Hey Chris, first off, this is a great idea! Cudos on the concept.
I've looked at this image for awhile now. For me it's not conveying lava pouring from a building or even lava being thrown on a building from an eruption. I get the motion element of the lava moving in the air, it's the lava on the building itself that is throwing me.
Only this little section looks believable to me.....
The rest is too transparent or something.
Also, (even though I have never seen this in person) I would think that anything that came in contact with the lava would instantly burst into flame. I also think that any structure beneath the lava would quickly burn through and leave a deficit.
The angle of the falling lava is the same as the lava on the building, this makes it very difficult to distinguish the difference between the two. It's almost like a curtain of falling lava between the viewer and the building if that makes any sense. Maybe change the angle of the flying lava verses that of the lava hitting, sticking and flowing down the building.