A full eruption will shroud the zone known as "earth" in darkness. Its that enormous of an eruption. Some people apparently think its a joke but the earth has about a dozen super-volcanoes that erupt periodically. Modern man hasn't seen that yet but we will eventually and when we do, it won't be like seeing Mt. Saint Helens on television.
I don't know if there are any math people in here but they say a full Yellowstone eruption would be equivalent to Mt. St. Helens X1000-2000 range. I can't even wrap my mind around that because wasn't St. Helens was like a small nuclear bomb going off, 1000 times St. Helens would be mind boggling in terms of energy release.
One of the main concerns would be that the bread basket of the U.S. which produces a massive amount of the world's food would be destroyed and that is just for starters, that's just the warm up. Even the super-volcanoes at the bottom of oceans would cause calamity when they erupt.
Like I said, modern man hasn't seen this yet, but it's going to happen sooner or later.