And this little fire cracker was only a 6 (the Volcanic Explosivity Index (VEI) - St Helens was a VEI of 5. Each digit of increase is 10 times the previous.
1815 A.D. - Mount Tambora (VEI) = 7, (100x greater than Mt. St Helens)
1912 Novarupta, Mount Katmai, Alaska, VEI=6
1991 Pinatubo, Luzon, Philippines, VEI=6
Yellowstone would be a VEI of 10 or higher... or 100,000 times Mt St Helens ...