Nah, just craft a self replicating dirt golum that feeds off dirt and rock. If it doubles in population in a given amount of time, then eventually, sooner then you imagine, the world will be nothing but Air, fire, and water.
Depending on your campaign setting, either the golums all tumble into the planet's firey core and melt thus forming a sea of magma with a few missed chunks of inhabitable rock and a few eternally flying things surviving (which would be one cool campaign world, now that I think about it.), or if it's FR, you go down deep enough, everything falls into the plane of shadow. So on FR, the world would fall, chunk at a time, into the shadow realm. until only an inky blot remains orbiting the sun.
Hey... I think I have my next world spanning threat for the players to face.