Near the bottom level of my map, I dug out some space for magma shops, dug some tunnels on the level below to carry magma from the magma pipe, and then channeled out the holes in the floor to place the magma shops over. I've done this several other times on other forts, no problems. Unpumped magma can't flow upwards, so it doesn't leak into the magma shop rooms despite the fact that there are a dozen z-levels of magma above it. Except for this time.
Every so often, a splash of magma will "burp" up out of the hole closest to the magma pipe, leaving 3-5 tiles of 1/7 magma on the floor. It doesn't burp up out of any of the others, which also have 7/7 magma beneath but are more remote. Also, the "burping" hole is directly over a wider section of the actual magma pipe on the level below, not over an "artificial" magma tunnel (not sure if this would matter).
I can't build a shop over this hole because there's always some 1/7 magma laying around. Even if I could I don't think that I'd want to, since an untimely burp would probably set the worker on fire. The only thing different with my current setup (compared to my past setups) is that I used no floodgate, grates, etc this time, just digging, since I had the final system planned out from the beginning, and had already killed the nasties in the pipe. No pumps are involved. The pipe hasn't completely refilled from draining into the tunnels; the top level has a bunch of 6/7s that are cycling around. I imagine that the burps also create some top-level 6/7s.
Anyone seen this before, or know what causes this?