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.

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Earlier this week, a group of scientists working at the VU Amsterdam University in the Netherlands conducted an experiment in which they learned how the Moon’s interior is truly structured: below the thin rocky surface (otherwise known as a lithosphere) churns a thick mantle of liquid magma.

The team was led by Mirjam van Parker and Wim van Westrenen and consisted of scientists from the Universities of Paris 6/CNRS, Lyon 1/CNRS, Edinburgh, and the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Using copies of 380 kilograms worth of lunar rock samples that were collected by the astronauts from the Apollo missions, the scientists melted them with a high electric current at a temperature of 1,500˚F, then compressed them at a pressure of 4,500 bar. The aforementioned temperature and pressure are thought to be at the same intensity as the ones underneath the Moon’s surface.

After this, the team measured the samples’ density with powerful x-ray beams emitted from a synchrotron, provided by the ESRF. They learned that the molten rock (the magma) was quite dense – much denser than they had assumed – and that it was liquid, filled with titanium.

These results have disproven a commonly referenced hypothesized cross-section that scientists constructed in the past: first is the thin crust, which is not uniform in thickness around the surface; below is a thick, solid mantle; following is a thinner mantle known as the moonquake zone, which is slightly less solid than the upper mantle, so that seismic waves can travel; and, finally, a solid iron-rich core lies in the center.

The Moon lacks current volcanic activity because the magma is too dense, or just a bit too firm; lighter liquid tends to be pushed up more, similar to the magma under the Earth, and there must be a difference in density between the magma and the surrounding solid material for any eruptions.

“Today, the Moon is still cooling down, as are the melts in its interior,” Wim van Westrenen, the chair of the Netherlands Platform for Planetary Science, states in the ESRF’s news release. “In the distant future, the cooler and therefore solidifying melt will change in composition, likely making it less dense than its surroundings.

This lighter magma could make its way again up to the surface forming an active volcano on the Moon – what a sight that would be! – but for the time being, this is just a hypothesis to stimulate more experiments.” He and Mirjam van Parker have published the findings of their experiment in the journal, ‘Nature Geosciences’, on February 19.

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