Global seismic analysis reveals a layer of partially melted rock beneath approximately 44% of Earth’s surface, yet this molten material contributes very little to long-term tectonic plate movement. Researchers from Brown University, the University of Texas at Austin, and Cornell University published findings in Nature Geoscience showing that while partial melt explains slow seismic waves, it does not drastically weaken the mantle’s viscosity over geological time scales.
Seismic Evidence From 716 Stations Worldwide
To map what lies beneath the crust, the research team gathered data from 716 permanent seismic stations across the globe. After rigorous quality checks, they analyzed 184,340 receiver functions—specialized measurements isolating the signals generated when seismic waves encounter shifts in Earth’s interior composition. The approach examined S-waves converting into P-waves at underground boundaries. These wave conversions carry distinct data regarding changes in material properties, allowing researchers to estimate depths and wave speed shifts.
Clear signals emerged at roughly 150 kilometers below many of the observation stations, marking an increase in seismic velocity with depth. Where the boundaries appeared, the low-velocity layer measured an average thickness of about 65 kilometers. The team found strong evidence across 160 stations and weaker signs at 240 others, pointing to widespread partial melt rather than a continuous molten shell encircling the entire planet.
Molten rock alters seismic responses without driving plate motion
While slower seismic waves indicate molten rock, they do not dictate faster plate motion. Researchers tested whether melting creates a heavily weakened layer by comparing seismic anisotropy—the variation of wave speeds with direction—across regions with and without the detected boundary.
Earthquake waves probe material properties in a matter of seconds, whereas tectonic plate movement involves cumulative deformation over millions of years. Consequently, the melt alters rapid seismic responses without driving the slow flow of the asthenosphere. Gradual variations in temperature and pressure remain the primary factors controlling mantle weakness, according to the study’s conclusions.
Melt boundary underlies nearly half of Earth surface
What is the asthenosphere and where does it sit?
The asthenosphere is a weaker region of Earth’s mantle situated directly beneath the lithosphere, which consists of the rigid crust and uppermost mantle. It accommodates the slow movement of tectonic plates over geological time scales.
How much of Earth’s surface sits above this newly mapped melt boundary?
By extrapolating data from seismic stations to areas with comparable mantle speeds, researchers estimate that the detected boundary may underlie approximately 44% of Earth’s surface, though ocean coverage remains limited in the dataset.
Who led the international research team?
Junlin Hua initiated the research as a doctoral student at Brown University and led the project as a postdoctoral researcher at the University of Texas at Austin, working alongside colleagues from Cornell University.
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