A recent study suggests that a new explanation for plate tectonics – the motion of the Earth’s crust plates – may be needed, according to NASA.
Plate tectonics have been responsible for some of nature’s most amazing visuals. Plate tectonics have contributed to the development of the Himalayas, the Swiss Alps and The Andes, the United States Geological Survey posits.
Unfortunately, plate tectonics have also caused some of Earth’s deadliest natural disasters, such as earthquakes and volcanic eruptions.
A NASA-sponsored researcher believes that a stratum of halfway molten rock approximately 22 to 75 miles below the Earth’s surface is not the only instrument that gives continents the ability to shift their position over millions of years.
“This melt-rich layer is actually quite spotty under the Pacific Ocean basin and surrounding areas, as revealed by my analysis of seismometer data,” says Dr. Nicholas Schmerr, a NASA Postdoctoral Program fellow, in a statement.
“Since it only exists in certain places, it can’t be the only reason why rigid crustal plates carrying the continents can slide over softer rock below,” Mr. Schmerr adds.
Mr. Schmerr, who works at NASA’s Goddard Space Flight Center in Maryland, is the author of a study on this research that appeared in a March 23rd issue of the journal Science.
During the course of more than four billion years, the processes of plate tectonics have moved continents many thousands of miles. Along the way, mountain rages have been created out of continental collisions and valleys and oceans have been created when continents were ripped apart.
Scientists also believe that continental drift may have contributed to climate change by altering the currents in the ocean and atmosphere.
Many details of plate tectonics are known while many are still a mystery. The extraneous layer of Earth, known as the lithosphere, is made up of the crust and a bottom layer of cool and rigid mantle. The lithosphere is thickest beneath continents.
Below the lithosphere is the asthenosphere, which is a layer of rock that is lethargically deforming. Heat in the Earth’s core warms mantle rocks above. The rocks in the mantle rise and sink depending on their temperature respective to their surroundings. This movements causes the continental plates to shift.
Up until now, however, scientists have been unable to explain how the lithosphere moves freely of the asthenosphere.
“Something has to decouple the crustal plates from the asthenosphere so they can slide over it,” says Mr. Schmerr.
“Numerous theories have been proposed, and one of those was that a melt-rich layer lubricates the boundary between the lithosphere and the asthenosphere, allowing the crustal plates to slide. However, since this layer is only present in certain regions under the Pacific plate, it can’t be the only mechanism that allows plate tectonics to happen there. Something else must be letting the plate slide in areas where the melt doesn’t exist,” he adds.
According to the Carnegie Institution, Mr. Schmerr is referring to the lithosphere-asthenosphere boundary (LAB), which represents the transition from the hot asthenosphere to the cold and stiff lithosphere.
In studies of seismic waves moving across the LAB, scientists have found that seismic waves have higher wave speeds in the lithosphere and lower wave speeds in the asthenosphere. In a boundary referred to as the Gutenberg discontinuity (named after Beno Gutenberg), seismic waves slow down by as much as 5 to 10 percent.
The introduction of water to the rock and small variations in composition, temperature, or the grain size of mineral in this region may allow for the decoupling of crustal plates from the asthenosphere, but the data to confirm these theories is not ready for examination.
Mr. Schmerr posited that a different explanation for plate tectonics may be needed after looking at the arrival times of earthquake waves at seismometers throughout the world. S-waves, one of the kinds of waves produced by earthquakes, will spring or rebound off material interfaces inside the Earth. S-waves will appear at seismometers at varying times based on where they connect with these interfaces.
Mr. Schmerr was able to calculate the depth and seismic properties of melt layers under the Pacific Ocean basin by looking at the arrival times, heights, and forms of the primary and melt-layer-reflected waves at different locations. An S-wave that confronts a deeper melt layer at the lithosphere-asthenosphere boundary will arrive earlier to the seismometer, because it travels a shorter route.
“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,” Mr. Schmerr posits.
“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,” he professes.
Mr. Schmerr believes that water may be a key ingredient to explaining how plate tectonics work. He cites Venus as an example of a planet that lacks oceans and has no attestations of plate tectonics.