Maybe you weren't going to Europe anyway, what with those high airfares, but if you did buy a ticket you might not be going anyway, what with that volcano thing. Here are some things to consider:
1. Travel insurance will cover some of your expenses, but only as long as the hazard, in this case the volcano, wasn't a known hazard when you bought your insurance. No travel insurance covers all expenses, but can help a bit.
2. U.S.-based airlines are not responsible by law for expenses you might incur, however in Europe during the last big volcanic eruption the airlines did put people in hotels for various time periods, but not for ever. It was on a case by case basis. After all, an eruption is an "act of God" and not within the airline's control.
3. Many airlines refunded fares with no problem. If your flight is cancelled, you're entitled to a full refund. But you must wait until the flight is cancelled. You can't cancel your flight yourself in anticipation of not being able to fly and expect a refund.
4. The best fare to buy is a fully refundable one when flying during a known hazard. These are more expensive, but you get your money back even if you cancel.
5. You can also buy "cancel for any reason" insurance, which is more expensive, and the cancellation must be made 48 hours or more before your trip, but in most cases that's doable with a situation like this.