The steam, as evidenced by the geysers and bubbling springs are surface phenomena. As Thorne has related, the energy contained within the caldera is stored in gases as well as the upwelling magma. When Mt St. Helens popped, it wasn't steam that blew out the side of the mountain. When - not if - Yellowstone pops; which it has many times in the past, the east coast north of Miami will be toast, at least as far east as Pittsburgh, and the rest of the world will be impacted by the amount of ash sent to the stratosphere. But hey- melanoma cases will drop like a rock!