Daksin is correct. Here is a good description:
http://en.wikipedia.org/wiki/Superheating
If it was just the solution coming in contact with the superheated neck of the vessel, only the portion of the solution in contact with that surface would boil, and you would not get the geyser described...
So if your first hop addition is at 20 minutes left in the boil, is there any reason to boil the traditional 60 minutes? In other words, can the entire boil be reduced to 20 minutes?