Scientists have discovered a strange patch of Earth deep in the Pacific Ocean that may explain why the East Pacific Rise, the world’s fastest-growing ocean ridge, formed in this region. Using seismic data, University of Maryland geologist Jingchuan Wang and colleagues found ancient oceanic plates hidden deep within the Earth that may have contributed to ocean spreading dating back to the time of the dinosaurs.

“Our discovery opens up new questions about how the depths of the Earth affect what we see at the surface, at great distances and on different time scales,” says Wang.

By sending reflected sound waves deep into the earth to create seismic maps, Wang and his team discovered a strange cluster of surprisingly slow-moving mantle beneath the Nazca plate, which precisely borders the South American continental plate.

Most of the Earth’s volume consists of heated silicates sandwiched between a cold, thin outer shell and a red-hot core. This layer of partially molten minerals, called the mantle, cycles over tens of millions of years due to extreme temperature changes above and below. Denser, colder material is pulled into the warmer interior by a process called subduction.

In this region the Nazca Plate is currently subducting beneath South America as shown in the diagram below. But on the western side of the plate lies a rapidly rising oceanic ridge and a hotspot of geological activity beneath the Easter Islands, as well as a mysterious structural break between the central and eastern Pacific Ocean.

“We found that material in this region sank twice as slowly as we expected, suggesting that the mantle transition zone may act as a barrier and slow the movement of material through the Earth,” explains Wang. The team determined that this plate structure was colder and denser than surrounding areas and appeared to be a fossilized part of the ancient seafloor.

“This thickened area resembles the fossilized scar of an ancient piece of seabed that sank beneath the Earth about 250 million years ago,” explains Wang. “This gives us a look into Earth’s past that we’ve never had before.”

Remnants of what was once the Triassic ocean floor are protruding deeper into the hotter layers of the mantle, without melting as completely as the surrounding mantle, causing material to bulge out into structures called superplumes. The Easter hotspot is believed to be on one of these plumes.

“Geodynamic modeling explains the geometry and stability of lower mantle structures through their direct interaction with the subducting plate,” the paper’s authors write.

Researchers suspect that this series of east-west anomalies may help tell the story of the Nazca Plate and how it has moved throughout Earth history. By unraveling the historical traces of these ancient collisions underground, geologists can learn more about how our planet’s internal processes shape our world’s surface today.

“Looking at the ancient subduction plate from this perspective gave us a new understanding of the relationship between very deep Earth structures and surface geology that was not previously clear,” Wang explains. This research was published in the journal Science Advances.