Scientists have discovered a hidden layer of partly molten rock located about 100 miles under Earth’s surface that may resolve long-standing mysteries about the movements of tectonic plates, reports a new study.
Previous studies have revealed hints of this layer in isolated areas, but the new research shows that it extends across a much larger portion of the planet’s subterranean regions than expected. The discovery of this strange zone suggests that melted rock flowing through the upper mantle–the part of Earth that sits right below the surface and crust–may play only a minor role in the shifts of tectonic plates compared to other forces, such as the transfer of heat in this underground realm. It’s important to nail this down, because tectonic plate movement contributed to the flourishing of life on Earth, for example. Better knowledge of this domain could even help us investigate alien worlds with similar dynamics.
We are familiar with Earth’s current map, which includes seven continents and an ocean. However, this is just one face Earth has worn for billions of years. Our planet is tectonically active, meaning that huge plates of rock shift across its surface over time. The global patterns of oceans and landmasses change as the plates smash into one another or sink into the mantle.
For example, the majority of Earth’s land collapsed into Pangaea, a supercontinent formed by dinosaurs just 250 millions years ago. According to certain projections, Asia and North America may collide in another 300million years and create a new landmass ,. These plates help maintain the planet’s habitability and provide a steady climate.
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. Particularly, the details of the asthenosphere’s lower boundary, located about 100 below Earth’s surface have been elusive.
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. According to a study published Monday by Nature Geoscience, this finding will allow for improved models of Earth’s moving components.
“The low-viscosity layer of the mantle that divides the lithosphere and the deeper one, called the asthenosphere (low-viscosity, or mantle) not only allows plate tectonics to take place by accommodating plate movements with respect to deeper mantles, but also helps to maintain the existence of tectonic plates today,” Hua said along with his co-authors in the study.
“While temperature and pressure variations with depth contribute to creating low-velocity and low-viscosity asthenosphere, the distribution and effects of partial melt remain debated,” the team continued. “To fundamentally understand the origins of the low-viscosity asthenosphere, resolving the global distribution of partial melt, both laterally and vertically, is required.”
In other words, scientists are not quite sure how partially melted rocks in the asthenosphere contribute to the flow of tectonic plates above it, in part because there’s still a lot to learn about the abundance and distribution of these gooey rocks in this layer. While one might expect that big patches of melted rock would make the asthenosphere softer, thereby producing an easy glidepath for plates to flow over, the exact relationship between the layers and tectonic motion remains a puzzle.
Hua stumbled across a potential piece of this puzzle while he was assembling a global map of the asthenosphere with the help of seismic waves produced by earthquakes at hundreds of different locations around the globe. The waves interact with different materials within each layer to reveal details about the properties of the material. They travel throughout the Earth’s interior.
While making the map, Hua noticed that the seismic waves slowed down when they hit a hidden layer of melted rock that spans much of the globe at a depth of 150 kilometers, or 93 miles, below the surface. The team called the zone “PVG-150” for “positive velocity gradient at 150 kilometers.”
The researchers then examined whether the presence of the PVG-150 at certain locations had any impact on the tectonic flow in the same areas. Interestingly, they did not find a correlation between the melted rock and the movement of the plates, 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 variations.
“The PVG-150 mantle boundary–documented in this study to be common in high-temperature regions of the mantle on a global basis–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 said in the study.
“However, the lack of variations in seismic anisotropy and deformation that are spatially correlated with the PVG-150 indicates that the effects of variations in melt fraction on mantle viscosity are minimal,” the researchers added. “Our results thus imply that although the presence and distribution of partial melt vary substantially within the asthenosphere, the low viscosity that defines the asthenosphere is controlled primarily by gradual temperature and pressure variations.”
In addition to exposing a new layer of Earth, the study could simplify models of plate tectonics by limiting the influence of melted rock. New research sheds light on the lower, murky layer of the asthenosphere. This could allow scientists to unravel mysteries about how plate tectonics developed on Earth and on other planets.
Considering that these moving parts have helped to enable life on Earth, understanding them fully will be an essential part of our search for extraterrestrial life elsewhere in the universe.
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