Mt Pinatubo started with a lot of seismic activity and it didn’t take long for it to let go.
http://en.wikipedia.org/wiki/Mount_Pinatubo
Pinatubo had been dormant for hundreds of years when it reactivated. After the eruption it left a 3 mile wide caldera. Remember all the nasty weather we had for several years thereafter. The 500 year flood of the Mississippi, the major winter storms from the Carolinas to Canada, etc. Imagine what a 30 mile diameter caldera event would do to the climate. Just checked the Yellowstone earthquake activity map again, and there have been more than 20 in the past 24 hours, including one 3.0.
http://www.seis.utah.edu/req2webdir/recenteqs/Maps/Yellowstone.html