Question: @James V - How many super-volcanoes are there in the world? How much damage would each one do seperatley and how much damage would they all do together?

James Verdon answered on 26 Jun 2012:

Hi babbyhoff,
Volcanologists don’t really use the term supervolcano – we use the Volcanic Eruptive Index (VEI) to describe the power of eruptions. The largest point on the scale would be a VEI of 8, so I guess these could be considered what you would call supervolcanoes. There are probably about 5 or 6 volcanoes that could erupt with a VEI of 8. Yellowstone is the most well known. Others include Toba in Indonesia and Taupo in New Zealand.

We can look at the geologic record to see what happens during these large magnitude eruptions. Obviously, anything nearby gets obliterated by the blast waves and clouds of scalding hot gasses and ash flowing down the sides of the volcano (called pyroclastic flows). In a wider region, things get smothered by the ash falls. Ash from the Toba eruption, which happened in Indonesia about 70,000 years ago, covered most of South Asia to a depth of 15cm, and in some places the ash was several meters thick.

Volcanic eruptions of such size affect the whole planet. The ash and gasses in the atmosphere block out the sun, leading to ‘volcanic winters’, which can last for several years with reduced temperatures all year round, as well as reduced crop growth that can lead to famines.

So it would be pretty unpleasant if we had a super volcano eruption, but the human race would be able to survive. These things don’t happen very often though (neither Toba nor Yellowstone have erupted in the last 50,000 years at least), so the chances of them all happening at once are pretty slim.