Most supereruptions occur in areas that remain active over millions of years but enjoy a long repose period, so don't place too much confidence in Yellowstone's apparent calm. Broadly speaking, the longer the dormancy, the bigger the boom [source: Geological Society of London].
Like other supervolcanic areas, Yellowstone sits on a long-active tectonic zone, a weakened and thinned crust overlying a 2,500 F (1,370 C) magma dome rising from the upper mantle. This dome has melted and broken into the crust to create two magma chambers roughly 5-7 miles (8-11 kilometers) underground, each measuring 30-plus miles (48-plus kilometers) across [source: Encyclopedia Britannica]. These magma chambers are filled with an amalgam of magma, semisolid rock and dissolved gases like water vapor and carbon dioxide.
Over centuries and millennia, additional magma builds up, delivering more heat and pressure, pushing overlaying ground upward little by little. If the chamber receives a steady and substantial supply of hot magma, pressure builds in an often cyclical process called incubation. If it doesn't, then some material solidifies and sinks, removing pressure. The sheer volume of a supervolcano's magma chamber means that incubation requires a heat delivery rate 2-3 orders of magnitude greater than that of a traditional volcano [sources: Achenbach; Klemetti].
Eventually, overpressure creates fractures along the dome's periphery, venting pressure from the chamber. The gas-filled magma blasts skyward, raining ash and debris over hundreds of miles and releasing deadly pyroclastic flows -- fast-moving, thick clouds of gas, ash and rocks boiling away from the eruption at 1,470 F (800 C) – across tens of thousands of square miles [sources: Achenbach; Geological Society of London].
Additional blasts pop off periodically for weeks. Ash drifts down on a regional scale, filling the sky with pollutants and blanketing tens of millions of square miles in inches of crop-killing ash [sources: Geological Society of London; Klemetti]. Until it settles, anyone within thousands of miles around risks breathing tiny glass needles, bursting pulmonary blood vessels and drowning in a slurry of ash and lung moisture [sources: Achenbach; Geological Society of London; Tyson]. Ash collapses roofs, pollutes vital water sources and gums up vehicle engines, sparking a crisis of food production, transportation, communication and economics lasting months to years [sources: Geological Society of London; Klemetti].
Within weeks, dust and sulfate aerosols encircle the globe, filtering out sunlight and cooling global average temperatures by an estimated 5-9 F (3-5 C) for several years afterward [sources: Geological Society of London; Klemetti; Marshall]. One-third of the U.S., particularly the states of Montana, Idaho and Wyoming, remain uninhabitable for months, possibly years [sources: Tyson; USGS].
Thankfully, the odds argue against this happening any time soon. But another supereruption, someday, somewhere in the world, is inevitable. Maybe it's time we got started on that Mars colony after all.