How BIG are Volcanic Eruptions?

Every year about 60 volcanoes erupt, but most of the activity is pretty weak. How do volcanologists measure how big an eruption is? There is not any single feature that determines the "bigness", but the following eruption magnitude scale - called the Volcanic Explosivity Index or VEI - is based on a number of things that can be observed during an eruption. According to this scale, really huge eruptions don't happen very often, luckily!

VEI Description Plume Height Volume Classification How often Example
0 non-explosive < 100 m1000s m3 Hawaiian dailyKilauea
1 gentle 100-1000 m 10,000s m3 Haw/StromboliandailyStromboli
2 explosive 1-5 km 1,000,000s m3 Strom/VulcanianweeklyGaleras, 1992
3 severe 3-15 km 10,000,000s m3 VulcanianyearlyRuiz, 1985
4 cataclysmic 10-25 km 100,000,000s m3 Vulc/Plinian10's of yearsGalunggung, 1982
5 paroxysmal >25 km 1 km3 Plinian100's of yearsSt. Helens, 1980
6 colossal >25 km 10s km3 Plin/Ultra-Plinian100's of yearsKrakatau, 1883
7 super-colossal >25 km 100s km3 Ultra-Plinian1000's of yearsTambora, 1815
8 mega-colossal >25 km 1,000s km3 Ultra-Plinian10,000's of yearsYellowstone, 2 Ma