Using seismic waves, scientists detect widespread partially melted rock hidden under the Earth’s tectonic plates in a new way, a step in settling a long-held geological debate that has big implications in understanding plate motion.
One of the most important questions in geoscience revolves around when the subterranean interaction of tectonic plates started, helping to set the stage for life on Earth and shaping the planet’s surface as it’s known today.
An equally important question is: “What makes plate tectonics possible to begin with?” A new study from researchers at Brown University, the University of Texas at Austin and Cornell University offers fresh insights into that question by adding some of the sharpest evidence yet that hidden beneath the Earth’s tectonic plates is partially molten rock.
The research, led by Brown Ph.D. graduate Junlin Hua, shows that at about 100 miles beneath the Earth’s surface, a layer of partially melted rock appears whenever temperatures exceed about 2,640 degrees Fahrenheit in parts of an interior portion of the planet known as the asthenosphere.
The researchers say the new findings mark the first time this partially melted rock has been detected at this scale in the asthenosphere, a layer of Earth that is made of mostly solid but malleable rock that the planet’s tectonic plates rest on and glide over as they shift. The findings also show, rather surprisingly and counterintuitively, that this layer of partly melted rock doesn’t make the asthenosphere any softer or easier for tectonic plates to traverse, suggesting it plays little role in enabling plate tectonics.
“When we think about something melting, we intuitively think that the melt must play a big role in the material’s viscosity,” said Hua, who is now a postdoctoral fellow at U.T. Austin. “But what we found is that even where the melt fraction is quite high, its effect on mantle flow is very minor.”
The work is described in a new report in Nature Geoscience and is an important step toward resolving the long-held debate among geoscientists on whether the asthenosphere contains extensive patches of molten rock and what role this melt plays or doesn’t play in plate motion.
“This study is fundamental to understanding why the asthenosphere — the weak mantle layer below the tectonic plates that enables the plates to move — is in fact weak,” said Karen M. Fischer, a geophysics professor at Brown and an author of the study. “Ultimately, it provides evidence that other factors such as temperature and pressure variations can control the strength of the asthenosphere and make it weak enough for plate tectonics to be possible.”
Work on the study began in 2020 when Hua was a graduate student in Fischer’s lab at Brown. While there, Hua used seismic images of the Earth’s interior from more than 700 locations around the world to put together a global map of the asthenosphere.
The data let him take a close look at seismic waves that move through the body of the Earth. As these waves move through the different materials and compositions in the Earth, they change speed, direction and in some cases even how they vibrate, coming to the station’s seismic detectors at different times and as different signals. In regions where there is melt, for instance, seismic waves move slower and convert to a different type of wave.
Hua was able to analyze the changes and calculate the properties of the asthenosphere based on how the waves moved through the Earth. The calculations showed that small percentages of molten rock — about 1% or less — were present in a layer of the asthenosphere and only appeared where the asthenosphere was the hottest.
Hua then compared the map with seismic measurements of tectonic movement and calculated that while the partial melt makes the seismic waves slower, it doesn’t influence the rock in the long-term. Basically, it told the researchers that while the partial melt makes the asthenosphere slightly weaker in the moment the waves pass by, it doesn’t make it weaker at the much slower rates at which the plates move.
“What this shows us is that the melt is making the rocks in the asthenosphere weaker on the timescales of seismic waves — which are tens of seconds, more or less — but it’s not making the rocks weaker at the timescales where you will have big enough plate motion to produce a lot of strain in the asthenosphere,” Fischer said. “It’s making the waves move more slowly, but it’s not enabling the plates to move faster.”
By eliminating one major possibility, the research team hopes to use the new findings to continue to understand how the process of plate tectonics works and to probe what makes it possible in the first place.
The research was supported by funding from the National Science Foundation.
This story was originally published by Brown University on February 6, 2023.