Good info Jeephammer.
The voids created by the cavitation of an impeller are created by the low pressure on the backside of the impeller blade. The low pressure forces water to convert automatically from liquid to a gas, but it does not introduce air into the system because the pressure in the system and the (relatively) low temperature. It creates steam.
Since the water is at a near boiling point it only takes a little bit of negative pressure for the vapor pressure in the water molecules to exceed the system pressure. That negative pressure is created on the back side of the impeller. This creates little steam voids.
The steam voids collapse immediately on the other side of the impeller because the steam void is not created by heating (like a pot on a stove), it is created by negative pressure.