Water on top of slowly receding magma and obsidian-based floor formations - flow control?

Solution 1:

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.