Brian Wilcox of Nasa's Jet Propulsion Laboratory (JPL) at the California Institute of Technology came to the conclusion that the supervolcano threat is substantially greater than the asteroid or comet threat.
There are around 20 known supervolcanoes on Earth, with major eruptions occurring on average once every 100,000 years. One of the greatest threats an eruption may pose is thought to be starvation, with a prolonged volcanic winter potentially prohibiting civilization from having enough food for the current population. In 2012, the United Nations estimated that food reserves worldwide would last 74 days.
What is a Supervolcano?
A volcano which erupts and throws magma and rocky particles over an area greater than 240 cubic miles (1000 cubic kilometers) is considered a supervolcano. These massive eruptions dwarf typical volcanic eruptions. It is like comparing a small shock from static electricity to a lightning bolt: Mount Vesuvius produced 100,000 cubic yards of magma per second during its massive explosion in A.D. 79. The damage from this "ordinary" volcano was legendary. If Mount Vesuvius had been a supervolcano, it would have produced 100 million cubic yards of magma per second.