A Yellowstone "supereruption" would distribute ash over almost all of the United States, and into Canada, according to 9/9/14 guest Larry Mastin, a geologist at the USGS Cascades Volcano Observatory. His study of the subject, which used improved computer modeling, found that a large eruption at the National Park would create a type of ash cloud known as an "umbrella" (pictured in graphic), which expands evenly in all directions. More here.