There are about 500 active volcanoes in the world. There are three known supervolcanoes in the US, (including Yellowstone), Lake Toba in Indonesia, Taupo in New Zealand and Aira Caldera in Japan.
A supervolcano is a volcano capable of producing a volcanic eruption with an eruption radius greater than 240 cubic miles (1,000 cubic kilometers).
Supervolcanoes are the most dangerous, with a Volcanic Explosivity Index of 8. This is thousands of times larger than most historic volcanic eruptions. Geologists believe that Yellowstone is the most likely supervolcano to erupt next, as it has been exhibiting signs of unrest, with earthquake swarms, ground deformation and considerable heat and gas emissions.
Earthquakes are the primary indicator that a volcano is about to erupt. The Yellowstone caldera experiences about 1500 measurable earthquakes a year. The last eruption of Yellowstone was 640,000 years ago. If it did erupt, expect half of the USA to be wiped out, and major variations in global climate for many years. Residual volcanic ash in the atmosphere would result in the devastation of world agriculture, severe disruption of food supplies and mass starvation. Yellowstone could emit 2,000 millions of tons of sulphuric acid, and could produce the equivalent of a “nuclear winter”, when the dust and debris from the fallout of a nuclear war block out sunlight for several years, causing worldwide famines.