Research Stories

by Diane Boudreau

One hot summer day, I took my 7-year-old son and his best friend on an outing. In the back seat of my car, they started their usual game of one-upmanship. The boys try to outdo each other on just about any subject from Pokémon powers to Tyrannosaurus trivia. The topic this day was the inner Earth.

"The Earth's outer core is made of melted iron," said Tanner, an animated second-grader with a shock of bright red hair.

"I already know that," replied Nick in the most bored tone he could muster. Nick fancies himself the science expert of the elementary crowd.

"The mantle is made of really hot rocks," his friend added.

Nick paused, shedding his all-knowing façade for a moment. "But how do you know that's what's inside the Earth?" he asked.

"I read it in a book."

"But how did the people who wrote the book know?"

Tanner was silent for a moment. Finally he gave an exasperated sigh.

"Pshhhhhhh! That's so simple I'm not even going to answer it!" he responded, and turned to gaze out the car window.

Possessing slightly more maturity than a 7-year-old, I can admit what Tanner couldn't. I don't have a clue how we really know what's inside the Earth. Like him, I've read books that say our planet has a solid iron inner core, surrounded by a molten outer core, surrounded by a mantle of rock. But I have no idea how scientists know this about a place they can't visit or take samples from, a place they can't see even with tools like microscopes or telescopes or X-ray machines.

Unlike my young charges, however, I know how to find out.

Catching the perfect wave
I start with a visit to Ed Garnero, a geophysicist in the School of Earth and Space Exploration at Arizona State University. Garnero studies what happens deep inside the Earth and how it relates to what happens on the surface. His tool of choice is earthquakes.

Although scientists can't generate their own earthquakes, the Earth actually experiences hundreds of them every day. You don't read about them in the news because they are small or they happen in unpopulated areas. But scientists can use the information they provide to understand the makeup of the inner Earth. By measuring the seismic waves that an earthquake generates, they can learn a lot about what's deep under the surface.

"When earthquakes happen the waves propagate in all directions. It's like dropping a rock in a pond–the water ripples outward," explains Garnero.

"If you live five miles from your office and your average speed is 30 mph, we can predict how long it will take you to get to work. If you arrive later than expected we know that something held you up," he says. "It's the same with seismic waves. If we know the speed of the wave and the location of the earthquake's epicenter, we can tell how long it should take the wave to get to the point where the seismometer is measuring it. If it takes longer, then something slowed it down."

There are two main types of seismic waves–P waves and S waves. These waves tell us different things about the structure of the Earth. For example, S waves provide evidence that the outer core of the Earth is liquid, because S waves don't propagate in liquids. When an earthquake strikes, there is a "shadow zone" directly across the Earth in which no S waves are recorded.

Seismic wave studies also add to our knowledge of the mantle. The makeup, structure, and temperature of mantle rocks all affect the speed of waves passing through them.

Look at an old textbook cross-section picture of the Earth. The mantle probably looks like one homogenous layer. It is anything but. Although the mantle is solid rock, it behaves like a fluid over geologic time scales, slowly convecting and mixing. In fact, it is Earth's churning innards that drive actions on the surface such as earthquakes, volcanoes, and mountain-building.

Through seismic wave studies, and by examining pieces of the mantle that have come up through volcanic explosions, scientists have a pretty good idea of what the mantle is made of. It mostly consists of oxygen, magnesium, and silicon, with lesser amounts of iron. How these minerals respond to the intense pressure and heat deep below the Earth's surface, however, remains a mystery.

Squishing rocks
To find out more about how rocks behave under pressure, I visit Tom Sharp, an ASU mineralogist whose bookshelves are crowded with an impressive array of rocks.

He hands me a chunk of heavy rock flecked with green crystals. The rock is a piece of Earth's mantle that came to the surface through a volcano. The green crystals are olivine, the dominant mineral in the upper mantle.

