There are no universally accepted criteria for supervolcanoes. Volcanoes exist on a continuum, albeit a logarithmic one, so categorical edges tend to be fuzzy [sources: Achenbach; Geological Society of London; Tyson]. Consequently, estimates vary regarding the number of supervolcanoes, and of how often they blow their tops.
But a few common delineations do exist, including magnitude, the volume or mass of magma erupted, and intensity, the rate at which that magma erupts [source: Geological Society of London]. Magma is hot, molten material that comes from within Earth's mantle or crust and is forced out as lava during volcanic eruptions. It typically contains silicates, suspended crystals and dissolved gases [source: Oxford Dictionary of Science].
Another common categorization, called the Volcano Explosivity Index (VEI), classes volcanoes according to ash column height and the quantity of ash, pumice and lava ejected [source: USGS]. Supervolcanoes commonly occupy the highest VEI category, magnitude 8, meaning they produce more than 240 cubic miles (1,000 cubic kilometers) of erupted material and a plume more than 16 miles (25 kilometers) high [sources: Marshall; Rowlett; USGS]. Supervolcanoes rain destruction upon entire regions and leave behind calderas the size of Rhode Island [sources: Achenbach; Geological Society of London; Robinson; Tyson].
For such enormous and destructive wonders, supervolcanoes are surprisingly difficult to spot. Indeed, their size and power are part of the problem. Instead of building mountains, these giants blow them up. In fact, the Yellowstone National Park supervolcano was discovered partly due to a gap it created in the otherwise rugged landscape. Even then, its sheer extent -- 30 by 45 miles (50 by 70 kilometers) -- beggars the mind's ability to take it all in [sources: Achenbach; Geological Society of London; Tyson].
Add to that the vastness of time -- the hundreds of thousands to millions of years in which a caldera can erode, fill with lava from smaller eruptions or become a tree-lined lake -- and it's not hard to understand how supervolcanoes can hide in plain sight. But researchers remain stymied by yet another immensity, namely the scale of the processes that feed them -- mechanisms that reach deep into Earth and extend hundreds of miles across [sources: Friedman-Rudovsky; Geological Society of London; Marshall; Tyson; USGS].
So don't think of them as scaled-up volcanoes. Supervolcanoes are a phenomenon all their own, a deep process that we still struggle to comprehend [sources: Achenbach; Malfait et al.]. To better understand how they work, researchers turn to potential supervolcanoes like Uturuncu in Bolivia, which has grown half an inch (1.3 centimeters) per year for the past two decades, and to known hot spots of upwelling magma like the one beneath Yellowstone [source: Friedman-Rudovsky].