Long chain of causation from A to Z, where A and Z are not intuitively connected.

Also, volcanos are pretty chaotic themselves.

Non-butterfly would be: due to increasing population size and a growth of cities and long distance commerce, people invented cars. A very stable reason, not reliant on something fragile.

Butterfly: the balance of magma happened to be in a certain particular configuration, which ultimately lead to a revolution in our lifestyle. Had the magma been slightly different, we'd be on horseback.

Again, maybe the story above does not add up, but if it does, it deserves to be called butterfly.