As Yellowstone has no summit for lava to flow from, sounds like localized pyroclastic (sp) flow and ash clouds are the problem?
A caldera is the hole left after the mountain gets blown away. Sometimes a resergent dome will form in the middle as the magma chamber pushes its way back to the surface or there is a bit of leakage from the dome.
If you fly over Colorado before the snow completely covers it, the calderas stand out.