http://www.wired.co.uk/news/archive/2012-09/06/mount-fuji [video=youtube;CFWX0hWCbng]http://www.youtube.com/watch?v=CFWX0hWCbng[/video]
You can measure the amount of compression an object (erupted rock) was under based on distance from its ejection point. Pretty simple physics equation.
umm, yeah that works for a bullet out of gun barrel. It is commonly known a ballistics. But we have no gun barrel here nor anything quite as directional. Seems like swag is closer to the mark.
Maybe Reptilicus lived in the volcano before becoming a movie star, and he kept good records. That's why he needs no agent.
You don't have a barrel per se, but it's not that difficult to look at a large sample of ejected material, measure deformation, average distance from the center of the crater and come up with a pretty good statistical model for force and pressure. You can also look at recent measured eruptions of mountains with similar geology (like St Helens) and get an idea of the kind of forces that were at play in the 1707 eruption. It's never going to be 100% accurate, like a true measurement, but people that study this stuff aren't just pulling these numbers out of their ass either.