I'm adding bells & whistles to my recent map processor, that simulates a meteor storm hitting the map. Basically, meteors are made of obsidian and very hot. So, basic small craters are made of sand with an obsidian bottom, and some amount of lava so trees get set on fire and what not. The maximum number of meteors that can hit a chunk is configurable, so you could pretty much turn the planet into a giant burning ball of disaster. Good times. Screen cap below -- you can see trees burning from their recent encounter with a meteor.