High Resolution Imaging Reveals Puzzling Features Deep in Earth’s Interior

The first detailed 'image' of an unique pocket of rock at the boundary layer with Earth's core, some three thousand kilometers beneath the surface, has been obtained by new study led by the University of Cambridge.

The enigmatic rock formation, which lies almost immediately beneath the Hawaiian Islands, is one of numerous ultra-low velocity zones, so named because earthquake waves slow to a crawl as they travel through.

The study, which was published in the journal Nature Communications on May 19, 2022, is the first to demonstrate in detail the complex internal variability of one of these pockets, offering information on the geography of Earth's deep interior and the processes that operate within it.

“Of all Earth’s deep interior features, these are the most fascinating and complex. We’ve now got the first solid evidence to show their internal structure — it’s a real milestone in deep earth seismology,” said lead author Zhi Li, a PhD student at Cambridge's Department of Earth Sciences.

The iron-nickel core is at the center of Earth's interior, which is surrounded by a thick layer known as the mantle and a thin outer shell — the crust we live on. Despite the fact that the mantle is solid rock, it is hot enough to flow very slowly. Internal convection currents transport heat to the surface, causing tectonic plate movement and triggering volcanic eruptions.

Seismic waves from earthquakes are used by scientists to'see' beneath the surface of the Earth; the echoes and shadows of these waves offer radar-like images of deep inner topography. However, 'images' of the structures at the core-mantle border, a vital area for analyzing our planet's internal heat flow, have been grainy and difficult to decipher until recently.

The researchers discovered kilometer-scale features at the core-mantle boundary using cutting-edge computational modeling techniques.“We are really pushing the limits of modern high-performance computing for elastodynamic simulations, taking advantage of wave symmetries unnoticed or unused before,” says co-author Dr Kuangdai Leng, who pioneered the methods while at the University of Oxford. According to Leng, who is currently employed at the Science and Technology Facilities Council, this means they can enhance the image resolution by an order of magnitude over prior efforts.

The speed of seismic waves passing at the base of the ultra-low velocity zone beneath Hawaii was reduced by 40%, according to the researchers. This backs with previous claims that the zone contains significantly more iron than the surrounding rocks, making it denser and slower. “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,” stated study leader Dr. Sanne Cottaar of Cambridge Earth Sciences.

Conceptual cartoons of the Hawaiian ultra-low velocity zone (ULVZ) structure. A) ULVZ on the core–mantle boundary at the base of the Hawaiian plume (height is not to scale). B) a zoom in of the modeled ULVZ structure, showing interpreted trapped postcursor waves (note that the waves analyzed have horizontal displacement).

Scientists may be able to learn more about what lies beneath volcanic chains like the Hawaiian Islands as a result of this research. Scientists have discovered a link between the position of the descriptively named hotspot volcanoes, such as Hawaii and Iceland, and the ultra-low velocity zones near the mantle's base. The origins of hotspot volcanoes are unknown, but the most prominent idea suggests that plume-like structures transport hot mantle material all the way to the surface from the core-mantle barrier.

The researchers can now gather rare physical evidence from what is presumably the root of the plume feeding Hawaii, thanks to photographs of the ultra-low velocity zone beneath the island. Surface observations would be backed up by their discovery of dense, iron-rich granite beneath Hawaii. “Basalts erupting from Hawaii have anomalous isotope signatures which could either point to either an early-Earth origin or core leaking, it means some of this dense material piled up at the base must be dragged to the surface,” Cottaar explained.

To determine if all surface hotspots have a pocket of dense material at the base, more of the core-mantle boundary must be scanned. Where earthquakes occur and where seismometers are positioned to capture the waves determine where and how the core-mantle boundary can be targeted.