Volcanic rocks can tell us a lot about Earth's innards. But once released to the surface, they are no longer under intense pressure and heat. Sharp conducts experiments that mimic the conditions rocks face far belowground. In effect, he squishes rocks for a living.

He uses a tool called a multi-anvil press. ASU's largest press can create pressure equal to that found at the top of the lower mantle. That is 700 kilometers (about 435 miles) beneath the Earth's surface. At this depth, the pressure is about 270,000 times the normal air pressure around you right now. The press can also heat samples up to 2,500 degrees Celsius (4,572 degrees F)–hot enough to melt mantle rock at that pressure.

Sharp puts minerals under intense pressure and heat, cools them very quickly, then decompresses them and removes them from the press. He studies what comes out under a high-resolution electron microscope.

He is particularly interested in studying change. Although the entire mantle is made up of the same elements, their mineral structures change as the temperature and pressure change. This change in mineral structure is called a phase transition. For example, the upper mantle is mostly composed of a mineral called olivine. But at about 410 km down, the olivine turns into a mineral called wadsleyite. Deeper still (around 550 km) it becomes ringwoodite. It finally turns to magnesium silicate perovskite past 660 km, the top of the lower mantle.

Sharp wants to know how water affects these phase transitions. The plates that make up Earth's crust–the uppermost layer–are always in motion. Where plates pull apart, mantle material rises up to the surface. In other areas, plates dive beneath the surface, or "subduct," returning to the mantle. Most subduction zones lie beneath an ocean, which means that some water sinks into the mantle as well as rock.

"Scientists who study the mantle now realize that the mantle can hold a lot of water. Under high pressure, rocks can take on more water–the solubility of water increases with pressure," he says.

In other words, you can't squeeze water from a stone, but you can squeeze water into one. In the mid-mantle, rocks can be up to 3 percent water. That means the mantle could be holding enough water to fill all the world's oceans several times over!

Some scientists have suggested that phase transitions might happen later in subduction zones because the water lowers the temperature. However, Sharp's research shows that water also speeds up reactions in rocks.

"Everything happens faster with water," he explains. "Water speeds up the reaction so much that it eliminates any slowdown from the lower temperature in the cool center of a subducting slab."

Letting in the sun
Water might be the key to a mysterious layer of molten rock lying about 410 km beneath Tucson, Ariz. The layer is unique, and doesn't exist at that level in most other places. James Tyburczy believes water is a key factor in explaining why the layer exists.

"Why there? One reason it seems likely is that the western United States is an area of great tectonic activity. It seems to be a potentially likely source for introducing water to make this occur," says Tyburczy, a mineral physicist at ASU. "You don't see that in other places where there is no subduction." It also doesn't happen in all subduction zones.

"Subduction is required, but also something more. There's something special about the Southwest," adds the ASU scientist.

Tyburczy and former ASU graduate student Daniel Toffelmier studied this phenomenon using electromagnetic waves coming all the way from the sun.

The sun emits a continuous flow of charged atomic particles called the solar wind. When gusts of these particles reach the Earth, they induce changes in our planet's magnetosphere. This causes weak but measurable electrical currents to flow through the rocks deep inside the Earth.

The scientists measure the changes in the rocks' conductivity at different depths. By comparing this data with what we know about the conductivity of rocks from lab studies, the scientists can estimate the composition of rocks deep in the mantle.

"Rocks are semiconductors," says Tyburczy. "And rocks with more hydrogen embedded in their structure conduct better, as do rocks that are partially molten."
A common source of hydrogen, of course, is water.

In 2003, scientists at Yale University proposed that mantle rock rising up through a depth of 410 km would give up any water mixed into its crystal structure. The release of water lowers the rock's melting point. As a result, the rock would liquefy.

Tyburczy and Toffelmier decided to test this hypothesis using electromagnetic data collected from five different regions: the American Southwest, northern Canada, the French Alps, a regionally averaged Europe, and the northern Pacific Ocean. What they found both supported and disputed the Yale scientists' proposal.

