Hm. I'm not sure if you'd end up with a geyser. Assume a chimney fire first. It's like a square pipe with 20cm - 30cm side length and - depending on how active

Hm. I'm not sure if you'd end up with a geyser. Assume a chimney fire first. It's like a square pipe with 20cm - 30cm side length and - depending on how active it is - 200 - 400 degrees in there. That's already a condition you cannot introduce water into, because if water flash-boils into steam, it can expand it's volume by a factor of up to 1700. This can cause the chimney to turn into pretty much a pipe bomb, with the additional bonus that it's going to distribute the fire previously contained inside the chimney all over the house. And now we're talking about some much, much hotter situation (melting rock), pretty much contained (because that 1m hole isn't going to vent much) hitting ideally a large amount of water? That's not a geyser, that'll be a bomb. For example, one of the largest steam explosion in the US was caused by a pipe 0.61 meters in diameter with steam at 400 degrees C. It left some 4.5 meter crater.