My friend Melanie loves to watch the Discovery Channel and share what she has watched with others. The problem is she can never get the story straight. Her latest tale is about Yellowstone Park sitting on top of a supervolcano that has been dormant for 6,000-ish years. She said the supervolcano is starting to show signs that point to its being active. Is there any truth to her tale?

Cut Melanie some slack. Yeah, she screwed up some details: it's been 600,000-ish years since Yellowstone's supervolcano has gone off full blast, not 6,000. She got the drift, though. Nobody's saying Yellowstone is going to blow in time to mess up your summer vacation plans. But it's a question of when, not if.

A supervolcano is one that explodes in (natch) supereruptions. Definitions vary, but usually we're talking a magnitude-eight (M8) eruption: one trillion metric tons of ash and other debris filling at least 100 cubic miles, typically upchucked over the course of about a week. Picture 1,000 Mount Saint Helenses, or 8 Tamboras. Besides causing regional devastation, supereruptions affect global climate. An Indonesian super 74,000 years ago kicked off a thousand-year drought that some contend caused a human population crash. One shudders to think what a similar blow would do now.

Yellowstone is both a supervolcano and a hotspot; the two don't always go together. A hotspot is the business end of what's known as a mantle plume, a stream of magma that rises hundreds of miles through a channel in the earth's crust like the blob in a lava lamp. The Yellowstone plume head, 50 miles underground, is several hundred miles wide. Over time, the hot head melts the overlying crust, forming a smaller magma chamber. Yellowstone's magma chamber is just a few miles down and contains partially melted granite viscous enough to trap gas, allowing pressure to build. Periodically the pressure cracks the surface, explosively ejecting gas and disintegrated rock into the surface world. After about a tenth of the chamber's contents have erupted, pressure falls and the show's over. Reheat and repeat.

The Yellowstone hotspot has produced dozens of large eruptions over the past 16 million years, the last three within the Yellowstone volcanic field: two supers and one M7.4 lightweight that created 68 cubic miles of debris. They left overlapping giant calderas, or craters, each 10 to 50 miles wide. Since filled with lava and eroded, the calderas are inconspicuous and went unrecognized till the 1960s.

These three big eruptions were 640,000 years ago, 660,000 years earlier, and 800,000 years before that. See a pattern? Lurid reports suggest we're almost due, even overdue, but the pattern is illusory. Over the past 4.5 million years, large Yellowstone eruptions have come at irregular intervals of 300,000 to 2.4 million years. Furthermore, a series of smaller eruptions (relatively; one was Krakatoa-esque in magnitude) between 170,000 and 70,000 years ago ejected as much material as a super -- perhaps enough to relieve the pressure, and danger, for a while.

And while you're waiting, why not enjoy some of our many fine geysers and hot springs? Yellowstone's thermal features obviously depend on the heat rising from the magma chamber, demonstrating that a supervolcano, when not wreaking global havoc, can be a lot of fun.

Yellowstone has been eruption-free for 70,000 years, neglecting occasional steam explosions. Two centers of resurgence within the last caldera, where the land heaves up and down several feet over the course of decades, bear watching, but there's no sign of imminent trouble. Volcanologists expect at least a few weeks of pre-eruption warning.

What's the chance of a supereruption anywhere in the world in the next 50 years? Estimates range from one chance in 1,000 to one in 10,000 -- hardly cause for panic. When it does happen, though, a lot of people are going to be toast. A supereruption could kill tens of millions. Within 10 to 50 miles of the next Yellowstone vent, you'll be Pompeii-ized beneath thousands of feet of hot ash. More than half the United States will experience ashfall, potentially fatal if inhaled. Ash and associated toxins could devegetate a third of the lower 48 (including some of the world's most productive farmland) for years or decades, leading to mass starvation. Grim, but so unlikely that the U.S. Geological Survey lists Yellowstone as only the 19th most dangerous American volcano. Pimples like Kilueha, Saint Helens and Rainier top the list.

Yellowstone's cousins are worth watching. The last supereruption at the Long Valley caldera in eastern California was 760,000 years ago. Another would be devastating, but as at Yellowstone, major cities are distant enough to avoid being completely buried. Elsewhere, some giant calderas are frightfully close to urban areas. Lake Taal in the Philippines is only 40 miles from Manila. In Italy the rim of Campi Flegrei lies only five miles from central Naples. The last near-supereruption there came 35,000 years ago; a much smaller one killed 24 in 1538. Pizza napoletana on my next Italian tour? Sure. But make mine to go.

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