Seismic wave studies already showed that Earth's density abruptly changes at 410 km, suggesting a phase transition. But Tyburczy and Toffelmier's work shows that rock is not always melting at this depth, as the Yale researchers proposed. However, it is happening beneath Arizona.

Clearly, more studies are needed to figure out what makes this region susceptible to melting. Tyburczy and Toffelmier's work is just one more clue to understanding what is really happening in the mantle. The typical cross-section image of Earth we all know is simply a global average, and doesn't reflect regional variations.

"There are a lot of perturbations where that simply spherical picture isn't perfect. We know a lot more about it now," says Tyburczy.

Writhing blobs of...data
Blobs of red and blue writhe about on Allen McNamara's computer screen like some kind of digital lava lamp. Actually, the analogy isn't too far off the mark. The animation simulates processes that occur in the mantle of an Earth-like planet. The blue blobs represent colder material, while the red blobs represent hotter material. The blue blobs sink downward, where they are heated, turn red, and start to rise again.

McNamara is a geodynamicist. He uses computer models to test ideas about the structure and dynamics of the inner Earth.

"The mantle is solid rock but it sometimes behaves like fluid," explains McNamara. "Basically, I use a bunch of computers working together to solve equations of fluid flow."

Even for powerful computers, this is heavy-duty work. It usually takes 30 computers more than three months to solve one equation. Although time-consuming, this kind of work wasn't even possible a couple of decades ago.

McNamara gathers all types of data from scientists like Garnero, Sharp, and Tyburczy. He then plugs it into a computer model to see how it all fits together.

"Someone will make new observations in seismology or mineralogy. Based on these observations, people come up with all kinds of ideas. I test these ideas to see if they are reasonable and consistent with the laws of physics," says McNamara.

McNamara started his career wanting to know why the plates on the surface of the Earth move the way they do. In order to understand this, he had to look at the mantle.

"To understand how the surface works–how mountains form or how oil is trapped–you need to understand the forces behind that, and that's mantle convection. It's the fundamental cause of everything we see," he says.

Computer models need to take many, many factors into account. Temperature, pressure, and chemical makeup are the obvious ones. McNamara also studies something called seismic anisotropy.

Mantle rock is made up of mineral grains thought to be about one millimeter in size. Each of these grains is anisotropic, meaning that a seismic wave moves through it faster in one direction than in other directions.

Normally, these grains are randomly oriented. As a result, the wave speed through the rocks is an average of all the different grains.

But as mantle material flows, sometimes the mineral grains start to orient in the same direction. Then seismic waves will travel through them faster.

McNamara works with mineral physicists and seismologists to understand how this affects flow. He models convection in the Earth. He then applies those forces to minerals in the lab to find out how the grains are oriented. Finally, McNamara creates a model showing how seismic waves will move through the grains.

Allen McNamara

This is just one of the areas where McNamara's expertise comes in handy. He is helping snap together some of the pieces of the puzzle of the inner Earth.

"It's really interesting that after all these years we don't have a good idea of what's going on," McNamara says. "We have lots of ways to 'see' inside the Earth but each observation is a little inconclusive. It's a skill to see those observations and come up with a hypothesis based on them all. I spend a lot of time trying to understand the possibilities. I find a lot of ideas don't make any sense. They look good on paper, but they don't bear out. The reject pile gets pretty big," he says.

But a big reject pile doesn't spell failure in the scientific world. In fact, it is an integral part of the process. Only by testing lots of hypotheses can scientists ever find the ones that turn out to be true.

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Research by ASU scientists to study the inner Earth is supported by the National Science Foundation, NASA, and other funding agencies. For more information, contact the School of Earth and Space Exploration, an academic unit of the College of Liberal Arts and Sciences. Call 480.965.5081. Send email to seseinfo@asu.edu