Google has just released a search tool called Wonder Wheel that helps me to exactly that. Here's what I found on earthquakes, for example. earthquakes - Google Search. You can see the topics that google lumped up under the heading "earthquakes". I'm not quite sure how they decide how many spokes or how categories get assigned, but I'm going to investigate that.
I can imagine using this at the beginning of a unit when we are trying to brainstorm topics that are related to the unit title. I do a bit of background vocab work and this will work so nicely on the SmartBoard. As you can probably tell, you can click on any of the spokes and it will take you to a sub-wheel that breaks that category down even further.
Can you imagine how easily students can begin to see how Google's Search engine "thinks" about topics? And how those topics would be great jumping off points for their own inquires? I sure can.