GEOLOGISTS ENTHRALLED BY IDEA OF GIANT YELLOWSTONE ERUPTION.

Byline: William Brock Twin Falls Times-News

Surrounded by mountains, the Snake River Plain is a sea of tranquillity in a tumultuous landscape, but the vast sweep of flatland has a history every bit as violent as the peaks.

Huge volcanic eruptions - up to 3,000 times larger than Mount St. Helens - have changed the face of southern Idaho over the past 14 million years.

The earliest was in what is now the tri-corners area of Oregon, Nevada and Idaho. Successive eruptions were strung out in a line stretching to the northeast. The latest, a mere 500,000 years ago, is the site of present-day Yellowstone National Park.

If history is a guide, the next big eruption will be northeast of Yellowstone - sometime in the next 2 million to 5 million years.

In addition to the heat, concussion and molten lava, the blast could produce ``horrendous, death-dealing clouds that roll over the landscape at almost supersonic speeds, frying everything in their path.''

Those sobering words come from Martha Godchaux, a geology professor at Mount Holyoke College and longtime student of southern Idaho's volcanic past.

Like her peers, Godchaux is careful to note that no one has ever seen an eruption of that size. Moreover, the odds favor smaller, rather than larger, eruptions.

Still, the prospect of The Big One entrances many geologists.

``My guess is, if one of those things went off, it would probably wipe out everything on the Eastern Snake River Plain - either from the blast, or the ash, or the heat,'' says Earl Bennett, Idaho's state geologist and leader of the Idaho Geological Survey.

``It would be the equivalent of somebody dropping a hydrogen bomb.''

Yellowstone's eruptions are among the largest documented by researchers anywhere, said Dennis Geist, associate professor of geology at the University of Idaho, ``and another one would produce absolute annihilation within 100 miles.''

Anyone lucky enough to survive the explosion probably would perish if the blast were accompanied by heavy ash flows. Geist paints a worst-case scenario of hurricane-force winds driving ash flows hundreds of meters high, with all the destructive power of an avalanche.

As if that wasn't enough, temperatures would exceed 1,000 degrees Fahrenheit.

Rhino fossils in Wyoming have been unearthed from beneath 50 feet of ash, Bennett says, and the animals apparently died a terrifying death. Judging by their contorted posture, he said, they evidently ``were struggling to get away, but suffocated right in place - like they couldn't get out of the stuff.''

That much ash also would obscure the sun, probably reducing temperatures on the surface of the Earth, Bennett says.

Such an eruption is wildly improbable in the lifetime of anyone alive today, geologists agree. Far more likely is an eruption on the scale of the one that produced the Craters of the Moon, roughly 2,000 years ago. There would be no volcanic pyrotechnics, and lava would simply ooze from fissures in the earth, covering the area like syrup on a pancake.

``If something like that did happen, it would be very tourist-friendly and easy to avoid,'' Godchaux says. Most people can out-walk a typical lava flow, but moving lava destroys anything that can't get out of its path.

Though extensive, basalt flows are a relatively recent chapter in southern Idaho's geologic history, covering earlier and far more violent volcanic birthmarks.

Southern Idaho's volcanic past is the result of a ``hot spot'' near the Earth's core - like an open door in a furnace - where rock is forced toward the surface by tremendous pressures. The hot spot has remained in the same place, but the surface of the Earth slowly has been moving southwest in a process known as ``plate tectonics.''

As the Earth slides over the hot spot, the enormous pressures produce an area of uplift, like a pimple on the face of the Earth. The uplift is accompanied by earthquakes over a period of hundreds, perhaps thousands of years. Deep beneath the surface, the rising plume of rock is presumed to be fairly solid, but it becomes molten as pressures ease near the surface.

When the first finger of magma finally reaches the surface, it probably spews an ash column 20 to 30 miles high, Godchaux says.

When the pressure eases off, the bulge recedes and cracks begin to form like a mud-caked balloon when air is released. The cracks can weaken an area 35 miles across, laying the groundwork for an eruption of colossal proportions.

It has happened before, Geist says, and it is going to happen again.

``Any rational geologist would be happy to make a prediction that, within the next 5 million years, there's going to be an enormous eruption from the Yellowstone area,'' he says. ``I think that's a given.''

Where it will happen is anyone's guess, Godchaux says, ``because we don't even know if Yellowstone is finished erupting yet.''