I remember something about the Colorado river valley from when I was studying geology. I think that the layers of rocks along the valley were created by massive lava flows, some which were hundreds of feet deep and as the river cut through these layers can be seen. If this same kind of volcanic outpouring were to happen again then I think much of Colorado would become uninhabitable. Also, did you know that Yellowstone national park contains one of the largest caulderas in the world. It is estimated that it blows it's top every 650,000 years and guess how long it has been since the last eruption. If this cauldera erupted it could cause worldwide effects like that of an asteroid hit........gives you something to think about. Catlin