, where upon it freezes again. If this water refreezes over empty space (like over the middle of a magma pipe after a portion of the water has solidified into obsidian and fallen in) however it will cause a cave-in, launching both free magma and water into the air, generating the magma mist and steam seen throughout. The magma lands on other ice, instantly melting it. the water launched into the air instantly freezes, falling to the ground in yet another cave-in, sending more magma and more water flying. the magma and newly unfrozen water mix forming an obsidian wall, possibly extending the violent reaction by melting tiles around them. This can go on until the magma is exhausted through producing obsidian, or anti-gravity runs out of luck (by a divine miracle...) and the reaction crawls to a halt. In this way that man's companions were sucked into the pit o' doom by the cave-ins, smashed against stone and ice walls, or melted in the magma mist, or even trapped in ice/obsidian as the floor melted out from under them and refroze/obsidianized. If this sort of thing commonly occurs over natural magma pipes, we have something worth being afraid of, besides the bogeymen. We know that magma pipes still occasionally get generated entirely submerged under water, and adding a few layers of ice is less of a delay and more of a fuel-source for the reaction.
I REALLY want to try causing something like this to occur in DF mode now... Just dig out a pit under a glacier and fill with magma, easy enough...