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ASU geoscientists explain puzzling pockets of rock deep in Earth's mantle

Isolated pockets of rock in Earth's mantle have puzzled scientists — until now.
August 2, 2017

A team led by geoscientists from Arizona State University and Michigan State University has used computer modeling to explain how pockets of mushy rock accumulate at the boundary between Earth's core and mantle.

These pockets, lying roughly 2,900 kilometers (1,800 miles) below the surface, have been known for many years but previously lacked an explanation of how they formed.

The relatively small rock bodies are termed "ultra-low velocity zones" because seismic waves greatly slow down as they pass through them. Geoscientists have thought the zones are partially molten, yet the pockets are puzzling because many are observed in cooler regions of the deep mantle.

"These small regions have been assumed to be a partially molten version of the rock that surrounds them," said Mingming Li, lead author of the study, which was published today in the journal Nature Communications. "But their global distribution and large variations of density, shape and size suggest that they have a composition different from the mantle."

Li joined ASU's School of Earth and Space Exploration this month as an assistant professor. He was a graduate student of former ASU Associate Professor Allen McNamara, also a co-author on the paper; McNamara is now at Michigan State's Department of Earth and Environmental Sciences. The additional co-authors are ASU Professor Edward Garnero and his doctoral student Shule Yu.

“We don’t know what ultra-low velocity zones are,” said McNamara. “They are either hot, partially molten portions of otherwise normal mantle, or they are something else entirely, some other composition."

Because seismic evidence allows both possibilities, he said, "We decided to model mantle convection by computer to investigate whether their shapes and positions can answer the question.”

Do pockets relate to blobs?

About year ago, Garnero, McNamara and School of Earth and Space Exploration Associate Professor Dan Shim reported that two gigantic structures of rock deep in the Earth are likely made of something different from the rest of the mantle. They called the large structures "thermochemical piles," or more simply, blobs.

"While the origin and composition of these blobs are unknown," Garnero said at the time, "we suspect they hold important clues as to how the Earth was formed and how it works today."

What the big blobs are made of and how they formed still remain unknown, said Garnero. "But the new computer modeling explains how these ultra-low velocity zones are associated with the much bigger blobs."

Li said, "The ultra-low velocity zones are generally around tens of kilometers tall, and hundreds of kilometers wide or less. They are mostly located near the edges of the much larger blobs, but some of them are detected both inside the blobs and well away from them."

Tiny regions of compositionally distinct rock (red material, known as ultra-low velocity zones), collect at Earth’s core-mantle boundary (tan surface), nearly halfway to the center of our planet. Small accumulations of this distinct rock collect near the margins of large thermochemical piles (green) that reside at the base of Earth’s mantle. Image by Edward Garnero/ASU

The outcome of the computer modeling showed that most of these ultra-low velocity zones are different in composition from the surrounding mantle, Li said. What's more, the modeling showed that pockets of rock with different compositions will migrate from anywhere on the core-mantle boundary toward the margins of the large blobs.

"The margins of the thermochemical piles are where mantle flow patterns converge," McNamara said, "and therefore these areas provide a 'collection depot' for denser types of rock."

Gathered by heat

The force driving this movement is heat, which powers convection in the mantle.

Earth's mantle is made of hot rock, but it behaves more like fudge simmering slowly on a stove. In the mantle, heat comes both from radioactivity within the mantle rock and from the planet's core, the center of which is about as hot as the sun's surface. Mantle rock responds to this heat with a slow churning — convective — motion.

"The details are not completely clear," Li said. But the modeling shows that rocks of different composition respond to the convection in a way that gathers compositionally similar materials together. This moves the small pockets of chemically distinct rocks to the edges of the hotter blobs above the core-mantle boundary.

"We ran 3-D high-resolution computer modeling, and we developed a method to track the movement of both the small pockets of ultra-low velocity zones and the much larger thermochemical piles." Li explained. "This allowed us to study how the small pockets move around and how their locations can be related to their origin."

McNamara said, "What was new about our approach — and also computationally challenging — was that the modeling simultaneously took into account vastly different scales of motion." These ranged from global mantle-scale convection patterns, to the large thermochemical piles in the lower mantle, and down to the very small-scale pockets of ultra-low velocity zone at the bottom.

"What we ultimately found," he said, "is that if ultra-low velocity zones are caused by melting of otherwise normal mantle, they should be located well inside of the thermochemical piles, where mantle temperatures are the hottest."

But he added, "If the ultra-low velocity pockets of rock have a composition different from the ordinary mantle rock, then mantle convection would continually carry them to the edges of piles where they collect.

"This is consistent with what we see in the seismic observations.”

Rocks diving deep?

Where do the different materials in the deep mantle come from in the first place?

"There are several possibilities," Garnero said. "Some material might be associated with former basaltic oceanic crust that got subducted deeply. Or it might be associated with chemical reactions between the outer core's iron-rich fluid and the crystalline silicate mantle."

Garnero said that where the rock in ultra-low velocity zones originally came from is currently unsolved. But the process of collecting this material into small pockets of rock is clear.

