text: This is a valid question, and should be one that is on the list of many government disaster preparation plans. Fortunately, it would be unlikely to take anyone by surprise, as most volcanic eruptions are preceded by periods of earthquake activity. However, a large eruption would require huge evacuations and would disable travel and transport for much of North America for days or weeks, so it is not an insignificant risk. It would also probably shut down all air transport almost worldwide for some period. It might be a small probability, but so are most natural disasters. \n\nThe best analog example for a Yellowstone eruption would be the Long Valley Caldera, which might actually erupt again before Yellowstone ever does. The Long Valley Caldera last had a major eruption about 760,000 years ago. About 140 cubic miles of rock and ash were ejected. Ash covered most of the western United States, as far east as Missouri. \n\nHere is a USGS map of the area affected:\nhttp://lvo.wr.usgs.gov/GeologicMap.html\n\nThe USGS has a page discussing the possible eruption scenarios at:\nhttp://lvo.wr.usgs.gov/eruptions.html\n\nThey estimate the probability of an eruption of some size in any given year to be one in a few hundred, which is about the same probability as a major earthquake on the San Andreas fault. The area has seen signs of activity since 1980, and has experienced tree kills from volcanic gases rising through the soil, and a large dome has been growing outside the town of Mammoth Lakes. \n\nHere is the home page for the Long Valley Observatory:\nhttp://lvo.wr.usgs.gov/index.html\n\nThe Yellowstone Observatory of the USGS has lots more material that you will be interested in. There have been satellite measurements of rising elevations in the area, and here is the map:\nhttp://volcanoes.usgs.gov/yvo/2006/uplift.html\n\nThere is a map showing the area affected by the last major eruption of Yellowstone about 2.1 mybp here, as well as many examples of recent volcanic activity in the area:\nhttp://pubs.usgs.gov/fs/2005/3024/\n\nThe home page for the Yellowstone Volcano Observatory is here:\nhttp://volcanoes.usgs.gov/yvo/index.html\n\nAs far as climate effects go, in North America the jet stream travels eastward, so much of the ash from any volcanic eruption, whether from Long Valley, Yellowstone, or the Cascade Range, will travel eastward. The largest ash falls are likely to range as far east as the Mississippi River, maybe further, and at some times of year may be pushed southward as well, leaving the Midwestern US and the Southern US as the most vulnerable areas outside of the immediate area. A large eruption could leave ash falls of several feet over much of that area. The polar jet stream that crosses North America averages between 50 and 100 mph so the spread of ash would take less than a day for most of the US. \n\nhttp://www.islandnet.com/~see/weather/elements/jetstream1.htm\n\nThe Mt Pinatubo eruption in 1991 spread a plume of sulfuric acid across one hemisphere in about 9 days. Here is a map of that plume.\nhttp://www2.sunysuffolk.edu/mandias/lia/possible_causes.html \nThe overall climate effects of such an eruption would likely be global cooling of a few degrees and would affect the northern hemisphere more severely, depending on the size of the eruption. The Pinatubo eruption is said to have had a greater effect on global climate during the two years after its eruption than either El Nino, or anthropogenic greenhouse gases. Here is the data:\nhttp://www.cmar.csiro.au/e-print/open/greenhouse_2000e.htm\n\nI'm sure you can go a lot further with this research. There is lots of material out there on the effects of volcanic eruptions. USGS, the Hawaiian, Alaska, and Cascade Volcano Observatories, as the Smithsonian Global Volcanism Program have a wealth of m