The highest quality images of the Earth's interior have just been captured

The new technique could help scientists peer into the very depths of the Earth.
Christopher McFadden
An illustration of Earth's coreRost-9D/iStock

A joint research project from the UK has recently published a study into one of the least known or understood, parts of the Earth's interior - the core-mantle boundary. Focusing their work on a large mantle plume underneath the Hawaiian archipelago, they have made some interesting observations about the most enigmatic parts of the Earth's geological system. 

The study was first published in the journal Nature Communications

Using new imaging techniques, the team was able to gain some valuable insight into this ultra-low velocity zone that lies around 1,864 miles (3,000km) below the Earth's surface.

Until now, we've known this area exists from analyzing seismic waves that flow through the planet. The name for the zone(s) comes from the way seismic waves slow right down as they pass through them.

So far, it has been difficult to make much more sense of them beyond some grainy and hard to analyze images. However, this new study of the mantle below Hawaii has produced some much clearer and more high-definition images. 

"Of all Earth's deep interior features, these are the most fascinating and complex," says geophysicist Zhi Li, from the University of Cambridge in the UK and a contributor to the study.

"We've now got the first solid evidence to show their internal structure – it's a real milestone in deep Earth seismology," he added.

To create the images, the team developed new computational models that take the high-frequency signals from the study area to generate a comprehensible image. Using this technique is was able to produce a kilometer-scale look at the rock pocket, at resolutions magnitudes better than using conventional techniques.

It is now hoped that this technique can be used to study the boundary between the Earth's iron-nickel core and surrounding mantle to better understand one of the major engines for plate tectonics, volcano formation, and other related processes like earthquakes. 

Currently, it is believed that extra iron in these unusual zones might be creating the additional density that shows up on seismic wave patterns. Whether correct or not, the study of this region is a top priority for some geologists.

"It's possible that this iron-rich material is a remnant of ancient rocks from Earth's early history or even that iron might be leaking from the core by an unknown means," says seismologist Sanne Cottaar, from the University of Cambridge.

A possible link between ultra-low velocity zones and volcanic hotspots

Other scientists also believe there is a link between ultra-low velocity zones and volcanic hotspots, such as those in Hawaii and Iceland. One hypothesis is that these hotspots might be caused by material shooting up from the core to the surface called "mantle hot spots.

This new technique could help revolutionize this field of study too. Yet others can now better focus on the effusions of lava that sit above these hot spots to look for evidence of so-called "core leaking.". 

While the use of ultra-low velocity zone seismic data is limited in some respects by where earthquakes occur and where seismographs are installed, the team is very much keen to apply their high-resolution imagery enhancements to other deep pockets of Earth.

"We are really pushing the limits of modern high-performance computing for elastodynamic simulations, taking advantage of wave symmetries unnoticed or unused before," says data scientist Kuangdai Leng, from the University of Oxford in the UK.

"The lowermost mantle right above the core-mantle boundary is highly heterogeneous containing multiple poorly understood seismic features. The smallest but most extreme heterogeneities yet observed are ‘Ultra-Low Velocity Zones’ (ULVZ). We exploit seismic shear waves that diffract along the core-mantle boundary to provide new insight into these enigmatic structures. We measure a rare core-diffracted signal refracted by a ULVZ at the base of the Hawaiian mantle plume at unprecedentedly high frequencies. This signal shows remarkably longer time delays at higher compared to lower frequencies, indicating a pronounced internal variability inside the ULVZ. Utilizing the latest computational advances in 3D waveform modeling, here we show that we are able to model this high-frequency signal and constrain high-resolution ULVZ structure on the scale of kilometers, for the first time. This new observation suggests a chemically distinct ULVZ with increasing iron content towards the core-mantle boundary, which has implications for Earth’s early evolutionary history and core-mantle interaction."

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