Researchers have used seismic waves to peer deep below the Earth’s surface and study the mysterious structures between the mantle and liquid iron core.
Down here in the depths of the planet, there are places where seismic waves slow to a crawl. Named “ultra-low velocity zones” (ULVZs), these are enigmatic places hundreds of kilometres in diameter and tens of kilometres thick. But we know little about their composition or origin – or even whether they have a single origin or several different sources.
Now, new research has shown that these zones are made from different material than their surroundings – and that they have remained largely unchanged for billions of years, suggesting that they are leftovers from the processes that formed the Earth.
“For a long time, no one really knew for sure what these mysterious ULVZs were made up of,” says Hrvoje Tkalčić from The Australian National University (ANU), who co-authored the study in Nature Geoscience.
“Now, we’ve developed the clearest picture yet. Using advances in seismology and mathematical geophysics made at ANU, we’ve shown that ULVZs are made up of layers.
“Over billions of years of the Earth’s shaping and reshaping, these zones have churned close to the planet’s core, but largely remained intact.
“It’s like an egg in a cake that doesn’t get mixed in with the rest of the ingredients but stays as yoke and egg white, despite the constant mixing all around it.”
The team – led by geophysicist Surya Pachhai from the University of Utah – focused on ULVZs beneath the Coral Sea between Australia and New Zealand. Earthquakes are common in this region, frequently sending seismic waves through the core-mantle boundary where the ULVZs are located, making it an ideal study spot.
But instead of directly measuring seismic waves through nearly 3000 kilometres of crust and mantle, the team used a reverse-engineering approach.
“We can create a model of the Earth that includes ultra-low wave speed reductions, and then run a computer simulation that tells us what the seismic waveforms would look like if that is what the Earth actually looked like,” explains Pachhai.
“Our next step is to compare those predicted recordings with the recordings that we actually have.”
Over hundreds of thousands of model runs, the method produced a mathematically robust model of the interior of the planet.
It showed that these ULVZs likely have layers, which gives us clues into how the Earth formed and evolved.
Back in its infancy, Earth was a hot and violent world. The Solar System itself was still forming, with rocks and planetoids constantly colliding in their orbit around the Sun. Then, around 4 billion years ago, an object the size of Mars is thought to have smashed into the Earth, throwing up debris that could have formed the Moon and raised the temperature of Earth.
“As a result, a large body of molten material, known as a magma ocean, formed,” Pachhai says.
A jumble of rock, gases and crystals would have been suspended in this magma. As it cooled, the denser materials sank down to the bottom of the Earth’s mantle.
As the centuries turned to millennia turned to eons, the mantle churned around and forced these denser bits into small patches, forming ultra-low velocity zones.
But the most surprising finding, according to Pachhai, is that ULVZs are more diverse than previously thought.
“ULVZs are not homogenous but contain strong structural and compositional variations within them,” he explains.
“We found that these types of ULVZs 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.”
There may still be other kinds of ULVZs, too, with different research suggesting that these zones could also be the result of melting ocean crust, sinking back into the mantle.
And so the scientific journey to the centre of the Earth continues.