What happens if I move a cap such that the first erupting piece would land in an empty space?

That's an eruption
Caps (and erupting pieces) can be moved into any empty space, including the space in the center (called the Caldera) which only remains empty until a piece erupts into it. Of course you won't capture the piece which landed in the empty space, since it did not land on a piece of the same size.
Related Rule(s)