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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 m||1000s m3||Hawaiian||daily||Kilauea|
|1||gentle||100-1000 m||10,000s m3||Haw/Strombolian||daily||Stromboli|
|2||explosive||1-5 km||1,000,000s m3||Strom/Vulcanian||weekly||Galeras, 1992|
|3||severe||3-15 km||10,000,000s m3||Vulcanian||yearly||Ruiz, 1985|
|4||cataclysmic||10-25 km||100,000,000s m3||Vulc/Plinian||10's of years||Galunggung, 1982|
|5||paroxysmal||>25 km||1 km3||Plinian||100's of years||St. Helens, 1980|
|6||colossal||>25 km||10s km3||Plin/Ultra-Plinian||100's of years||Krakatau, 1883|
|7||super-colossal||>25 km||100s km3||Ultra-Plinian||1000's of years||Tambora, 1815|
|8||mega-colossal||>25 km||1,000s km3||Ultra-Plinian||10,000's of years||Yellowstone, 2 Ma|