Birth of Yellowstone caldera wasn't from hot-rock eruption, scientists find By Scientific American, adapted by Newsela staff on 03.21.16 Word Count 869

The Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the United States, and the third largest in the world. Below the hot springs at Yellowstone lies a sleeping supervolcano. Photo: Wikipedia

Deep below the geysers and hot springs of the Yellowstone caldera is a sleeping supervolcano. The caldera, which looks similar to a crater, is the result of a massive volcanic eruption in the past. If the supervolcano erupted again, the blast would be powerful enough to blanket the western U.S. with ash.

Scientists have long debated the origins of the supervolcano under Wyoming's Yellowstone National Park. The most widely accepted idea suggested that it was formed by a mantle plume. Such plumes — columns of hot rocks emerging from deep below the Earth's crust — are what many people think of when they imagine a volcano erupting. A new computer simulation has disproved that idea. A mantle plume could not have reached the surface at Yellowstone because it would have been blocked by a slab of rock from an ancient tectonic plate, a huge moving section of the Earth's crust.

A Newer, More Sophisticated Geological Tool

The simulation was based on a computer model of the geological processes at work underneath Yellwwstone. It was the first time scientists created a model of the complicated relationship between a mantle plume and a sinking slab of solid rock.

Lijun Liu is a geologist at the University of Illinois at Urbana-Champaign. He built the model with his graduate student Tiffany Leonard. By inputting information about Yellowstone into a computer, they were able to recreate the history of the supervolcano in their model. The model included both the plate tectonic history of the surface and what happened closer to the Earth’s interior. They wanted to understand both the movement of the Earth's plates and how those movements related to the movement of the mantle below the plates.

“No one had modeled it this vigorously,” says Brandon Schmandt, a University of New Mexico geologist who was not involved in the study. Liu's model did more than simply create a three-dimensional view of Yellowstone’s volcanic underbelly. It extended this view back 40 million years to recreate the eruptions that occurred in the states surrounding Yellowstone.

Big Bully Of A Rock Slab

No matter how they tweaked the settings of their model, Liu and Leonard could not recreate most of the recent eruptions. The reason for that is simple. It is hard to get anything, even a mantle plume, past a giant slab of rock. “Slabs are the bully,” says Eugene Humphreys, a University of Oregon geophysicist who was also not involved in the study. “Plumes are just pretty wimpy in comparison.”

The slab in question was once part of a larger tectonic plate. It was driven deep into the Earth’s mantle about 100 million years ago when the Pacific and North American plates began converging. Like a canoe paddle pushing through water, the mantle flowed around the slab as it sank downward. As more mantle surrounded the slab, more pressure built toward the front of the slab.

The model shows that 15 million years ago the pressure became too great and the slab began to break. A hole formed. A mantle plume below the slab shot through the hole, leading to massive outpourings of magma on the Earth's surface. Evidence of this magma flow remains visible in multiple states close to Yellowstone. Disproving Widely Accepted Idea

That’s where the similarities between the model and the Earth's surface end. Despite the hole in the sunken slab, the plume did not continue to rise through it. That is because the mantle is more like honey than water — it is very thick. As the slab continued to sink, it pulled the surrounding mantle down with it. Ultimately this mantle sealed the hole and blocked the plume from reaching the surface for the next 15 million years.

The idea that the slab blocked the underground plume changes our understanding of Yellowstone. It shows that the theory that a mantle plume created the Yellowstone supervolcano is incorrect. It cannot even explain the formation of Yellowstone’s caldera, which happened only 2.1 million years ago. “Ultimately, we need a new explanation for Yellowstone’s formation,” Liu says.

Looking At Juan De Fuca Ridge For Answers

Liu's team needed to find an additional heat source. Leonard thinks this could come from the Juan de Fuca ridge. The Juan de Fuca ridge is a volcanic area in the Pacific Ocean where magma oozes up between tectonic plates. It is located off the West Coast of the U.S. Despite being almost 1,600 kilometers (nearly 1,000 miles) away from Yellowstone, the ridge can easily affect the middle of the North American plate.

The Juan de Fuca ridge lies at the western edge of the Juan de Fuca tectonic plate, where the plate meets the Pacific plate. It is likely that millions of years ago the Juan de Fuca plate was pushed underneath the North American plate. This push would have created a string of volcanic eruptions and helped form Yellowstone’s gigantic caldera.

Scientists will continue to argue over the origin of Yellowstone. However Liu's model makes it clear that slabs are much more important than previously thought. “It's like smoke from a chimney that's getting swept up in some sort of windstorm,” says the University of Oregon's Humphreys. “But it's not this vigorous plume that just blasts through everything.” Quiz

1 Which section explains a new theory of how Yellowstone was formed?

(A) "A Newer, More Sophisticated Geological Tool"

(B) "Big Bully Of A Rock Slab"

(C) "Disproving Widely Accepted Idea"

(D) "Looking At Juan De Fuca Ridge For Answers"

2 Read the sections "Big Bully Of A Rock Slab" and "Disproving Widely Accepted Idea." Which paragraph explains why the mantle stopped flowing through an opening in the rock slab?

3 Which of the following BEST expresses two main ideas of the article?

(A) Scientists discovered through a simulation that a mantle plume could not have led to Yellowstone's caldera. Instead, they think the volcano is due to a giant slab of rock.

(B) Scientists thought, for a long time, that a mantle plume led to the formation of the Yellowstone supervolcano. They were completely wrong, but now they know what did cause the volcano.

(C) Scientists used a computer simulation to learn that a mantle plume could not have created Yellowstone's supervolcano because of a giant slab. They suspect the Juan de Fuca plate might be the real culprit.

(D) Scientists thought that there was a clear explanation for Yellowstone's supervolcano: a mantle plume. However, they have learned that the true reason for the supervolcano is actually the Juan de Fuca plate, which was pushed underneath the North American plate. 4 Which paragraph would be MOST important to include in a summary of the article?

(A) Deep below the geysers and hot springs of the Yellowstone caldera is a sleeping supervolcano. The caldera, which looks similar to a crater, is the result of a massive volcanic eruption in the past. If the supervolcano erupted again, the blast would be powerful enough to blanket the western U.S. with ash.

(B) The slab in question was once part of a larger tectonic plate. It was driven deep into the Earth’s mantle about 100 million years ago when the Pacific and North American plates began converging. Like a canoe paddle pushing through water, the mantle flowed around the slab as it sank downward. As more mantle surrounded the slab, more pressure built toward the front of the slab.

(C) The idea that the slab blocked the underground plume changes our understanding of Yellowstone. It shows that the theory that a mantle plume created the Yellowstone supervolcano is incorrect. It cannot even explain the formation of Yellowstone’s caldera, which happened only 2.1 million years ago. “Ultimately, we need a new explanation for Yellowstone’s formation,” Liu says.

(D) The Juan de Fuca ridge lies at the western edge of the Juan de Fuca tectonic plate, where the plate meets the Pacific plate. It is likely that millions of years ago the Juan de Fuca plate was pushed underneath the North American plate. This push would have created a string of volcanic eruptions and helped form Yellowstone’s gigantic caldera.