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Although geologists and seismologists have known and understood the basics of plate tectonics since the concept was put forth and proved, they are still baffled by many aspects, including the nature of the “boundary” between Earth’s two outermost layers: the lithosphere and asthenosphere.
Just this week, however, seismologist and NASA Postdoctoral Program Fellow Dr. Nicholas Schmerr, who is stationed at NASA Goddard Spaceflight Center in Greenbelt, Maryland, has found an actual layer between the two aforementioned layers that effects the movement of plate tectonics and seismic waves.
Plate tectonics describes the movement of the seven broken plates of the Earth’s crust. These movements are a result of the magma churning in the mantle and cause continental drifts (i.e. the continents move toward or away from one another), earthquakes, and volcanic activity.
The lithosphere (the crust) is the outermost layer of the Earth. The asthenosphere lies between the lithosphere and the mantle and contains viscous magma. This thin layer – thinner than the lithosphere – acts as the transition from cold, solid rock to hot, liquid rock.
The imaginary boundary between the lithosphere and asthenosphere is called the LAB, where the abrupt change in temperature occurs. For many years, there has been a mystery as to what causes the continents to slide over the asthenosphere. Recently, it has been speculated that there is a layer at the LAB that is designated the ‘Gutenberg discontinuity,’ which provides lubricant for the plates to move with ease. The Gutenberg discontinuity is composed of partially molten rock.
“This melt-rich layer is actually quite spotty under the Pacific Ocean basin and surrounding areas, as revealed by my analysis of seismometer data,” Dr. Schmerr said in the NASA press release. He hypothesized that the existence of the Gutenberg discontinuity is the result of decompressed hot rock, or hot mantle, plumes that cause the lower portion of the lithosphere to melt.
“Most of the melt layers are where you would expect to find them, like under volcanic regions like Hawaii and various active undersea volcanoes, or around subduction zones – areas at the edge of a continental plate where the oceanic plate is sinking into the deep interior and producing melt,” he continued. Essentially, the Gutenberg discontinuity is located in only certain areas, mainly in parts as deep as the LAB and where there has been recent volcanic activity.
“However, the interesting result is that this layer does not exist everywhere, suggesting something other than melt is needed to explain the properties of the asthenosphere.”
To find an answer, Dr. Schmerr analyzed shear waves (S-waves) with a seismometer. S-waves, which are a type of wave produced by earthquakes, bounce off different interfaces inside the Earth and arrive at certain locations and times depending on the type of interface. Dr. Schmerr measured their arrival times, heights, and shapes.
From his data, he determined that S-waves having longer paths travel all the way up to the surface without being reflected on any interface. Meanwhile, S-waves with shorter paths are reflected from the melt layers right at the LAB, causing them to travel faster. After comparing the arrival times, he was then able to ascertain the seismic properties and depth of the layers under the Pacific Ocean basin.
Dr. Schmerr will continue his study to see if he can find a presence of the melt layers in other oceans. If he and others are able to determine the exact nature of plate tectonics, the discovery would allow scientists to understand the evolution of the Earth and those of other rocky planets in the solar system.