100 miles below the surface of the Earth, scientists find the Hidden Zone

 By Becky Ferreira

February 07, 2023

According to a new study, scientists have found a buried layer of partially molten rock roughly 100 miles beneath the surface of the Earth. This discovery could help to explain long-standing puzzles regarding the motions of tectonic plates.

The layer has previously been suggested by isolated investigations, but the most recent work demonstrates that it covers a significantly broader area of the planet's subsurface regions than previously thought. The finding of this odd zone shows that other forces, such as the transfer of heat in this underground world, may be more important in the movement of tectonic plates than the melted rock flowing through the upper mantle, the section of Earth that is directly beneath the surface and crust. It's crucial to establish this because, for instance, tectonic plate movement facilitated the emergence of life on Earth. We could even be able to examine other worlds with comparable dynamics with better knowledge of this field.

The current map of the earth, which features seven continents and one ocean, is one of several that Earth has had over the course of billions of years. Huge rock plates constantly move across the surface of our planet because it is tectonically active. As a result, as these plates spread, collide, or are driven deeper into the mantle, the global pattern of landmasses and seas changes.

For instance, the majority of Earth's surface was compressed into the supercontinent Pangaea when dinosaurs were just beginning to appear 250 million years ago. According to some predictions, Asia and North America may merge and create a new landmass in 300 million years. These shifting plates aid our planet's habitability by helping to maintain a stable climate in addition to keeping it looking good.

There are many unanswered concerns regarding the precise dynamics of this crucial process, which involves tectonic plates drifting over the asthenosphere, a region of the upper mantle of the Earth. Particularly, information regarding the asthenosphere's lower border, which is located roughly 100 miles below the planet's surface, has been difficult to come by.

Now, a team of researchers led by Junlin Hua, a postdoctoral fellow in geosciences at the University of Texas at Austin, has found what looks to be a secret layer of soft rock at the base of the asthenosphere that covers at least 44 percent of the planet and possibly more. This partially melted zone "has no appreciable effect on the large-scale viscosity of the asthenosphere" in spite of this vast range, indicating that it probably has little impact on plate tectonics. According to a study that appeared on Monday in Nature Geoscience, this discovery will aid in the improvement of models of the Earth's moving parts.

"The asthenosphere, the low-viscosity mantle layer that separates the comparatively rigid lithosphere from the deeper mantle, may also contribute to stabilising the very existence of tectonic plates in their current form," Hua and his colleagues wrote in their paper.

Although low-velocity, low-viscosity asthenospheres are produced by temperature and pressure fluctuations with depth, the researchers said that the distribution and consequences of partial melt were still up for dispute. "Resolving the global distribution of partial melt, both laterally and vertically, is necessary to properly comprehend the genesis of the low-viscosity asthenosphere."

In other words, scientists aren't sure how partially melted rocks in the asthenosphere affect the movement of tectonic plates above them, partly because there's still a lot to learn about the abundance and distribution of these gooey rocks in this layer. While one might expect large patches of melted rock to soften the asthenosphere, creating an easy glidepath for plates to flow over, the precise relationship between the layers and tectonic motion remains a mystery.

Hua discovered a possible piece of this puzzle while assembling a global map of the asthenosphere using seismic waves produced by earthquakes in hundreds of different locations around the world. These waves travel through the earth's interior, interacting with the various materials in each layer, revealing information about their properties.

Hua noticed that the seismic waves slowed when they hit a hidden layer of melted rock that spans much of the globe at a depth of 150 kilometres (93 miles) below the surface while creating the map. The zone was designated as "PVG-150," which stands for "positive velocity gradient at 150 kilometres."

The researchers then investigated whether the presence of the PVG-150 at specific locations influenced tectonic flow in the same areas. Interestingly, they found no correlation between the melted rock and plate movement, implying that the presence of these rocks is not as important to tectonic flow as other asthenosphere forces such as temperature and pressure variations.

"The PVG-150 mantle boundary is best explained as the base of a layer within the asthenosphere in which the presence of partial melt significantly reduces seismic velocities," the team wrote in the study.

"However, the lack of spatially correlated variations in seismic anisotropy and deformation with the PVG-150 indicates that the effects of variations in melt fraction on mantle viscosity are minimal," the researchers added. "Our findings suggest that, while the presence and distribution of partial melt vary significantly within the asthenosphere, the low viscosity that defines the asthenosphere is primarily controlled by gradual temperature and pressure variations."

In addition to revealing a new layer of Earth, the research could simplify plate tectonic models by limiting the influence of melted rock. The new study also sheds new light on the murky lower layer of the asthenosphere, which could aid scientists in unravelling the mysteries of how plate tectonics evolved on Earth and how common they might be on other worlds.

Given that these moving parts have aided in the evolution of life on Earth, fully comprehending them will be critical in our search for extraterrestrial life elsewhere in the universe.