There are many more places where water destroys earth. It also doesn't explain what earth does to air.
I have no problem with this answer, but there shouldn't be an ssumption that it makes sense. If a fireball takes out water, then a lightning bolt shouldn't have a special effect on water either. Either maintain logic, or throw it out the window completely.
I'm not 100% positive, but I think it depends on the game. Some games have just the versus where Fire versus Water have bonuses against eachother, while Air and Earth get bonues against one another. While other games go in a circle, Air beats water, water beats fire, fire beats earth, and earth beats air.
Then there is Pokemon, which has so many different types that it's near impossible to keep track of them all.
Makes it dusty.
Water, given enough time, can destroy anything. Well, except Air, I suppose. In the hypothetical example of Fire mage versus Water mage, if the Water mage can control the temperature of the water as well, it'd probably be possible to just cast enough cold water that the fire can't stay lit. But then it's magic fire, so who can say? It depends on the setting - "Magic A is Magic A" and all that…
I see Earth as the opposite of Air because it's solid matter as opposed to Air's openness - you can move through both Water and Fire as well, though obviously unless you have the apropriate immunities, you might not find it too comfortable depending on the nature of that particular Water or Fire.
Hey, it's Fantasy - as long as the rules are consistent within the setting (ie Magic A is Magic A), I don't think it really matters how the stuff works or relates to other stuff, only that it does within the given "rules".
I quite like the Schools of Magic present in the Might and Megic games - there's the Elemental schools, which are of course Earth, Air, Fire, and Water. Then there's the Self, whcih consists of Spirit, Mind, and Body. Finally there's the Mirrored Path, which is comprised of Light and Dark. The effects of plenty of the Mirrored spells can overlap with other schools, particularly the Self (for example, a load of Self spells are buffs, while Light has one or two spells which buff the same things but in greater quantity or whatever). If we were to include the Self schools of magic in our Sentinels groupings, that'd give people like Visionary and Mr Fixer more obvious places to go ;).