What supports the Rocky Mountains in North America? Or put another way, Why are they there? and Why are they still there?
It might be tempting to think of compression, especially in an east-west direction. However at the south end, New Mexico is experiencing east-west extension (grabens including Rio Grande Rift Valley, plus associated volcanism).
Hot spot and/or mantle plume uplift doesn't appear to explain them either. Yes there's the Yellowstone Hotspot under NW Wyoming. Regardless of whether this is really caused by a mantle plume or not, the related volcanism does not correlate with the mountains.