The earliest chemicals formed on Earth might have migrated down to the lowest reaches of the mantle, creating an ultra-slow velocity zone around Earth's core.
Obviously, none of us are going to go down there and be affected by this zone, but seismic waves are, according to researchers at the University of Utah. In a new study published in the journal Nature Geoscience, the researchers describe a surprisingly layered region around the Earth's core that is slowing seismic waves down.
"Of all of the features we know about in the deep mantle, ultra-low velocity zones represent what are probably the most extreme," said Michael S. Thorne, associate professor in the University of Utah's Department of Geology and Geophysics, in a statement. "Indeed, these are some of the most extreme features found anywhere in the planet."
Between the thin outer crust and the iron-nickel core is Earth's incredibly hot but solid mantle. This mantle can move over time and is the driving force behind the planet's plate tectonics. It's also impossible to see, so how do we know what's there?
For this, researchers use seismic waves generated by earthquakes to measure when and where these waves are detected at various monitoring stations around the world.
Since the physical waves from an earthquake travel at different speeds through different materials and can also be distorted by certain materials as well, when and how we detect these waves tells us about the material it passes through inside the planet.
Using these waves, researchers have found that along the lowest part of the mantle, there is a region where these seismic waves slow by as much as 50%, and where the density is three times higher than the surrounding mantle.
Originally, researchers thought the mantle might be partially melted in this region—it is right up against the liquid outer core, after all—and that it might be where the magma that produces tectonic "hot spots" responsible for creating the Hawaiian Islands and Iceland, but that does not appear to be the case.
"But most of the things we call ultra-low velocity zones don't appear to be located beneath hot spot volcanoes," Thorne said, "so that cannot be the whole story."
The alternative hypothesis, that the ultra-slow velocity zones have a different composition from the rest of the mantle, makes more sense given the data.
"The physical properties of ultra-low velocity zones are linked to their origin," said postdoctoral scholar Surya Pachhai, "which in turn provides important information about the thermal and chemical status, evolution and dynamics of Earth's lowermost mantle—an essential part of mantle convection that drives plate tectonics."
A large part of that origin might be the hypothetical collision between Earth and a Mars-sized proto-planet named Theia around 4.5 billion years ago. The collision, which is believed to have carved out a portion of Earth's mantle and formed the Moon, would have generated incredible heat and created a vast ocean of magma across the Earth's surface.
This magma ocean would have had all kinds of minerals suspended in it, and the convection forces would have carried a lot of them down into the mantle, where over time they would have settled at the bottom. Over 4 billion years of convection processes, this uniform layer of material would have been pushed into smaller, layered patches, producing the low-velocity regions turned up in the seismic data.
"So the primary and most surprising finding is that the ultra-low velocity zones are not homogenous but contain strong heterogeneities (structural and compositional variations) within them," Pachhai said.
"This finding changes our view on the origin and dynamics of ultra-low velocity zones. We found that this type of ultra-low velocity zone can be explained by chemical heterogeneities created at the very beginning of the Earth's history and that they are still not well mixed after 4.5 billion years of mantle convection."