The natural beauty of Yellowstone National Park may appear serene, but it's rooted in a violent volcanic past. Now, geologists have identified which parts of the park are most likely to erupt again someday.

Yellowstone's next major eruption will probably be centered in one of three parallel fault zones running north-northwest across the park, a new study predicts.

Two of these areas produced large lava flows the last time the supervolcano was active—174,000 to 70,000 years ago—while the third has had the most frequent tremors in recent years.

Knowing this will help scientists determine which areas of the vast park to monitor most carefully, said study lead author Guillaume Girard, a visiting professor at Michigan State University in East Lansing.

The Yellowstone region is often referred to as a "supervolcano" because it has spewed more than 240 cubic miles (a thousand cubic kilometers) of ash and lava in a single event. The most recent of these massive blasts occurred some 640,000 years ago.

Smaller eruptions occur more frequently, said Girard, but the chance of one happening in any given year is still less than one in ten thousand. He described these eruptions as lava flows, which are not explosive: "They have very, very high viscosity and flow very, very slowly."

Similar flows have fed the slow-growing lava dome at Mount St. Helens in the years after that volcano's major eruption, but Yellowstone's lava flows occur on a much grander scale.

"Some of these flows traveled up to 20 miles [32 kilometers]," said Girard, whose study appeared in the September issue of GSA Today. "We have never seen a rhyolite eruption of this magnitude in human history."

(Find out what's really going on under Yellowstone and how the next super-eruption could unfold in "When Yellowstone Explodes" [National Geographic magazine, August 2009].)

Yellowstone Lava's Source

By studying the titanium content of the lava flows, Girard's team determined that the source of these flows had risen very quickly from a magma chamber some 4 to 7 miles (6 to 12 kilometers) deep.

The amount of titanium within the lava's quartz crystals marks the depth at which crystals formed out of the slowly cooling magma. So if the magma paused at intermediate levels during its ascent, the titanium content of each crystal should vary from its center to its outer edges, like the layers of an onion.

But, Girard said, the crystals have no such features. This means that the magma rose quickly to the surface, without pausing long enough at any intermediate level for the crystals to grow.

"Quickly" in terms of geology, anyway. By human standards, the lava's rise was probably fairly slow, taking place over hundreds or thousands of years.

Does this relatively fast-moving magma suggest Yellowstone could soon have more eruptions?

"It is not an imminent hazard," Girard said. "Every study has concluded that there is no magma that is ready to erupt within any foreseeable future."

Ben Ellis, a volcanologist at the Institute of Geochemistry and Petrology, ETH Zurich, Switzerland, finds Girard's study "really neat" but notes that eruptive patterns can also change unexpectedly.