Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

New study upends a theory of how Earth's mantle flows

07.07.2016

Small-scale processes may have big effects

A new study carried out on the floor of Pacific Ocean provides the most detailed view yet of how the earth's mantle flows beneath the ocean's tectonic plates. The findings, published in the journal Nature, appear to upend a common belief that the strongest deformation in the mantle is controlled by large-scale movement of the plates. Instead, the highest resolution imaging yet reveals smaller-scale processes at work that have more powerful effects.


Illustrations show how (a) pressure gradient-driven flow and (b) density-driven small-scale convection could work in the asthenosphere. At the top is the surface view showing the locations of the NoMelt seismometers. The red arrows indicated the flow direction.

Credit: Lin et al., Nature 2016

By developing a better picture of the underlying engine of plate tectonics, scientists hope to gain a better understanding of the mechanisms that drive plate movement and influence related process, including those involving earthquakes and volcanoes.

When we look out at the earth, we see its rigid crust, a relatively thin layer of rock that makes up the continents and the ocean floor. The crust sits on tectonic plates that move slowly over time in a layer called the lithosphere. At the bottom of the plates, some 80 to 100 kilometers below the surface, the asthenosphere begins. Earth's interior flows more easily in the asthenosphere, and convection here is believed to help drive plate tectonics, but how exactly that happens and what the boundary between the lithosphere and asthenosphere looks like isn't clear.

To take a closer look at these processes, a team led by scientists from Columbia University's Lamont-Doherty Earth Observatory installed an array of seismometers on the floor of the Pacific Ocean, near the center of the Pacific Plate. By recording seismic waves generated by earthquakes, they were able to look deep inside the earth and create images of the mantle's flow, similar to the way a doctor images a broken bone.

Seismic waves move faster through flowing rock because the pressure deforms the crystals of olivine, a mineral common in the mantle, and stretches them in the same direction. By looking for faster seismic wave movement, scientists can map where the mantle is flowing today and where it has flowed in the past.

Three basic forces are believed to drive oceanic plate movement: plates are "pushed" away from mid-ocean ridges as new sea floor forms; plates are "pulled" as the oldest parts of the plate dive back into the earth at subduction zones; and convection within the asthenosphere helps ferry the plates along. If the dominant flow in the asthenosphere resulted solely from "ridge push" or "plate pull," then the crystals just below the plate should align with the plate's movement. The study finds, however, that the direction of the crystals doesn't correlate with the apparent plate motion at any depth in the asthenosphere. Instead, the alignment of the crystals is strongest near the top of the lithosphere where new sea floor forms, weakest near the base of the plate, and then peaks in strength again about 250 kilometers below the surface, deep in the asthenosphere.

"If the main flow were the mantle being sheared by the plate above it, where the plate is just dragging everything with it, we would predict a fast direction that's different than what we see," said coauthor James Gaherty, a geophysicist at Lamont-Doherty. "Our data suggest that there are two other processes in the mantle that are stronger: one, the asthenosphere is clearly flowing on its own, but it's deeper and smaller scale; and, two, seafloor spreading at the ridge produces a very strong lithospheric fabric that cannot be ignored." Shearing probably does happen at the plate boundary, Gaherty said, but it is substantially weaker.

Donald Forsyth, a marine geophysicist at Brown University who was not involved in the new study, said, "These new results will force reconsideration of prevailing models of flow in the oceanic mantle."

Looking at the entire upper mantle, the scientists found that the most powerful process causing rocks to flow happens in the upper part of the lithosphere as new sea floor is created at a mid-ocean ridge. As molten rock rises, only a fraction of the flowing rock squeezes up to the ridge. On either side, the pressure bends the excess rock 90 degrees so it pushes into the lithosphere parallel to the bottom of the crust. The flow solidifies as it cools, creating a record of sea floor spreading over millions of years.

This "corner flow" process was known, but the study brings it into greater focus, showing that it deforms the rock crystals to a depth of at least 50 kilometers into the lithosphere.

