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[Rekov]

I am curious about where if anywhere obsidian floors will be created. When dropping magma onto water, do I get floor sections of obsidian above and/or below the wall section of obsidian? What about when I drop water onto magma? There are basically two situations I want to avoid:

If I am dropping water into a volcano, I assume the top z level of magma will turn into obsidian, capping off the volcano. If I mined out the obsidian, I would presumable be left with an obsidian floor below the z level to which the volcano would refill to. I'd have to channel out this floor with lava slowly rising through it as I did so.

If I am dropping lava into a river, I want to avoid flooring the bottom of the river with obsidian, which I would only be able to remove by channeling and thus deepening the river.

[Gwolfski]

THe river floor would be obsidian when you mine it out. Why do you not want the floor to be obsidian in a river? if the entrance and exit arent blocked, the river will be fine.

[Zuglarkun]

As far as I am aware of, if you dig out the obsidianized area, you'll get a floor of the same material as what the layer was originally. So if the area was originally sandy clay and you dug out the obsidian, you'll get a sandy clay floor. If it was a rock area, you'll get a rock floor. At least this was true as of version 40.24. Not so sure if this has changed since in 42.06.

You'll cap off the volcano if the adjacent tiles you are dropping water from are touching an adjacent wall, if not, it'll just cave in.

I don't think there is much difference whether it is magma onto water or water onto magma, though you might get steam only if you trickle water onto a magma filled area via bucket brigade. In that case, you'll want to designate a pond area one z-level above the top of the magma.

[Panando]

When casting magma you really want to pump it up into a dedicating casting space unless your goal is to actually block water/magma.

My normal simply casting strategy works like this:

Channel out a 2-deep room.

Pump magma into the bottom level of the room using a magma safe pump (powered by a dwarf).
Once the bottom room has a good layer of magma, pump water into the higher level (also powered by dwarf).


This design is very simple because it doesn't require any mechanisms. You just start and stop dwarfpumping as required. The obsidian can be simply channeled out and the process repeated as many times as you want.

[PatrikLundell]

Obsidian floors will be formed on top of all obsidian tiles you create PROVIDED the floor level is free. That level is NOT free if you have an up/down staircase there (which isn't uncommon when obsidianizing the magma above a magma flow). If you remove the staircase after obsidianization you end up with an empty space tile with an obsidian tile underneath (and you can't channel away that obsidian, removal can only be done by digging from the side). Removing the staircase before obsidianizing the tile below will provide an obsidian floor.

As others have said, casting obsidian and then channeling it away results in a bottom floor of the "natural" material in that area. This means that casting obsidian on the surface and channeling it away will produce a floor of the normal soil type (including any "contaminants" in the form of patches of stone normal to the area). Such a ground level soil floor supports vegetation, including herbs (probably saplings and trees as well, although my experiments were made on top of a built floor, so there was no room for roots). Casting obsidian above the ground level ("up in the air") and then channeling it out resulted in soil that did support grass but no herbs in my experiment. As far as I've seen, obsidianizing magma in it "natural" environment (the magma sea and magma pipes, which includes volcanos) always results in an obsidian floor when channeled away.

I'm not sure what you get if obsidianizing a stream and then channeling it away, but would assume you'd usually get soil, i.e. the same floor as was there previously. If you drain a murky pool, floor the bottom, and then remove the floor, the "murkiness" property of the pool floor goes away (e.g. allowing vegetation to grow there), and if there is some kind of property associated with a stream bed floor you might destroy that property through obsidianization and subsequent channeling, as I would expect would happen if it was done to a murky pool.

The reasons for the original questions are not clear, however. Why do you want to avoid converting a stream bed to obsidian (which I think won't happen, as per the above), i.e. why does the material matter?
Why do you care about what material a volcano floor is made of (I believe I've only seen obsidian, as per the above, but I haven't cared much)?

Magma pipes (volcanos are just magma pipes that reach the surface) have a "top level" to which it will fill the pipe up with magma. This means that if you lower the level it will fill up again, unless you prevent it from filling up. The process by which magma pipes are refilled is somewhat strange, however, as magma doesn't actually well up from below, but rather rain down from above. As far as I've seen from my experiments, magma pipes are refilled by 1/7 magma randomly forming at the pipe's top level (or the highest level reachable, if a floor blocks the level above) over tiles that are directly above a magma flow tile. From there it falls down to join the top level of the magma (or on top of the previous level, if the top one is 7/7 already). I suspect these 1/7 lots of magma can "flow" sideways while in the air, thus falling on tiles beside the one it was formed over.
The above means you can safely obsidianize the top level of a volcano and channel down into the resulting obsidian to reveal the (obsidian(?)) floor. When you channel down into this floor, however, you provide a path from the magma flow above to the level you channeled from (and, and exposed levels above that one), allowing magma to be generated and rain down above that tile.