I think I'm starting to hate magma. Tried going through the middle part, didn't work. There was still semi-molten rock. I have no idea how to control the magma, or get to the adamantine. I actually abandoned the fort, but I foresee this becoming a problem with ALL my forts if I don't find a way to work with magma, and control it.
Finding magma (without being murdered by cavern-dwelling monstrosities) is the hard part - unless you intend to bring the magma all the way up to the surface as well, that is. Digging down until you hit warm stone or semi-molten rock is the first step - once you hit warm stone (I think if you have temperature switched off there may be an effect on this?) the designation will auto-cancel to prevent unintentional flooding with fiery death. Magma always adjoins a tile marked as 'warm', although it won't specify where - the uppermost level of 'warm' stone is directly above the magma, so dig up and check for warm stone, then once you find non-warm stone, dig two spaces into the highest level warm stone and channel down - this should reveal the magma sea safely.
You can then floor over the gap (to prevent the residents getting into your fort and causing trouble), dig out the top-level warm stone and use this area as your forge / glassmaking / pottery / goblin incineration chamber.
If you hit semi-molten rock, just dig exploratory tunnels across until you find warm stone, then go up.
Mining adamantine safely is pretty much a contradiction; the only ways to prevent magma reclaiming the area are to drain the magma sea (...sort of impractical), obsidianize the area around the adamantine, or dig only in the centre. Each of these has its own fort-murderizing dangers attached.