Newswise — The largest and most-devastating earthquakes and volcano eruptions occur where one tectonic plate is shifted underneath another one. At these so-called subduction zones, the plates become “slabs” and sink into the Earth's mantle, a thick layer of hot and deformable viscous rock.
However, what governs the sinking motion of slabs within the mantle is not well understood.
A New Mexico State University researcher authored a paper published recently in “Nature Communications” that takes a look at how and why these giant slabs move. The article titled “Global Observations of Reflectors in the Mid-Mantle with Implications for Mantle Structure and Dynamics,” seeks to answer the question of why some slabs stop sinking at some depths, and become trapped there.
“In the mantle, there is hot material that rises up, and cold material that sinks back down, like in a pot of boiling stew. That is the plumes and the slabs. The downward-going material becomes trapped at a couple of depths,” said Lauren Waszek, NMSU assistant professor of physics who authored the paper. “We can see this in tomography models, which are analogous to CT scans of the Earth. Instead of X-rays, we use sound wave energy from earthquakes, or “seismic energy”. The tomography models map the speed that the waves travel. The slabs are cold from time spent at the surface, so waves travel faster, and they show up clearly in contrast to the warmer mantle material.”
“We can see that some of the downgoing slabs stop sinking at about 1,000 kilometers depth and begin to move horizontally, but there is no good explanation for it. That was our question. Why do the slabs get stuck there? Hot plumes rising from the Earth’s deep interior are deflected sideways at this depth too: why?”
Waszek compiled and analyzed a dataset of more than 45,000 reflections of seismic energy in the mid-mantle. Reflectors are regional features that represent sharp changes in rock properties. The energy reflection is like a reflection from a mirror. It means there is a horizontal surface, where some material could potentially be trapped.
“We detect reflectors globally, with variable depths, geographic size, and reflection strength,” Waszek said. “The results identify three distinct domains in the mid-mantle, which are linked to the different up and downwelling features in the tomography models. The large variability of the reflectors indicates that the properties of these domains vary greatly.”
Waszek’s research determined that the three domains have highly distinctive observational signatures. Downgoing slabs, which do not stagnate, generate very few reflections since they are oriented close to vertically. Hot, upwelling regions produce many reflections, indicating where they are deflected in the mid-mantle. Domains with no hot material coming up or cold material going down contain laterally large and coherent reflectors. This last category may correspond to the trapped slabs.
“The interesting observation is that in the latter domains – where the wave speed is intermediate between slabs and plumes – we find very large, flat features, consistent across thousands of kilometers. These could be related to a slab that became stuck: an ancient slab that has now warmed up to become the same background temperature as the Earth, but the slab itself remains structurally different. There could be potentially an additional compositional or structural transition in the background mantle at around 1,000 kilometers which prevents the slab from sinking further, and that has not been detected before,” Waszek said. “This indicating how the overall dynamics and mixing of the mantle might be evolving over time.”
While these events happen too deep to predict earthquakes, the research tells scientists about the compositional variations inside the earth. The next step in Waszek’s research is to compare slabs in different areas of the globe; for example, comparing a slab sinking under Central America to one sinking beneath Japan, and observing how these are seismically different. Her team also wants to find out why some slabs do not get stuck.
“A recent idea about mantle convection is that there are two giant donut-shaped features of strong rock, one each side of the earth, centered on Africa and the Pacific Ocean,” Waszek said. “These structures do not mix in with the flowing parts of mantle. The hot material comes up through the middle, and cold slabs travel down the outside. If any cold material hits this donut, it cannot get through, so it has to travel along horizontally, which corresponds to a stagnant slab. So we are trying to determine if that is what is happening in the mantle.”
Waszek’s paper, published on Jan. 26, is available online at https://www.nature.com/articles/s41467-017-02709-4.
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