In the asthenosphere, the patterns suggest two potential flow scenarios, both providing evidence of convection channels that bottom out about 250 to 300 kilometers below the earth's surface. In one scenario, differences in pressure drive the flow like squeezing toothpaste from a tube, causing rocks to flow east-to-west or west-to-east within the channel. The pressure difference could be caused by hot, partially molten rock piled up beneath mid-ocean ridges or beneath the cooling plates diving into the earth at subduction zones, the authors write. Another possible scenario is that small-scale convection is taking place within the channel as chunks of mantle cool and sink. High-resolution gravity measurements show changes over relatively small distances that could reflect small-scale convection.

"The fact that we observe smaller-scale processes that dominate upper-mantle deformation, that's a big step forward. But it still leaves uncertain what those flow processes are. We need a wider set of observations from other regions," Gaherty said.

The study is part of the NoMelt project, which was designed to explore the lithosphere-asthenosphere boundary at the center of an oceanic plate, far from the influence of melting at the ridge. The scientists believe the findings here are representative of the Pacific Basin and likely ocean basins around the world.

NoMelt is unique because of its location. Most studies use land-based seismometers at edge of the ocean that tend to highlight the motion of the plates over the asthenosphere because of its large scale and miss the smaller-scale processes. NoMelt's ocean bottom seismometer array, with the assistance of Lamont's seismic research ship the Marcus G. Langseth, recorded data from earthquakes and other seismic sources from the middle of the plate over the span of a year.

###

The lead author of the study is PeiYing (Patty) Lin, who conducted the analysis while a postdoctoral fellow at Lamont-Doherty Earth Observatory. The other coauthors are former Lamont graduate student Ge Jin; John Collins, Daniel Lizarralde and Rob Evans of Woods Hole Oceanographic Institution; and Greg Hirth of Brown University. The research was supported in part by the National Science Foundation.

The paper, "High-resolution seismic constraints on flow dynamics in the oceanic asthenosphere," is available from the author.

Scientist contact:

Jim Gaherty gaherty@ldeo.columbia.edu (845) 365-8450

More information: Kevin Krajick, Senior editor, science news, The Earth Institute kkrajick@ei.columbia.edu 212-854-9729

Lamont-Doherty Earth Observatory is Columbia University's home for Earth science research. Its scientists develop fundamental knowledge about the origin, evolution and future of the natural world, from the planet's deepest interior to the outer reaches of its atmosphere, on every continent and in every ocean, providing a rational basis for the difficult choices facing humanity. http://www.ldeo.columbia.edu | @LamontEarth

The Earth Institute, Columbia University mobilizes the sciences, education and public policy to achieve a sustainable earth. http://www.earth.columbia.edu.

Kevin Krajick | EurekAlert!

Further reports about: Earth Earth Observatory crystals earthquakes lithosphere sea floor

More articles from Earth Sciences:

nachricht Global study of world's beaches shows threat to protected areas
19.07.2018 | NASA/Goddard Space Flight Center

nachricht NSF-supported researchers to present new results on hurricanes and other extreme events
19.07.2018 | National Science Foundation

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Future electronic components to be printed like newspapers

A new manufacturing technique uses a process similar to newspaper printing to form smoother and more flexible metals for making ultrafast electronic devices.

The low-cost process, developed by Purdue University researchers, combines tools already used in industry for manufacturing metals on a large scale, but uses...

Im Focus: First evidence on the source of extragalactic particles

For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.

To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...

Im Focus: Magnetic vortices: Two independent magnetic skyrmion phases discovered in a single material

For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.

Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...

Im Focus: Breaking the bond: To take part or not?

Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.

A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...

Im Focus: New 2D Spectroscopy Methods

Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.

"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Leading experts in Diabetes, Metabolism and Biomedical Engineering discuss Precision Medicine

13.07.2018 | Event News

Conference on Laser Polishing – LaP: Fine Tuning for Surfaces

12.07.2018 | Event News

11th European Wood-based Panel Symposium 2018: Meeting point for the wood-based materials industry

03.07.2018 | Event News

 
Latest News

A smart safe rechargeable zinc ion battery based on sol-gel transition electrolytes

20.07.2018 | Power and Electrical Engineering

Reversing cause and effect is no trouble for quantum computers

20.07.2018 | Information Technology

Princeton-UPenn research team finds physics treasure hidden in a wallpaper pattern

20.07.2018 | Materials Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>