The question is about the interaction of magma and water, and ensuing changes in topography. The DF wiki says that when magma is mixed with water you end up with steam and some obsidian from the interaction; it also states that volcanoes will always refill themselves.
Context
What you have here is a volcano at ground level and I didn't pay attention when digging the moat around the fortress entrance, so there was a cave-in and I decided to channel the river to the volcano instead, dedicating the fortress to geological "science". Since the dwarves deconstructed the trade depot twice when the elves came (next time I'm building an ashery next to the depot, blocks of coal and oak wood goblets), the wealth must have triggered a megabeast attack so our tribute was in blood. The idea to build an ever-lasting sauna platform atop the volcano came to mind, but safety first, so we dug a channel to bring the joy of magma to the caves below.
To summarize, you have the river going into the volcano and the volcano emptying itself some 15 layers below into the caves. The water is on top of slowly receding magma. My issue is whether a floor will entirely form itself out. As you can see top left or center only a few tiles seem to connect to the underlying magma at this point and this is barely 3-4 levels below the surface.
I'm trying to gain insight and analyze possible outcomes.
- Should I increase the magma flow in the caves to prevent such a floor
from forming or is it unavoidable? - How many tiles of magma flow are required to "fend off" (stop floors
from forming) a 3-tile(x2) water flow on top of it? - Is it possible to control the process to steer how the topography
gets shaped? - Generally, had I not channeled the volcano into the caves, would a
floor have formed immediately on the surface where the water meets the
magma, or do you end up with unlimited steam; can you have a water flow that is little enough to not generate floors (only steam)?
Best Answer
You raise multiple questions, but it all does boil down to how does magma react with water.
1.There will always be a surface of obsidian forming where water and magma meet. In your case that means that a surface of obsidian is unavoidable. From what I can see, this may have already happened. (I do not see an indication that the water can currently reach the magma anymore)
2.In order to for magma to 'fend off' the water, the water essentially needs to evaporate before reaching the open magma. When that happens depends upon the height of the water in your channels, and the temperature. At higher temperatures, water can evaporate at greater heights (not just 1/7). Temperature here will initially be based of the climate (Temperate, Warm, etc) and Season (summer/dry). However, once some obsidian has formed, additional water must flow over it to reach magma. The temperature of the obsidian is raised by the magma, which in turn allows water to evaporate more readily.
3.The position of the magma as it gets hit by water creates the topography. Normally this creates a relatively level surface of obsidian, because the magma should be fairly level in a magma pool. Diverting magma (channels, cave-ins) and choosing where it touches water (controlled flooding) will be the only way to steer the process.
4.Mostly answered with point 1, diverting water into a volcano will always cause an obsidian 'surface', but according to the wiki only steam (no surface) forms when rain hits magma, but as rain does not cause 1/7 water on open ground, it may be less water than can normally be moved.
As a *fun* note, your volcano will continuously attempt to fill (with magma) up to its original height, which will flood your caverns up to the height of the volcano if it can't flow off the edge of the map.