I do wonder...someone led that child to exogeology, don't you think? And where's the line between unschooling and homeschooling, anyway, if you are helping the child along and providing materials?
I'm sure quite a few of us have kids who have interests we never considered and that's true whether kids go to school or not. I can't say I even remotely understand most of my teenager's interests so I sure didn't lead him to them. I don't think someone has to lead a kid to exogeology. Bright kids read, follow weblinks, meet other people with similar interests... and on it goes.
Personally I don't find the distinction between unschooling and homeschooling to be a particularly important one. But, as a general answer I'd say the different is in who is leading the education. Unschoolers aren't against parents helping their kids, it is more about avoidance of compelling the child to learn a particular topic in a particular way.
