I have not tried it, but i'd assume it'd create piles. It will collapse, yes, but becuase it will create a solid *natural* obsidian wall, it should reform at the bottom after it collapses.
I think this could be remedied by setting up a seal-able chamber, floored by a bridge, where you want to create the pile. drop the lava for the entire layer all at once.
I've done a very similar thing before, when I was creating a giant bridge out onto very large lake that was several z levels deep. I wanted my giant bridge to have pilings that went down to the lake bottom, but I didn't really want to try to pump a hole in the water, and it never froze on that map.
My method was to build a sealed chamber with a floor (back before we could build more than a 1x1 floor all at once) that was one square larger than the piling I wanted to drop. I then filled it with magma and poured water over it to make solid stone.
I then "ramped out" that outer ring (thus removing the floor on the level above), removed those ramps, and then channeled out the floor (because mining out lava created walls that were built on constructed floors apparently leaves "natural" stone floors.) I left one floor section attached and carefully removed that with a disposable miner, causing the section of stone to collapse and form a "layer" of the piling. Sometimes the miner fell in, sometimes he didn't.
Oooo I've anted to do the tower in the ven t thing for a while too, it allways seems like a real hassle though. Can you upload the map I'd love to see it.
Given that the lighthouse is quite far from the magma vent my problem is going to be flow control because magma moves so slow. Does anyone know if pumps increase magma speed? Also does anyone know if the mechanisms will get melted or are they on the outside (and are the only axle materials wood?)?
(I knew I forgot to bring something, I wonder if I can get bauxite some other way...)
All wooden pumps can pump magma so long as the "back end" (the end where the dwarf stands) isn't touching the magma.
As for flow control, a simple way to get more magma moving is to breech the magma vent a few z-levels below the surface, which will allow the magma above the breech to act as a reservoir and will cause the magma to flow "faster". ("faster" in that magma will fall down from the reservoir and fill the tunnels so you don't have to wait for magma on the same z-level to slowly meander over to where you want it to.)
As for breaching the vent safely, I use this technique:
I find a magma vent layer that is smaller than the one directly below it. (Layer marked with (1))
Side view
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~~~######(1)
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I then dig the tunnel I want the magma to flow in on the level below
(Marked with (2))
Side view
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~~~######
~~~~~~# (2)
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I then dig an access tunnel above it, or at least right over the wall that separates the magma from my tunnel.
(Marked with (3), _ is the floor)
Side view
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~~~###___(3)
~~~~~~#
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Simply channel away the remaining stone wall, from above. Because magma doesn't flow up, the access tunnel is safe.
(_ is the floor)
Side view
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~~~### __
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