"You can have various mechanisms, such as plate tectonics, that push rock of differing chemistries into the deepest mantle anywhere on Earth," he said.

"But once these different rocks have gone down deep, convection wins and sweeps them to the hot regions, namely, where the continental-sized thermochemical piles reside."

Robert Burnham

Science writer , School of Earth and Space Exploration


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Parents, teachers need to be creative in finding ways to interact

ASU program prepares teachers to be part of the community where they teach.
New ASU project to engage parents in culturally relevant way to share knowledge.
Mary Lou Fulton curriculum teaches that parent-teacher communication is two-way.
August 4, 2017

ASU expert on teacher training says every family has something to contribute to school community; it might just take fresh approach

As families send their children back into the classroom, these first weeks of the school year are a critical time for parents and teachers to create connections that will make it easier to address any sticky issues that come up later, according to an expert on teacher training at Arizona State University.

But work and family responsibilities coupled with the demands on teachers’ time can make that difficult.

“It’s about being creative,” said Margarita Jimenez-Silva, an associate professor in the Mary Lou Fulton Teachers College at ASU. “There are a lot of ways to make families feel like they’re part of the school community.”

Jimenez-Silva researches parent engagement, especially among underserved populations, and was a middle-school teacher in California before she became a professor.

She talked to ASU Now about parent-teacher interactions.

Question: Why is it important for parents to engage with their children’s schools?

Answer: All of the research indicates that parental engagement helps the child feel part of the community, it helps with retention rates, it helps with academic achievement. The research is conclusive that it’s beneficial to both the child and the teacher because communication is easier.

Margarita Jimenez-Silva

For the family, engagement shows that school is a priority and is the important work of the child. And the parents feel like they’re in this journey of education in partnership with the teacher and not working at different purposes. It can be alienating for parents to not feel part of the community.

Q: What are some roadblocks to parent-teacher interaction?

A: There are a lot of cultural differences in how parents engage with schools. For a lot of first-generation parents or parents who are not as familiar with our school system, there are not a lot questions about academics. It’s very much about behavior. I’m a former classroom teacher, and the parents would say, “Is my kid being respectful?” Very rarely would I get questions like, “Is his reading on grade level?” 

That has to do with the teacher being seen as the expert on academics, and it’s disrespectful to ask about that.

Also, a lot of teacher preparation focuses on one-way communication. We prepare our teachers on how to communicate expectations with parents or when there’s discipline problems. There isn’t enough focus on how do you really reach out to parents to find out what their questions and concerns are.

Even with back-to-school night or parent-teacher conferences, it’s very much a delivery of information as opposed to establishing a relationship with the parents.

Q: So how do both sides creatively connect?

A: It’s about cultural community wealth. Every family has a source of wealth to contribute.

Teachers should understand that there are different ways to engage parents, and it’s not just about who can be a room mom or help with the worksheets. And parents must be creative in offering what they can to the classroom and to the school.

I worked at a charter middle school, and every family had to donate a few hours a month. As a teacher, I had to be very creative about how the parents could help me that didn’t involve them coming to the school because they had little ones at home or transportation issues.

I made a list. I need curtains. Is there a parent who can sew? I had a dad who was a semi-professional soccer player. Could he come in and do a clinic? That helps him feel a part of the community and helps other families see his role and say, “Hey, I have something I can contribute that I didn’t realize was valuable.”

My greatest asset were parents who had business connections who could hook us up for the carnival or other things.

I had a parent who could never go on a field trip because she had six kids at home. So she volunteered to call the other parents the night before. She felt like part of the field trip even though she wasn’t actually going.

The other thing that was really successful was having a core group of parents who share the cultural and language backgrounds of the families I was trying to engage. So if I had a family that was not participating, they would be a liaison between me and that other family.

Q: So parents should speak up about what they’re willing to offer?

A: Teachers should know your skills so they can tap into the resources you have.

When I had my doctorate I was teaching five classes, and it took somersaults for me to get the time to go volunteer at my sons’ school. And they would say, “We need you to sharpen pencils.” And not that I think I’m better than sharpening pencils, but I could have helped the kid who needed reading intervention, which I knew how to do.

I know teachers are frantic, but it’s a matter of being organized to make use of the resources they have.

Q: Are teachers being better prepared in how to cultivate parent relationships?

A: At ASU, we have a program that prepares teachers to be a part of the communities where they teach, so it’s not just drive in and drive out. That builds trust with parents.

Wendy OakesWendy Oakes is an assistant professor in Mary Lou Fulton Teachers College who researches practices that improve educational outcomes in children with emotional and behavioral disorders. and I are doing a a cool project in the next few weeks in an early childhood special education setting to engage parents in a culturally relevant way to engage families to share knowledge.

A few years ago we did a systemic review looking at our syllabi and how we prepared teachers and found it was very one-way. Now we are embedding in multiple courses the idea that the communications between parents and teachers has to be two-ways. And we’re seeing our graduates now who have gone through that are now thinking in this way.

Parents should be empowered to say, “These are the skills I have. Can this be of service to you?”

For more information on ASU's program to prepare teachers to better interact with parents, click here.

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Mary Beth Faller

reporter , ASU Now