What scientists know about Earth, still largely confined to the planet’s skin and surface, gives us a barest understanding of how geological forces cause the fractured crust to collide and rub against itself.
Recently, researchers discovered something new about the layer of partially molten rock that sits just beneath Earth’s cold outer shell. This finding could help us better understand the mechanisms behind the swirling currents far beneath our feet.
This flexible layer of hot material is known as the asthenosphere, and is generally considered to be mostly solid, with some liquid material weakening the overall structure. The top layer, however, appears to be softer than scientists had suspected.
A study led by researchers at the University of Texas has identified different qualities in the flow and density of a thin section of the asthenosphere, thus resolving a boundary zone within the layer that could extend worldwide.
Having a clear map of the different echoes of seismic waves passing through the bowels of the earth can help us understand the activities driving the movement of the tectonic plates floating on the planet’s surface.
The discovery adds important details to the global structure of Earth’s uppermost mantle, allowing geologists to exclude any influence this upper mantle soft zone may have on the entire asthenospheric churn.
“We cannot rule out that local melting is not a problem,” admits geophysicist Thorsten Becker of the University of Texas, quoted from Science Alert, Tuesday (7/2/2023).
“But I think it encourages us to look at these observations of melting as a marker of what’s happening on Earth, and not necessarily an active contribution to anything.”
Practically speaking, Becker says that there is one variable to worry about in future models of the Earth’s bowels.
Several previous studies have suggested the asthenosphere is disrupted by occasional bursts of liquid activity. Meanwhile, in this study published in Nature Geoscience, so far it is not known how widespread this phenomenon is.
Becker and his colleagues created a global map of the asthenosphere using mantle seismic imagery, collected from seismic imaging stations around the world.
When seismic waves sent from these above-ground stations hit the top of the asthenosphere, they slow down significantly, indicating that the upper layer is more fluid than the rest.
Materials with greater fluidity usually allow greater flow, but this does not appear to be the case here.
The asthenosphere maps compiled by scientists don’t really line up with the movement of the tectonic plates above. For example, areas where seismic waves move more slowly do not show greater tectonic activity.
“When we think about something melting, we intuitively think that melting must play a big role in the viscosity of the material,” said Junlin Hua, who led the research.
“But what we found is that even though the melt fraction is quite high, the effect on mantle flow is very small,” he said.
Surprisingly, there appears to be some liquid material scattered throughout the asthenosphere and not just at the summit, where hot magma tends to pool at depths of around 100 to 150 kilometers.
At the bottom of the asthenosphere, for example, there is usually a collection of molten material, possibly due to dehydration melting, which can occur when rock is not saturated with water.
In contrast, a middle layer about 260 kilometers deep is not widely distributed but appears sporadically and may be the result of carbon-assisted mantle melting.
Scientists have long suspected that Earth’s tectonic plates move based on currents of molten rock that lie deep beneath the surface, but the precise dynamics of the rising and falling of gases, liquids, or rock has been unclear.
Based on the current results, researchers from the University of Texas suspect that gradual temperature and pressure variations in the asthenosphere are what drive the flow in semi-molten rock. The overall viscosity of this region is not a big factor in terms of the movement of the tectonic plates across it.
“This work is important because understanding the properties of the asthenosphere and its weak origins is fundamental to understanding plate tectonics,” said seismologist Karen Fischer, who now works at Brown University.
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