Volcano eruptions can cause major able to alter global climate and even threaten the existence of human civilization. It is likely that if such an eruption would occur, the amount of ash in the atmosphere would be large enough to significantly reduce solar radiation. As a result, the climate would cool even with more than 20 degrees in temperate zones, and photosynthesis would be greatly diminished. The result could be extinction of numerous species of plants and animals.
Like other cataclysmic natural phenomenon (asteorizi massive impact, ice age etc..), such events are relatively rare. However, the likelihood of supereruptii is assessed to be 5-10 times higher than that of an asteroid impact mileage.
An example is considered Yellowstone super volcano, located in the U.S. namesake park. It is estimated that an eventual eruption of the super volcano would cover approx. half of the United States with a layer of ash about 1 meter. Yellowstone super volcano eruption frequency is assumed to be 600,000 years, but already have about the past. 620,000 years after the most recent. Recent data show an increase in activity in the Yellowstone area, but geologists believe that they are not necessarily signs of an impending eruption. Another is the Toba super volcano, located in the Indonesian island of Sumatera, which erupted 10,000 years ago and led to the disappearance of three quarters of the northern hemisphere plants.
Unlike other global threats, the situation that a super volcano would erupt no scripts or ideas that technology can prevent or at least, can mitigate the effects.