The discovery could help scientists learn more about the movements of Earth’s tectonic plates, which not only create mountains and earthquakes, but also contributed to forming environments with the right chemical and physical conditions to support life on early Earth.
Our planet’s outermost layer is the crust — which we live on — and below it are the mantle, the outer core and the inner core. The world’s oceans and continents sit on 15 major blocks that move and shift, called tectonic plates, that make up the lower crust and upper mantle.
The newly identified molten layer is located 100 miles (161 kilometers) below the Earth’s surface. This layer is a part of the asthenosphere, which sits beneath the tectonic plates. The asthenosphere exists as a soft layer of solid but malleable rock that can cause the tectonic plates to move and shift.
Researchers have wondered what factors make the asthenosphere soft and considered molten rocks to be part of the equation. Even though Earth’s interior is largely solid, rocks can shift and move slowly over time.
Junlin Hua, a postdoctoral fellow at the Jackson School of Geosciences at the University of Texas at Austin, was studying seismic images of Earth’s mantle located beneath Turkey for his doctoral research when he spotted signs of partly molten rock. He began his work in 2020 while a doctoral student at Brown University.
Scientists had previously spotted parts of this rock layer and thought it to be an anomaly, but Hua and his fellow researchers found evidence that it had a broader presence.
The research team confirmed that the asthenosphere is comprised of both solid and melted rock and that even though the rock later is partially molten, it doesn’t contribute to the movement of the plates or make it any easier for them to move.
When we think about something melting, we intuitively think that the melt must play a big role in the material’s viscosity,” Hua said. “But what we found is that even where the melt fraction is quite high, its effect on mantle flow is very minor.”
In the mantle, convection, or the transfer of heat, takes place as hot, less dense material rises and cooler, denser material sinks. The researchers believe the presence of solid rocks and convection contribute to plate motion.
Analysis of seismic activity
The main challenge of studying Earth’s internal layers is gathering the data because most of it can only be collected at the surface, and it’s difficult to directly sample the planet’s interior, Hua said.
Therefore, scientists have been using seismic waves generated by earthquakes that travel through the Earth interior to study the seismic wave traveling speed at these internal layers, similar to CT scans in the hospital,” Hua said.
He collected more than 700 images taken from seismic detectors around the world and created a global map of the asthenosphere.
By analyzing the data, Hua saw how the seismic waves moved through the different materials beneath Earth’s crust, including changes in speed, direction and time of arrival at the detection sites. The presence of melt in the partially molten layer meant that seismic waves moved more slowly.
The molten rock appeared on seismic readings in areas where the asthenosphere reached its highest temperatures, about 2,640 degrees Fahrenheit (1,450 degrees Celsius).
Hua is the lead author of a study detailing the findings that published Monday in the journal Nature Geoscience.
“This study is fundamental to understanding why the asthenosphere — the weak mantle layer below the tectonic plates that enables the plates to move — is in fact weak,” said study coauthor Karen M. Fischer, a distinguished professor of geological sciences at Brown University, in a statement.
“Ultimately, it provides evidence that other factors such as temperature and pressure variations can control the strength of the asthenosphere and make it weak enough for plate tectonics to be possible.”
The findings can help researchers understand how different layers beneath the Earth function.
