Kaboom!

The BBC has written a report about the supervolcano under the Yellowstone National Park, explaining that it is 2.5 times bigger than previously thought. It stretches for 55 miles and could contain up to 600 cubic km of molten rock, meaning that if an eruption occurred today, the consequences could be catastrophic. Yellowstone has had three major eruptions so far; 2.1 million years ago, 1.3 million years ago and 640,000 years ago, suggesting an average frequency of one every 700,000 years. This is a very rough estimate, as it is only based on two gaps between the three eruptions.
National Geographic explains that the most recent eruption was 1,000 times the size of the Mt St Helens eruption in 1980, with a pillar of ash rising 100,000 feet. Pyroclastic flows (dense fogs of ash, rocks and gas with a temperature of 800 degrees celsius) rolled along the landscape, welding itself to the ground. The eruption 2.1 million years ago was twice as strong, and the one 1.3 million years ago was slightly smaller, but still as devastating.
The Examiner goes on to explain that no supervolcano has erupted in human history, so it is hard to predict what will happen. They also say that there is an imminent danger, and that the magma is "ready to erupt", causing an eruption that would be 2,000 times larger than the Mt St Helens eruption in 1980.
However, the Yellowstone National Park Service believes that there is no evidence of an imminent catastrophic eruption, as geological activity has remained constant throughout the 30 years that it has been monitored. Instead the eruption is expected to occur in the next 1,000-10,000 years. This eruption could be in the form of a lava flow, oozing slowly for months and years, allowing people to have plenty of time to escape. Scientists also believe that they would examine the precursors to a catastrophic eruption for months or even years. These precursors include smaller volcanic eruptions, strong earthquake storms and ground deformation. The intensity of the precursors would increase closer to the time of the catastrophic eruption.
National Geographic gives a detailed explanation of what to expect from from a supervolcano. First, a hot plume would upwell, melting the rocks beneath the Earth's crust, forming a chamber filled with magma, rock, water vapour, carbon dioxide and other gases. The land would begin to dome upward as this material accumulates over thousands of years forming several fractures. The pressure in the chamber would build up, before being released through the fractures. The surface of the dome would collapse, leaving behind a caldera.
So it seems there is some controversy over the timescale in which Yellowstone will erupt, and the severity of the eruption, but hopefully us geologists will be able to keep an eye on things to give people enough warning about the impending doom!