WHAT WOULD IT BE LIKE?
Lurking beneath Yellowstone National Park is a massive underground reservoir of magma, capped by the park's famous caldera. 640,000 years ago, a super eruption rocked the region. What would happen if another such event blasted the park today? USGS geologist Jake Lowenstern, scientist-in-charge of the Yellowstone Volcano Observatory, answers.
Most of the real damage comes from ejecta that's airborne. But it's not fiery death from above. Instead, most damage would come from "cold ash" and pumice borne on the wind, which would be disastrous.
Read more: http://io9.com/what-will-really-happen-when-yellowstone-volcano-has-a-508274690
One of the most destructive effects of a supervolcano eruption is ash falling on agricultural fields. It does not only fall on the area surrounding the eruption, but it travels with the wind all around the globe, disrupting climate and spoiling agricultural fields everywhere. Depending on the area affected, famine would follow.
Let's also remember that air and ground transportation would be severely disrupted by ash during the first few weeks after the eruption, thus impeding efforts to assist those most affected.
Additional references -
Yellowstone volume of molten magma is much larger than previously thought
Yellowstone, where will it erupt - http://news.nationalgeographic.com/news/2012/09/120920-yellowstone-supervolcano-prediction-volcanoes-science-environment/