Much has been said about the future of Yellowstone volcano in Wyoming, as movies, articles, tabloids, and more have come out in recent years. Movies like "2012", and various TV movies on Sci-fi channel, as well as numerous documentaries on Nat Geo and other channels have put into our field of vision the grim specter of a super eruption in the United States which would have global ramifications.
Yellowstone super volcano last erupted more than 600,000 years ago. It is part of a "mantle plume", a column of magma that reaches from the mantle of the Earth up through the crust, occasionally punching holes in the surface that end up as a super eruption. This particular mantle plume has been active for many millions of years. The plume actually travels Eastward over time, as is evidenced by the Snake River Plain. Looking at Google Earth or other topographical data, you can easily see the trail of destruction and creation it has left from Utah all the way East to Wyoming and Montana.
Numerous calderas, cinder cones, volcanoes, and finally the Yellowstone Caldera leave a trail of evidence of past catastrophe. The current Yellowstone Caldera was formed a couple million years ago, with eruptive activity ceasing around 600,000 years ago, the time of the last great eruption. A few smaller phreatic eruptions have occurred since then, but nothing on the scale of what we see in the movies.
The Yellowstone caldera typically erupts every 600,000 years and the last time it erupted was 680,000 years ago. Some scientists say it is overdue for an eruption, and indeed, uplift has occurred since quake swarms in the 70's marked new activity for the caldera. The uplift has actually moved the shores of Yellowstone Lake by several feet, and uplift continues at around 3cm a year. In 2010, there was a large quake swarm in the northern region of the caldera which had scientists extremely worried.
Even more worrisome were the results of low velocity zone imaging which showed that the mantle plume's magma chamber was far, far larger than originally thought, and a lot closer to the surface. The mantle plume actually extends under parts of Montana, and is thousands of cubic miles in volume. This, is a big volcano.
Whether or not the volcano will erupt within our lifetime is anyone's guess, but scientists are not losing any sleep over it. Yellowstone, if it were to erupt, would likely give plenty of warning before it does so. The way Yellowstone's mantle plume has behaved in the past is not actually a good indicator for how it will erupt in the future (and it WILL erupt), however scientists state that a super eruption would occur if several criteria are met:
First, the mantle plume must inflate the crust to the point of a magmatic eruption. This would be an obvious sign that the S is about to hit the fan. Second, the eruption would have to extrude so much lava onto the surface as to nearly empty our the magma chamber, leaving a void of gas underneath the caldera, that would theoretically need to collapse under the weight of the surface's new lava. This collapse would cause the crust to actually fall into the magma chamber, resulting finally in the much feared super eruption.
There are plenty of smaller scale examples to look at. Crater Lake in Oregon used to be a large Cascades mountain called Mount Mazama. When Mazama had a large scale eruption that emptied out its chamber, the mountain collapsed under its own weight creating a gigantic crater in the earth visible from space. It is now the United State's deepest, and purest blue lake.
Yellowstone's evil cousin, Toba volcano in Indonesia, has had a much larger effect on humanity than Yellowstone. Toba erupted a mere 70,000 years ago, and was responsible for not only changing the global climate and throwing the Earth into a mini Ice Age, but it nearly extinguished the human species. What was a thriving colony of over 500,000 individuals at that time was reduced to a genetic bottleneck of a mere 5,000 individuals worldwide!
An eruption of either Toba or Yellowstone today would be catastrophic, resulting in severe economic damage, environmental turmoil, not to mention a very pesky 6ft of volcanic tephra that would accumulate over most of the United States and the rest of the Northern Hemisphere. While people would not be killed by pyroclastic flows or falling lava bombs (as Yellowstone isn't exactly near any major population centers) the fallout from the eruption would kill most of the crops in the USA (which produces the most food exports of any country in the world) and would result in many health issues due to breathing in volcanic ash (imagine breathing in pulverized glass...).
In any case, no need yet to have any nightmares about Yellowstone, but as we can be hit in this world by literally ANY disaster, it's always important to be prepared. Earthquakes, tsunamis, floods, hurricanes, tornadoes, blizzards, asteroids/meteors, wildfires, cyclones.... just some of the things that go on in this world that we all should be prepared for. Make sure you have lots of canned food, batteries, flashlights, candles, blankets, and other essentials in an easy to access, but safe place. The life you end up saving could be your own.
If you would like to monitor the Yellowstone volcano on your own, you can do so at http://volcanoes.usgs.gov/yvo/ and see quakes as they happen as well as great scientific reporting from the USGS.