Geoscientists have leveraged earthquake data to investigate the makeup of the lower section of the Earth's mantle beneath the Pacific Ocean, leading to some intriguing findings. They've identified regions where seismic waves exhibit unusual movement patterns, indicating the presence of structures that are either colder or possess a distinct composition compared to the adjacent molten rocks. The researchers characterize these structures as a significant enigma.

It is unclear what these structures are. If they were anywhere else, they could be portions of tectonic plates that have sunk in a subduction zone. But the Pacific is one large plate, so there should be no subduction material under it. The researchers are also uncertain about what kind of material these deep structures are made of or what this means for the internal structure of the planet.

"That's our dilemma. With the new high-resolution model, we can see such anomalies everywhere in the Earth's mantle. But we don't know exactly what they are or what material is creating the patterns we have uncovered," Thomas Schouten, first author and doctoral student at the Geological Institute of ETH Zurich, said in a statement.

The existence of numerous cold areas within the mantle implies that they may have originated at various points in time and from diverse sources. These regions are not solely the result of subduction plates from the past 200 million years.

Schouten elaborated, "We believe that the irregularities in the lower mantle stem from multiple sources. They might be remnants of silica-dense substances that have existed since the mantle's formation around 4 billion years ago, managing to endure the convective shifts within the mantle. Alternatively, these anomalies could represent areas where iron-laden rocks have gathered due to the mantle's movements over the span of billions of years."

Further efforts are required. The team relies solely on one characteristic of seismic waves—their speed—to deduce a great deal about the processes occurring beneath our surface. Despite having impressive models, it remains a challenge to derive all necessary information from this single aspect. We cannot physically access the Earth's interior to observe its contents.  

"Schouten explained, 'We need to assess the various material parameters that could account for the observed velocities of the different wave types. In essence, we must investigate the material characteristics that influence wave speed more thoroughly.'"

A paper describing these findings is published in the journal Scientific Reports.