This Idea is short and simple: Make the Geogen output one Obsidian for every 4 Lava it proceeds.
If Lava cools down you get Obsidian. So what does a Lava in a Geothermal Generator? Yes it cools down!
So isn't that realistic enough to say more?
Edit: The Geogen could use Water to increase the Obsidianproduction or can use Water in general for generation, so the Geogen needs two Liquids to run.