Earth’s Mantle Contains Melted Rock Beneath 44 Percent of the Surface
Global seismic analysis reveals a partial melt layer in the Earth’s mantle that sits beneath roughly 44 percent of the planet’s surface. This newly detected layer does not significantly weaken long-term tectonic plate movement, separating the physical presence of underground magma from the primary forces that drive planetary flow. Researchers from Brown University, the University of Texas at Austin, and Cornell University published these findings in Nature Geoscience, demonstrating that temperature and pressure variations remain the main drivers of mantle weakness rather than widespread liquid rock.
Seismic Waves Reveal Hidden Melt Layers at 150-Kilometer Depths
Junlin Hua initiated the study as a doctoral student at Brown University and led the project as a postdoctoral researcher at the University of Texas. The team analyzed data collected from 716 permanent seismic stations worldwide. After quality checks, researchers isolated 184,340 receiver function measurements that recorded how S-waves convert into P-waves when encountering underground material boundaries. These conversions highlighted a clear signal at approximately 150 kilometers below the surface, marking an increase in seismic velocity beneath a slower layer. The low-velocity layer has an average thickness of about 65 kilometers where detected.
Researchers Map Continental and Oceanic Melt Distribution Across the Globe
The seismic boundary does not appear everywhere beneath the Earth’s crust. Researchers found strong evidence at 160 stations and weaker signals at 240 others, indicating a patchy distribution of partial melt rather than a continuous global shell. This boundary occurs frequently beneath both oceans and continents, appearing well beyond active volcanic hotspots. By comparing basalt samples erupted worldwide, the team matched the estimated temperatures and depths of original magma formation with the depths of the seismic boundaries. Extrapolating these station observations suggests the melt-bearing layer underlies roughly 44 percent of the Earth’s surface.
Long-Term Viscosity Remains Unchanged Despite Slow Seismic Waves
While the underground melt slows down earthquake waves passing through in seconds, it does not reduce the long-term viscosity of the surrounding rock. Researchers tested this mechanical impact by examining seismic anisotropy, which measures how wave speeds vary with direction due to mineral crystal alignment during deformation. Mantle flow models showed that if the melt created a heavily weakened layer, the directional wave patterns would change significantly. The observed data failed to show this expected difference, proving that the melt slows down seismic pulses without controlling the slow, multi-million-year creep of tectonic plates.
Frequently Asked Questions About Mantle Melt and Tectonic Plates
How thick is the newly mapped low-velocity layer in the Earth’s mantle?
The layer of melted rock has an average thickness of about 65 kilometers where detected by global seismic station measurements.
What percentage of the Earth’s surface sits above this mantle melt?
Researchers estimate that the detected boundary layer rests beneath approximately 44 percent of the planet’s total surface area, primarily based on model extrapolations from 716 seismic stations.
Does the presence of underground melt cause tectonic plates to move faster?
No, the partial melt slows down seismic waves over short timeframes, but it does not act as the primary mechanical driver for long-term plate movement or mantle flow.
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