What scientists know about our planet is mostly superficial, giving us only the vaguest understanding of how geological forces cause the fractured crust to bump and rub against itself.
Just recently, researchers discovered something new about the layer of partially molten rock that lies just beneath Earth’s cold outer skin, and it could help us better understand the mechanisms behind the turbulent currents deep beneath our feet.
This flexible band of hot material is known as the asthenosphereand it is generally considered to be mostly solid, with some liquid material weakening the overall structure.
However, its top layer appears to be softer than scientists once thought.
A study led by University of Texas researchers has identified distinct properties in the flow and density of a thin section of the asthenosphere, and resolved the boundaries to zones within the layer that could extend around the world.
Having a clear map of the differences in the echoes of seismic waves passing through the bowels of the Earth could help us pinpoint the activities driving the movements of the tectonic plates floating on the planet’s surface.
The discovery adds important detail to the global structure of the uppermost layers of the mantle, allowing geologists to rule out any influence that this particular soft zone of the upper mantle might have on the overall movement of the asthenosphere.
“We cannot rule out that local melting does not play a role,” admits Geophysicist Thorsten Becker from the University of Texas.
“But I think it drives us to see these observations of the melt as an indication of what’s going on on Earth, and not necessarily as an active contributor to anything.”
In practice, Becker says that’s one less variable to worry about in future models of Earth’s interior.
While some previous studies have suggested that the asthenosphere is punctuated by occasional bursts of molten activity, it wasn’t clear until recently how widespread this phenomenon might be.
For the new study, Becker and his colleagues created a global map of the asthenosphere using seismic images of the mantle collected from stations around the world.
When the seismic waves sent from these surface stations hit the upper part of the asthenosphere, they slowed down significantly, suggesting that the top layer is more molten than other parts.
Material with greater fluidity usually allows for greater flow, but that doesn’t necessarily appear to be the case here.
The map of the asthenosphere that scientists have compiled doesn’t really match the movement of the tectonic plates above it. For example, areas where seismic waves move more slowly do not show greater tectonic activity.
“When we think of something melting, we intuitively think that the melt must play a big part in the viscosity of the material,” says Junlin Hua, who led the research.
“But what we found is that even where the melt fraction is quite high, its effect on sheath flow is very small.”
Curiously, there appear to be multiple bands of molten material distributed throughout the asthenosphere, and not just at the top, where hot magma collects at a depth of about 100 to 150 kilometers (about 60 to 90 miles).
For example, a band of molten material usually appears at the bottom of the asthenosphere, possibly as a result of dehydration melting that can occur when rock is undersaturated with water.
A middle layer about 260 kilometers deep, on the other hand, is not as widespread but occurs sporadically and could be the result of carbon-assisted mantle melt.
scientists have long suspected that the Earth’s tectonic plates move based on the flows of molten rock that lie deep below the surface, but the precise dynamics of the rise and fall of gas, liquid, or rock is unclear.
Based on the current results, UT researchers suspect that gradual temperature and pressure fluctuations in the asthenosphere drive the deep flow of semi-molten rock. The overall viscosity of this region isn’t nearly as important when it comes to moving the tectonic plates across it.
“This work is important because understanding the properties of the asthenosphere and the causes of its weakness is fundamental to understanding plate tectonics,” says Seismologist Karen Fischer, who now works at Brown University.
The study was published in nature geosciences.
