Volcanic park: some of the world's largest volcanic eruptions have blasted Yellowstone National Park. Discover what scientists know about these supervolcanoes.

DID YOU KNOW?

* Scientists know that Yellowstone's volcano is active from these signs: 1,000 to 3,000 earthquakes occur there each year; the area also features active ground deformation and more than 10,000 thermal features.

* "Supervolcano" usually describes a volcanic eruption of more than 1,000 cubic kilometers (240 cubic miles) of magma. Besides Yellowstone, supervolcanoes have occurred in Long Valley in eastern California, Toba in Indonesia, and Taupo in New Zealand.

CRITICAL THINKING:

* Yellowstone is an active volcano. Why do government officials think that it is safe for people to visit the national park?

CROSS-CURRICULAR CONNECTIONS:

LANGUAGE ARTS: Research a historic volcanic eruption and how it affected a nearby community. Then, write a play chronicling the community before, during, and after the eruption.

RESOURCES

* To see a virtual model of the Yellowstone supervolcano, check out the companion Web site to the Discovery Channel program Supervolcano: http://dsc.discovery.com/ convergence/supervolcano/

* The U.S. Geological Survey has an online teaching guide on volcanoes. The lesson plans are targeted for teachers of grades 4 through 8. Visit: http://interactive2.usgs.gov/ learningweb/teachers/volcanoes.htm

Anyone who visits Yellowstone National Park knows that it's a geologic hot spot.

Stinking mud pots, near-boiling hot springs, seething steam vents, and raging geysers all attest to the magma (mixture of molten rock and dissolved gases) that lurks beneath the surface of the park. Geologists know, however, that this vast pool of magma has not always remained underground. Three times in the past 2.1 million years, volcanic eruptions have ripped apart this land and spewed magma so violently that geologists refer to the explosions as supervolcanoes.

Yellowstone last erupted as a supervolcano about 640,000 years ago. It ejected 2,500 times more magma than the 1980 eruption of Mount St. Helens, which killed 57 people. Another supereruption took place in Yellowstone 2.1 million years ago. This one left a hole more than 97 kilometers (60 miles) across.

VIOLENT LANDSCAPE

Although it may not look like one, Yellowstone is a volcano. It lies over a hot spot, or a stationary plume of heat deep within Earth. Like a giant blowtorch, this fiery plume heats the rocks under Yellowstone until they liquefy. Unlike most volcanoes, such as the cone-shaped Mount St. Helens, Yellowstone's volcano is flat. In fact, the volcano lacks a top. When the magma shot out during the last supereruption, it left a void under the volcano's "roof" and the top caved in. This formed a caldera, or collapsed volcano.

What causes the supereruptions? Yellowstone's magma is rich in silica, a compound composed of silicon and oxygen. This makes the magma very thick, so it flows less easily than magma found in many other volca noes. Yellowstone's magma also lies close to Earth's surface. Together, these features create the perfect recipe for a supervolcano.

"Think of the magma chamber as a soda bottle. When [the bottle is] closed, it is filled with dissolved gas," says Jake Lowenstern, a geologist in charge of the Yellowstone Volcano Observatory. "If you shake [the soda bottle] and then take off the cap, the gas explodes Violently. The same thing happened at the Yellowstone volcanoes--but because the magma is [so thick] it can blast out even more explosively." Result: a supervolcano.

DANGER AHEAD?

The big question is whether the 3 million people who Visit Yellowstone annually are in danger of getting caught in a supereruption. Lisa Morgan, a research geologist for the United States Geological Survey, doesn't think a supereruption will occur for tens of thousands of years. More often, Yellowstone's magma escapes slowly as lava flows, the last of which occurred 70,000 years ago.

Plus, Morgan points out, the volcano sits in the middle of the North American plate, a slowly-moving slab of rock that makes up Earth's outer layer. The plate's motion has reduced the odds of an eruption. How? As the plate has moved, new, thicker crust has slid over the hot spot, making it harder for magma to punch through.

READING THE SIGNS

If an eruption were imminent, Morgan says that geologists would see signs. There would be swarms of earthquakes in the area, big changes in the ground's shape, and increased releases of heat and gas--all of which geologists monitor regularly.

"I'm not worried about an eruption," says Morgan. "We have a good monitoring system and a good understanding of the [volcano's] geology."

DIRECTIONS: Fill in the blanks to complete the following:

1. Yellowstone is a volcano that lies over a--, or a stationary plume of heat deep within Earth. Like a giant blowtorch, this fiery plume heats the rocks under Yellowstone until they

2. Yellowstone volcano is a--, or a collapsed volcano.

3. Yellowstone's--(mixture of molten rock and dissolved gases) is rich in--a compound made from silicon and oxygen.

4. Yellowstone sits in the middle of the--, a slowly moving slab of rock that makes up Earth's outer layer.

ANSWERS

1. hot spot; liquefy 2. caldera 3. magma, silica 4. North American plate