National Geographic video
(copied from National Geographic website*)
According to the United States Geological Survey, a volcano is considered “super” if it has had at least one explosion that released more than 240 cubic miles of material—a little more than twice the volume of Lake Erie. That places it at a magnitude of eight, the highest ranking on the Volcanic Explosivity Index, or VEI, which is used to measure the explosiveness of an eruption.
These are very large eruptions, the impacts of which would be widespread—from avalanches of hot rock and gasses racing down the volcano's flanks to global changes in climate. But there's an important caveat about supervolcanoes that most people commonly overlook: Just because a volcano has had a super-eruption once or even twice in its past doesn't mean its future eruptions will be just as big.
What's more, very few volcanoes reach such a super-status. For the some 5,000 eruptions with an assigned VEI that took place during the last ten thousand years, not a single one ranked a VEI 8, according to the USGS.
