This is one of the very few situations where I tend to recommend protective filters. The problem is that you don't know what's in the air (let alone what's in the ash, should any land on your lens). Sometimes the chemical mix of a volcanic eruption can be toxic to lens coatings, but very small particles in the air can also be highly abrasive. Thus, if you expose the front element of your lens to the volcanic air for any length of time, it's possible that you end up with stuff on your lens that "normal" cleaning will simply mush into the coating or lens surface and abrade. It's best to avoid that possibility at all by letting a filter take the hit.