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.
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We are all familiar with the current world map with seven continents and a global ocean, but this is only one of many faces that Earth has worn over billions of years. Our planet is tectonically active, meaning that huge plates of rock shift across its surface over time. As a result, the global pattern of landmasses and oceans changes as these plates spread, smash into each other, or get pushed down 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 boundary of the asthenosphere, which sits about 100 miles below the planet’s surface, have remained 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. This finding will help refine models of Earth’s moving parts, according to a study published on Monday in Nature Geoscience.
“We were amazed by how well the observation…matches our intuition,” Hua said in an email to Motherboard. “I was also a little surprised by how widespread it is.”
“Another surprising finding is the second part of the paper that we found the extra melt does not affect the plate motion…very much, since intuitively one may feel something melting (like a partly melted chocolate) would be easier to deform,” he continued. In this way, “the physics of solid rock deformation would likely be what defines the relatively weak asthenosphere, instead of the presence of melting.”
To that point, scientists have long debated 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. These waves travel through the interior of Earth and interact with the various materials in each layer, revealing details about their properties.
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.”
“I was studying Turkey at that time, and found a seismic signal that marks the lower reach of a seismic low-velocity layer there,” Hua explained. “People have been talking about the upper boundary of the low-velocity layer a lot, but the lower reach is less mentioned, and I was surprised by that clear lower reach in Turkey.”
“So I decided to study it across the globe to see how prevalent such lower reach is, and we found in this study that it is actually pretty widespread, and the lower reach just corresponds to the bottom of the layer with partially molten rocks,” he added.
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 major finding of the paper is divided in two parts,” Hua said. “The asthenosphere that lies beneath the lithosphere and enables the present-day plate motions can be generally categorized in two types: about half of them are hot enough, so in addition to solid rock, some rock are melted above 150 kilometers; while the other half are mostly pure solid.”
“Though being partially molten, counter-intuitively, those melt does not significantly influence how plate tectonics is expressed, which means the physics of solid rock deformation is still what controls the plate motion,” he noted.
In addition to exposing a new layer of Earth, the study could simplify models of plate tectonics by limiting the influence of melted rock. The new research also helps to shed new light on the murky lower layer of the asthenosphere, which could help scientists unravel the mysteries of how plate tectonics emerged on our planet, and how common they might be on other worlds.
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. In the meantime, however, Hua and his colleagues hope to continue probing the mysterious layers of the asthenosphere with seismic data captured in more remote regions, such as the oceans.
“One future direction would be better sampling oceanic regions,” Hua concluded. “In this study, we are mainly using seismic instruments on continents, and though we have also used some instruments from ocean islands, there are certainly some degrees of data gap in the ocean. Hence, a nice follow-up study would be using other types of data or seismic instruments located on ocean bottoms to bridge this gap.”
Update: This article has been updated to include comments from lead author Junlin Hua.
