A sleeping giant is nestled in the western part of the United States. Though it stirs occasionally, it has not risen from slumber in nearly 70,000 years. But when it finally awakes it may roar and heave with unprecedented force.

This giant is the “supervolcano” that lies beneath Yellowstone National Park, the wildlife and forest preserve positioned on a sprawling expanse that extends through the states of Wyoming, Idaho and Montana. The volcano itself is actually located in northwestern Wyoming, which is where the bulk of Yellowstone is contained.

The ground above the Yellowstone supervolcano sits on a hot spot made of molten and semimolten rock called magma. As magma feeds into a magma chamber, or reservoir situated about six to 10 kilometers (four to six miles) beneath the park, the ground swells. When the magma begins to solidify and cool, the ground falls.

Volcanologists, who have been measuring this activity since 1923, say the ground rose about 25 centimeters (9.8 inches) between 2004 and 2009. However, in 2010 the land began to subside.

The period of slow, steady rise has many scientists wondering whether Yellowstone might erupt in the near future. And if it does, there is concern about how intense that eruption may be.

“The big question is if Yellowstone started shaking tomorrow, what is there to expect?” says Dr. Steve Anderson, a volcanologist and earth sciences professor at the University of Northern Colorado. “I don’t think we know exactly what to expect.”

While scientists may not know exactly what to expect, they have an idea—and most say it’s unlikely to be doomsday.

In fact, Dr. Jacob Lowenstern, research geologist and Scientist-in-Charge at the Yellowstone Volcano Observatory, says that Yellowstone is currently a dormant volcano, with low levels of unrest.

“There is no current activity that is going on that would indicate anything is happening. If there was something coming, there is nothing to show at this point in time,” he says.

Learning from the Past
Still, the level of recent underground activity fuels speculation about the intensity of an eruption. Within the past decade, the volcano has continued to rise at the fastest rate ever recorded.

Yellowstone also averages between 1,000 and 3,000 earthquakes a year. Most are virtually unnoticeable, with a magnitude of three or less. Still, these quakes give scientists insight into just how fast the magma chamber beneath the park is filling up. An increase in the shaking and rattling throughout the park might indicate a fresh batch of magma was recently fed into the reservoir.

Even with the increase in temblors, scientists don’t think the rumblings in the magma chamber pose a threat anytime soon. However, since people haven’t been around to analyze absolutely everything that happens in Yellowstone, it’s hard to predict what exactly is going on, making it difficult for geologists to predict Yellowstone’s next move.

Examinations of the volcano’s distant past do provide something of a clue. Geologic evidence suggests that Yellowstone has produced three colossal eruptions within the past 2.1 million years. Volcanologists say the eruptions occurred at gaps of about 600,000 to 800,000 years. Evidence from the last big event, estimated to have to been about 640,000 years ago, is sprawled throughout the park and across thousands of kilometers of the surrounding landscape.

Each of the previous eruptions spewed enormous amounts of volcanic ash, gas, magma and other volcanic debris that covered most of the continental U.S. Some material has been found as far away as Louisiana.

After each of these eruptions, the Yellowstone supervolcano collapsed on itself, sucking in trees, mountains and everything else in the landscape. The depression formed by this phenomenon is called a caldera. (In fact, the Yellowstone supervolcano is also called the Yellowstone caldera.) A caldera-forming eruption would create a massive natural hazard in Yellowstone.

Scientists say the last Yellowstone eruption was 1,000 times greater than the notorious 1980 Mt. Saint Helens eruption that killed 56 people and thousands of animals, and scorched hundreds of square kilometers of land in Washington and Oregon.

Thousands of years ago, the last blast from the Yellowstone supervolcano shot a fatal plume of hot ash, molten rock, and lethal gases thousands of meters into the air. A third of the continent was likely plunged into complete darkness. Pyroclastic flows (fast-moving currents of hot, dry rock fragments and gases) raced along the region at alarming speeds, burying or shattering anything in their path. Magma spewing out of the ground charred the once-charming landscape for kilometers.

Some evidence of the last eruption can be found in the Yellowstone caldera itself, 50 kilometers (30 miles) wide and 70 kilometers (45 miles) long. The thick volcanic debris that remained after the eruption can still be seen in an area referred to as the Lava Creek Tuff.

Unlikely to Erupt
Officials of the United States Geological Survey (USGS) say a massive eruption like the last one is an unlikely scenario. In fact, officials at the Yellowstone Volcano Observatory say the most likely activities that might take place in the future are hydrothermal explosions (eruptions of steam and hot water, rather than molten rock) or lava flows.

Although lava flows are a type of magmatic eruption, they are not as devastating as the caldera-forming explosions. Instead of instant destruction, lava flows slowly ooze out of the ground over a period of days, months or even years.

They are also relatively rare. The last Yellowstone lava flows took place about 70,000 years ago. Yet even today, hikers can see evidence of those eruptions in the form of distinct rock layers along the park’s trails. Some evidence of younger lava flows can be found near the cliffs surrounding the Upper Geyser Basin, near Old Faithful. (Old Faithful is a geyser and one of the park’s most popular tourist attractions.)

Today, Yellowstone sleeps, with scientists checking its every hiccup or cough in an effort to predict its next move. While the brewing force beneath the park has been restrained for thousands of years, Yellowstone’s dormancy does not mean it will not one day awaken. The question remains: When and with what force?

“We are getting into the time period where it is supposed to become more unstable,” says Anderson, “but I am not holding my breath.”