Chemicals near the center of Earth reveal the planet’s past

On a journey towards the center of the Earth, seismic waves become very slow near the core. Scientists descriptively call these areas the “ultra low velocity zones.”

In a new study from the University of Utah, researchers have discovered that these ultra low velocity zones contain layers which may provide insights into the early Earth. In fact, these zones may be leftovers from the processes that formed our planet. 

“Of all of the features we know about in the deep mantle, ultra-low velocity zones represent what are probably the most extreme,” said Professor Michael S. Thorne. “Indeed, these are some of the most extreme features found anywhere in the planet.”

Initially, scientists thought that perhaps the ultra low velocity zones might be partially melted mantle, and the source of volcanic hotspots. 

However, this cannot be completely true, said Professor Thorne. He explained that most of the things we call ultra-low velocity zones do not appear to be located beneath hotspot volcanoes.

In collaboration with post-doctoral researcher Surya Pachhai and other colleagues, Professor Thorne explored an alternative hypothesis – that the low velocity zones are made of different rocks, rocks with composition dating from early Earth. 

“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,” explained Pachhai.

The researchers looked at the Earth’s deep layers by using imaging from seismic waves. However, this method can be imprecise and hard to use. So, the scientists used a modeling approach with a Bayesian Inversion method. 

“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,” said Pachhai..”Our next step is to compare those predicted recordings with the recordings that we actually have.”

The modeling method found that it’s likely that the ultra low velocity zones are layered and that some of the layers do represent the Earth’s origins, perhaps an interesting piece of the puzzle of our planet’s past.  

The study is published in the journal Nature Geoscience.

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By Zach Fitzner, Earth.com Staff Writer