Yeah, it does. Where was the volcano in the world?
Here it is:
The island is a wilderness temperate conifer forest on peat and clay, with enough muddy pools to get started with fresh water. The ocean is a wilderness temperate ocean.
As Jachim says (thanks Jachim!), be prepared for multiple collapses, as the ring of obsidian gradually moves inwards and seals off the magma in a vast flurry of steam. For maximum awesomeness edit your announcements.txt to take out the pausing-and-recentering, then watch it happen from the surface. When it's all over you have the ocean surface, then a z-layer of water, then the ocean floor with a sunken crater, then obsidian in the crater, then magma below the obsidian. The volcano is just a vertical magma pipe going all the way down to the magma sea. Oh yes, and instant adamantine, but can the king get to you since you're on a tiny island??
I'm trying to figure out how to build an offshore fortress above the magma. The ultimate no-bridge method would be to dig down below sea level, tunnel under the seabed to reach the volcano, bring a magma channel back to shore, construct a pump-stack to pump it up to the surface, then start piping it out over the ocean surface to form an obsidian walkway. Once over the cone, drop more magma to lay your foundations. Then move all your dwarves over, and deconstruct the walkway behind you. Voila, dwarves on a microscopic obsidian offshore island, sitting atop an active magma pipe.