Layer of molten rock is found under Earth’s tectonic plates

Researchers have detected an unknown layer of partially melted rock under the Earth’s crust.

The discovery could help scientists learn more about Earth’s tectonic plate movements, which not only create mountains and earthquakes, but also contribute to forming environments with the right chemical and physical conditions to support life on early Earth.

The outermost layer of our planet is the crust – in which we live – and below that are the mantle, outer core and inner core. The world’s oceans and continents sit on 15 major moving and shifting blocks, called tectonic plates, that make up the lower crust and upper mantle.

The newly identified molten layer is located 161 kilometers below the Earth’s surface. This layer is a part of the asthenosphere, which lies beneath the tectonic plates. The asthenosphere exists as a soft layer of solid but malleable rock that can cause tectonic plates to shift and shift.

Researchers wonder what factors make the asthenosphere soft, and consider molten rock as part of the equation. Although the Earth’s interior is largely solid, rocks can change 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 the Earth’s mantle beneath Turkey for his doctoral research when he spotted signs of partially melted rock. He started his work in 2020, while a PhD student at Brown University.

Scientists had previously seen parts of this rock layer and thought it to be an anomaly, but Hua and his fellow researchers found evidence that it had a wider presence.

The research team confirmed that the asthenosphere is composed of solid, molten rock and that, although the rock subsequently is partially molten, it does not contribute to the movement of plates or facilitate their movement.

“When we think of something melting, we intuitively think that the melt must play an important role in the viscosity of the material,” said Hua. “But what we found is that even where the melt fraction is quite high, its effect on mantle flow is much smaller.”

In the mantle, convection, or heat transfer, occurs when hot, less dense material rises and cooler, denser material sinks. The researchers believe that the presence of solid rocks and convection contribute to plate movement.

Analysis of seismic activity

The main challenge of studying Earth’s inner layers is collecting the data because most can only be collected at the surface and it’s difficult to sample directly from the planet’s interior, Hua said.

“Therefore, scientists have been using seismic waves generated by earthquakes traveling through the Earth’s interior to study the speed of propagation of seismic waves in these inner layers, similar to CT scans in the hospital,” said Hua.

He collected over 700 images taken from seismic detectors around the world and created a global map of the asthenosphere.

By analyzing the data, Hua saw how seismic waves moved through different materials beneath the Earth’s crust, including changes in speed, direction and time of arrival at detection sites. The presence of melt in the partially molten layer meant that the seismic waves moved more slowly.

Molten rock showed up in seismic readings in areas where the asthenosphere reached its highest temperatures, around 1,450 degrees Celsius.

Hua is lead author of a study detailing the findings published Monday in Nature Geoscience magazine.

“This study is critical to understanding why the asthenosphere — the weak layer of the mantle beneath tectonic plates that allows plates to move — is in fact weak,” said study co-author Karen M. Fischer, a distinguished professor of science. geological studies at Brown University, in a note.

“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.”