Given how they are positioned hundreds of kilometres beneath the ground, we often remain oblivious to the gliding and sliding of tectonic plates – the slabs of irregularly shaped rocks whose movements cause natural disasters like earthquakes and tsunamis.
But the devastating earthquakes that shook Turkey and Syria on February 6 and claimed nearly 8000 lives have brought tectonic plates and the stuff beneath them to the spotlight once again.
And a new study – coincidentally published on the same day as when the earthquakes hit Turkey – has revealed a previously unknown layer of partially molten rock beneath Earth’s crust that might help settle a long-standing debate about how tectonic plates move.
The Earth is composed of an outermost crust, the mantle just underneath it, followed by the outer and inner core. Now the area between the lower crust and the upper mantle is where the tectonic magic happens, with about 15 major blocks that move and shift.
But these plates require some sort of lubrication to move, and that is provided by a soft layer of solid but malleable rock – or so we thought.
Junlin Hua, a postdoctoral fellow at UT’s Jackson School of Geosciences, was studying seismic images of Earth’s mantle located beneath Turkey for his doctoral research when he spotted signs of partly molten rock. This was a part of the asthenosphere, which consists of both solid and molten rocks.
But a deep dive into the study made him realise that this was no anomaly and that this layer, present about 161 km below the Earth’s surface, had a broader presence.
Until recently, scientists believed that the molten rocks might be a contributing factor to tectonic movement. However, this study shows that melt, in fact, does not appear to notably influence the flow of mantle rocks.
“When we think about something melting, we intuitively think that the melt must play a big role in the material’s viscosity. But what we found is that even where the melt fraction is quite high, its effect on mantle flow is minor,” said Hua, the study’s lead author.
Convection, or the transfer of heat, occurs in the mantle when hot and less dense material rises while cooler, denser material sinks. And this study’s authors believe that it is the presence of solid rocks and convection that contribute to plate motion.
Further, the molten rock appeared on seismic readings in areas where the asthenosphere reached its highest temperatures, about 1,450 degrees Celsius. And the findings of this study provide 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.
“This work is important because understanding the properties of the asthenosphere and the origins of why it’s weak is fundamental to understanding plate tectonics,” said coauthor Karen Fischer, a seismologist and professor at Brown University.
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