If calderas were brag-prone creatures with bumper stickers, La Garita's would read "my volcanic eruption kicked your volcanic eruption's butt." La Garita in Colorado's San Juan Mountains cut loose with a massive magma explosion about 27 million years ago during the Oligocene Epoch, spewing out an estimated 1,000 cubic miles of ash, its center collapsing under the force. To date, La Garita is the largest known single volcanic eruption in the world. In only a few weeks, it pumped out about enough material to fill Lake Michigan. "Ash from La Garita killed everything for at least 100 miles in every...