Scientists have discovered a hidden layer of partially molten rock, located at a depth of about 150 km below the Earth’s surface, which may solve ancient mysteries about the movement of tectonic plates. This was evidenced by the results of a new study.

Previous studies have found hints of this layer in isolated areas, but the new study shows that it extends through much more of the planet’s underground regions than expected.

The discovery of this zone suggests that the molten rock that flows through the upper mantle (the part of the Earth that lies just below the surface and crust) may play only a minor role in the shifting of tectonic plates compared to other forces, such as heat transfer in this underground region. It is important to understand this, because the movement of tectonic plates contributed, for example, to the flourishing of life on Earth. Better knowledge of this area may even help to explore alien worlds with similar dynamics.

We are all familiar with the modern world map with the seven continents and the world ocean, but this is only one of the many faces that the Earth has changed over billions of years. Our planet is tectonically active, which means that huge slabs of rock move over its surface over time. As a result, the global pattern of land and oceans changes as these plates move apart, collide with each other, or sink into the mantle.

For instance, when dinosaurs were just starting to emerge 250 million years ago, most of Earth’s land was smushed together into the supercontinent Pangaea. In another 300 million years, Asia and North America may collide and form a new landmass, according to some projections. In addition to keeping Earth looking fresh, these moving plates support the habitability of our world by helping to maintain a stable climate, among other benefits.

Tectonic plates drift over a region of Earth’s upper mantle called the asthenosphere, but there are many open questions about the exact dynamics of this critical process. In particular, details about the lower limit of the asthenosphere, which is located at a depth of about 150 km below the surface of the planet, remained unknown.

Now, scientists led by Junlin Hua, a postdoctoral fellow in geosciences at the University of Texas, Austin, have discovered a hidden layer of soft rock at the bottom of the asthenosphere that appears to extend across at least 44 percent of the planet, and perhaps more. Despite this enormous range, this partially melted zone “has no substantial effect on the large-scale viscosity of the asthenosphere,” meaning it probably does not play a major part in plate tectonics. This finding will help refine models of Earth’s moving parts.

While mapping, Hua noticed that seismic waves slow down when they enter a hidden layer of molten rock that covers most of the globe 150 kilometers below the surface. The team named this zone “PVG-150,” which stands for “positive velocity gradient at a depth of 150 kilometers.”

The researchers then examined whether the presence of PVG-150 in certain locations had any effect on the tectonic flow in those same areas. Interestingly, they found no correlation between molten rock and plate motion, suggesting that the presence of these rocks is not as important to tectonic flow as other forces in the asthenosphere, such as temperature and pressure fluctuations.

In addition to discovering a new layer of the Earth, the research could simplify models of plate tectonics by limiting the influence of molten rock. The new research also helps shed new light on the lower layer of the asthenosphere, which could help scientists unravel the mysteries of how plate tectonics originated on our planet, and how widespread it may be on other worlds.