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A series of strange underground formations, deep below the earth’s crust and close to the core of our planet, have piqued the curiosity of many geologists around the world. Now, a team of researchers from universities National of Australia, Utah, Arizona and Calgary he has studied them thoroughly with seismic waves. The results of his research are published in ‘
Nature Geoscience‘.
Down there, thousands of km deep below the planet’s surface, there are places where the speed of seismic waves is drastically reduced. Known as’ultra low speed zones‘(ULVZ), it is about enigmatic masses of rock denser than the rest. Masses that can be hundreds of km long and tens of km thick.
And very little is known about its composition and origin. Are they part of our planet from the beginning or are they fragments of alien worlds that collided with ours?
The new research has managed to shed some light on the mystery and show that these large areas are made of a completely different material from their environment, and that they have remained virtually unchanged for billions of years, suggesting that it could These are buried remains of the primitive Earth, fragments that come directly from the process of formation of our planet.
“For a long time,” he explains. Hrvoje Tkalčić, from the Australian National University (ANU) and co-author of the study, no one knew for sure what these mysterious ULVZ were composed of. Now, we have developed the clearest image yet. Using advances in seismology and mathematical geophysics made at ANU, we have shown that ULVZs are made up of layers. During billions of years of Earth’s formation and reshaping, these zones have been churned near the Earth’s core, but largely remained intact. It is like an egg in a cake, which does not mix with the rest of the ingredients and remains as a whole, with its yolk and its white, despite the constant mixing around it ».
Reverse engineering
The team, led by the geophysicist Surya Pachhai, from the University of Utah, focused on the ULVZs located under the Coral Sea, between Australia and New Zealand. Earthquakes are common in that region, often sending seismic waves across the core-mantle boundary where the structures are located, making it an ideal place to study them.
However, instead of directly measuring seismic waves through nearly 3,000 kilometers of crust and mantle, the team of researchers used a reverse engineering approach this time.
“We created a model of the Earth with ultra-low wave velocity reductions and then we ran a computer simulation that would tell us what seismic waveforms would look like if the Earth were actually like this,” Pachhai explains. Our next step was to compare those predictions with the actual recordings we have. ‘
Over hundreds of thousands of runs of the simulation, the method eventually produced a mathematically robust model of the planet’s interior, showing that ULVZs are likely to be layered. Which gives us clues about how the Earth formed and evolved.
A hot and violent place
In its infancy, Earth was a hot and violent world. The Solar System itself was still forming, with rocks and planetoids constantly colliding with each other as they orbited the Sun. Later, a few years ago 4 billion years, an object the size of Mars, known to scientists as Teia, crashed into Earth, throwing debris into space that ended up forming the Moon. The tremendous impact also turned a good part of the Earth’s surface into magma, raising the temperature greatly. “As a result,” Pachha says, “a large mass of molten material was formed, known as the magma ocean.”
A jumble of rocks, gases and crystals was suspended in the middle of this magma. And as it cooled, the denser materials sank deep into the Earth’s mantle. Time passed, centuries turned into millennia and then into eons. The mantle churned, forcing these denser fragments into small patches, forming ultra-low velocity zones.
Unexpected diversity
But the most surprising finding, according to Pachhai, is that ULVZs are more diverse than previously thought. “In fact,” he explains, “ULVZs are not homogeneous, but contain strong structural and compositional variations within them. We found that these types of ULVZ can be explained by chemical heterogeneities created at the beginning of Earth’s history and that even today, after 4.5 billion years of mantle convection, they are not well mixed.
According to the study, other types of ULVZ could also exist, for example as a result of the melting of the oceanic crust, sinking back into the mantle. But that will be the subject of further research.
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