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.
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.
Jim Gaherty firstname.lastname@example.org (845) 365-8450
More information: Kevin Krajick, Senior editor, science news, The Earth Institute email@example.com 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.
The Earth Institute, Columbia University mobilizes the sciences, education and public policy to achieve a sustainable earth. http://www.
Kevin Krajick | EurekAlert!
In times of climate change: What a lake’s colour can tell about its condition
21.09.2017 | Leibniz-Institut für Gewässerökologie und Binnenfischerei (IGB)
Did marine sponges trigger the ‘Cambrian explosion’ through ‘ecosystem engineering’?
21.09.2017 | Helmholtz-Zentrum Potsdam - Deutsches GeoForschungsZentrum GFZ
At the productronica trade fair in Munich this November, the Fraunhofer Institute for Laser Technology ILT will be presenting Laser-Based Tape-Automated Bonding, LaserTAB for short. The experts from Aachen will be demonstrating how new battery cells and power electronics can be micro-welded more efficiently and precisely than ever before thanks to new optics and robot support.
Fraunhofer ILT from Aachen relies on a clever combination of robotics and a laser scanner with new optics as well as process monitoring, which it has developed...
Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.
A warming planet
Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.
The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...
Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...
Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!
When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...
19.09.2017 | Event News
12.09.2017 | Event News
06.09.2017 | Event News
25.09.2017 | Power and Electrical Engineering
25.09.2017 | Health and Medicine
25.09.2017 | Physics and Astronomy