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 convection 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.
One process missing from this study, is what causes the ebb and flow of convection? This is to say, what is the mechanism which causes the Earth’s core to heat up, or in cycles when it cools down? This is fundamental process of the dynamo theory which is “convection. My research suggests it is the cyclical expansion and contraction of celestial charged particles.
Increase Charged Particles → Decreased Magnetic Field → Increase Outer Core Convection → Increase of Mantle Plumes → Increase in Earthquake and Volcanoes → Cools Mantle and Outer Core → Return of Outer Core Convection (Mitch Battros – July 2012)
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 plumes, 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 plume 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.
Looking at the entire upper mantle, the scientists found that the most powerful process causing mantle plumes 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.
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 plumes 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.
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