Volcanoes are measured and reviewed through the Volcano Explosivity Index, or VEI, which determines the quality of the volcano, the volume of the lava, how high the volcano measured at the time of eruption and how often the volcano explodes, according to the Old Farmer's Almanac. By reviewing each volcano against this criteria, it is simpler to determine which ones have had the most impact.
The VEI counts the volume of the lava in cubic centimeters. Depending on the amount of cubic centimeters of lava that a volcano produces, the volcano can be assigned a rating on a scale of one to eight. Naturally, volcanoes rated a one are the smallest in scale in regards to lava production, while those at an eight are the most intense and thus most dangerous.
The VEI also assigns number ratings based on how often that a volcano erupts. Those ranked on a zero or one scale often experience the most eruptions, Geology.com notes. As the numbers increase, the frequency of volcanoes decreases to anywhere between a few times each year to a few times each decade. A volcano rated at an eight in frequency only occurs at a rate of once per 100,000 years.
Conveniently, the VEI allows researchers and scientists to see how destructive a volcano can be through more numbered rankings. A volcano becomes truly dangerous at a ranking of three and is the most destructive at an eight, according to the Old Farmer's Almanac .