Stop and think about volcanoes for a second — do you think you know which ones were the biggest of them all? Mt. St. Helens — a mere amateur. Yellowstone — respectable, but just middling over all. Toba — lethal, but still a punk geologically-speaking.
The biggest of them all, at least as far as we now know, was the La Garita super-volcano — erupting some 27.8 million years ago in what is now southern Colorado. In less than a week, this beast erupted at least 1,200 cubic miles of magma (some sources give a number as high as 3,000) — enough to fill the basin of Lake Michigan to the brim. This is about double the volume of the largest Yellowstone Caldera eruption, and some 4,000 times as large as the Mt. St. Helens eruption of 1980.
This view is of the southern-most corner of the surviving caldera, from an overlook on the southwest end of Wolf Creek Pass on U.S. Highway 160.
When La Garita erupted, it formed a huge caldera — about 35 x 75 km (22 x 47 miles), but this was subsequently almost obliterated by later volcanism and (much later) glaciation. It took geologists over 30 years to determine the extent of the ancient caldera. Meanwhile, the eruption produced a rock known as Fish Canyon Tuff — you can see a nice example of it in the foreground of this image, as well as on the bluffs in the distance. At the time, though, the tuff covered (likely sterilizing) a large portion of what is now Colorado, and fell as far as the east coast of North America.