Researchers Discover Unknown Molten Layer Below Earth’s Surface

An Indonesian volcano called Anak Krakatau, known as the 'child' of the legendary Krakatoa, erupted on July 19, 2018, spewing a plume of ash high into the sky as molten lava streamed down from its summit. The ‘Child of Krakatoa’ erupted at least 44 times in the past week. (Ferdi Awed/AFP/Getty Images)

Scientists have recently discovered that there is a layer of partly molten rock under the surface of the earth’s crust—a discovery crucial to understanding the movement of tectonic plates.

The earth’s crust is the outermost layer where humans reside. Below the crust is the mantle, followed by the outer core, and finally the inner core. Continents and oceans exist on 15 moving blocks called tectonic plates which comprise the upper mantle and the lower crust. The newly discovered melt layer is located around 100 miles below the surface, according to a Feb. 6 study published in the journal Nature Geoscience. The molten layer is a part of the asthenosphere located under the tectonic plates in the upper mantle.

The asthenosphere forms a kind of soft boundary of solid but malleable rock that allows tectonic plates to move through the mantle, playing a key role in plate tectonics. Without this softness, the earth’s upper layers would be too rigid for plate movements.

The reason why the asthenosphere is soft has been a mystery. Scientists had proposed that molten rocks could be a factor.

However, this study found that the asthenosphere is made up of both solid and melted rock. They also discovered that the newly identified partly molten rock layer does not contribute to the movement of the tectonic plates.

Plate Tectonics

As part of the study, Junlin Hua, a postdoctoral fellow at the University of Texas’ Jackson School of Geosciences who led the research, made a global map of the asthenosphere.

When Junlin compared the melt map with seismic measurements of tectonic movements, he found that there was no correlation even though the melt layer covers almost half the earth.

In the mantle, the process of convection or heat transfer pushes down cooler, denser material while pushing up the hotter, less dense material. Researchers believe it is the presence of solid rocks and convection that contributes to the movement of tectonic plates.

“This work is important because understanding the properties of the asthenosphere and the origins of why it’s weak is fundamental to understanding plate tectonics,” said coauthor Karen Fischer, a seismologist and professor at Brown University, according to a Feb. 6 press release.

Previously, other researchers have identified melt patches at similar depths. However, this is the first time that a study has revealed the global extent of the layer.

New Metallic Center

The molten layer is the newest unveiled mystery about the earth’s composition. In February, scientists from Australia found that there is a surprise fifth layer inside the inner core of the planet—a solid, metallic ball.

Researchers made the discovery by analyzing seismic waves traveling to the center of the earth. They found that an earthquake in Alaska had triggered seismic waves in the south Atlantic before bouncing back to its origin.

After examining the speed and trajectory of seismic waves, scientists concluded that the new fifth layer is made of a hard, metal mass composed of an alloy of iron and nickel. The discovery suggests that a major event in earth’s history resulted in a significant change in the planet’s core.

Meanwhile, the earth’s magnetic field, generated by the planet’s outer core, is said to be weakening between South America and Africa. Between 1970 and 2020, the field’s strength in the region dropped by eight percent, according to data from the European Space Agency’s (ESA) Swarm constellation of satellites.

This weakness has moved to the West at a speed of 12 miles per year. The weakening magnetic field in the region is thought to be a potential signal that the earth’s overall magnetic field is set to reverse—a phenomenon that previously took place around 700,000 years back.