Chemical leftovers from the very earliest days of our planet could still be present near Earth's core, according to new research, and the discovery could improve our understanding of plate tectonics phenomena happening today.
The team behind the study compares these leftovers to clumps of flour at the bottom of a bowl of batter – elements that haven't been properly mixed over the course of billions of years, which show up as anomalies on seismic wave readings.
We know that seismic waves slow to a crawl near the core of the Earth, going through what are known as ultra-low velocity zones (ULVZs). The big question is what are these zones made out of – and scientists now think they might have found the answer.
"This finding changes our view on the origin and dynamics of ultra-low velocity zones," says seismologist Surya Pachhai from the Australian National University.
"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."
The way that seismic waves echo through Earth's mantle and crust give us clues about their composition, but measuring down through around 2,900 kilometers or 1,800 miles of rock isn't easy. To address that, the scientists used a reverse-engineering approach, running hundreds of thousands of computer simulations, using a process known as Bayesian inversion.
By comparing these models to actual readings taken from underneath the Coral Sea between Australia and New Zealand, the team was able to narrow down the possibilities of what the ULVZs just above the liquid metal outer core could be made of.
The researchers suggest that the ULVZs could be partly made of iron oxide – we know it as rust, but it acts as a metal deep in the mantle. It also now looks likely that this section of our planet is made up of several sublayers, something which hasn't been suspected for these zones before.
This layering could well have been caused by a planetary object the size of Mars smashing into the early Earth. The event is thought to have thrown up debris that formed the Moon, and it's also likely to have created an ocean of magma, made up of rock, gases, and crystals, which could have sunk down to its current position over billions of years.
"The physical properties of ultra-low velocity zones are linked to their origin, 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," says Pachhai.
Seismic waves are known to slow by up to a half in ULVZs, with corresponding density going up by a third. It has been suggested that these are partially melted areas of the mantle, providing magma for volcanic hot spots on the surface (such as Iceland).
However, not all of the high-density zones match up with places of frequent volcanic activity, which suggests something else is going on. That inspired the research team to take a closer look – revealing the surprising layers that make up these ULVZs, with the help of computer modeling.
The mantle and the ULVZs at the bottom of it can drive the movement of tectonic plates close to the surface, which means the new research doesn't just teach us more about the birth of the Earth, but also more about how it's behaving today.
"Of all of the features we know about in the deep mantle, ultra-low velocity zones represent what are probably the most extreme," says geologist Michael Thorne from the University of Utah.
"Indeed, these are some of the most extreme features found anywhere in the planet."
The research has been published in Nature Geoscience.