'Sunken worlds' discovered under the Pacific ocean in Earth's mantle

Many people imagine Earth’s interior mantle as separate layers stacked on top of each other, like a layered cake, with each layer representing a plate. In reality, those internal zones are complex and often full of surprises.

Seismologists have relied on specific types of earthquake waves to learn about Earth’s subsurface, but now a different technique is painting a far more detailed picture.

This fresh perspective has led scientists to wonder if there is hidden material in places once believed to be empty.

Understanding Earth’s mantle – the basics

Earth’s mantle is the thick layer of rock that lies between the planet’s crust and its core. It stretches about 1,800 miles (2,900 kilometers) deep, making up around 84% of Earth’s total volume.

The mantle is primarily composed of silicate minerals rich in iron and magnesium, and it’s mostly solid, but it behaves like a thick, slow-flowing fluid over long periods of time.

This flow is what drives the movement of tectonic plates on Earth’s surface, causing earthquakes, volcanic eruptions, and the shifting of continents.

Heat from the Earth’s core causes convection currents in the mantle, where hot material rises, cools near the crust, and then sinks back down, creating a continuous cycle. This process plays a key role in shaping the planet’s surface.

Tectonic plates in Earth’s mantle

The discovery stems from the use of a high-resolution method called full-waveform inversion. Scientists use this approach to interpret every type of seismic wave produced by earthquakes, rather than focusing on only one kind.

Because of that, they can build a more precise model of Earth’s internal structures. 

After applying this method to the lower mantle, researchers saw pockets that appear to be leftover plate fragments in areas with no known history of subduction.

Following their analysis, they were startled by just how common these hidden anomalies seemed to be.

This work was led by a team that includes doctoral student Thomas Schouten from the Geological Institute of ETH Zurich, who collaborated with experts from the California Institute of Technology.

Their combined efforts brought fresh insights that challenge assumptions about where old tectonic plate remnants might reside in Earth.

Interpreting seismic signals

To understand how researchers got here, it helps to remember how seismic waves work. When an earthquake strikes, it sends waves radiating outward in all directions.

These waves bounce, bend, and shift as they travel through the planet. In much the same way that doctors use medical imaging techniques, geophysicists measure the time these waves take to reach various seismic stations around the globe. 

The velocity of those signals reveals details about rock density and stiffness. In previous years, scientists relied heavily on particular seismic phases.

But by examining all available wave data instead, new shapes and densities were identified from Earth’s lower mantle.

Why is Earth’s mantle important?

Over hundreds of millions of years, plates have formed, shifted, and sunk back into Earth’s interior. This cycling plays a role in controlling phenomena like volcano formation, earthquakes, and the slow drift of continents. 

Researchers have long known that slabs of subducted plates cluster under regions where one tectonic plate slides under another.

So it came as a surprise when the new images revealed large slabs below oceans and continental interiors that lack a clear history of plate collisions.

That signals a possible complication in our understanding of how plates evolve and where they end up.

Mysterious plate under the Pacific Ocean

One of the biggest surprises was in a zone under the western Pacific. According to current plate tectonic timelines, there is no reason for old plate fragments to be there. 

“Apparently, such zones in the Earth’s mantle are much more widespread than previously thought,” said Schouten.

No geologic data points to historical subduction nearby. This indicates that the anomalies might not be chunks of plates at all, or at least not in the way scientists originally imagined.

Could it be something else?

Scientists now face questions about the origin and nature of these structures.

Some say they may be ancient, silica-rich pockets left over from the early mantle. Others suggest they might be iron-rich accumulations that have drifted over billions of years. 

Schouten explained that the new model reveals anomalies throughout the Earth’s interior, but the exact materials or plate fragments responsible for these patterns remain unclear.

This uncertainty suggests a more diverse range of compositions in the Earth’s mantle than previously understood.

Next steps for deeper understanding

Full-waveform inversion is a promising tool for these investigations, but it offers only a view of how fast or slowly waves move through the material.

Researchers say they need further refinement to tease out chemical and thermal differences. 

Some are turning to additional datasets, including electromagnetic signals and data from mineral physics experiments.

Combining those methods might highlight whether these mantle “blobs” are from primordial origins, from recycled ocean crust, or from something else entirely.

Implications for plate tectonics

If scientists confirm that more zones like these exist in Earth’s depths, they may have to adjust many theories about how heat moves through the planet.

Such hidden zones might alter patterns of convection and the formation of mantle plumes. 

The story of plate tectonics could also see updates, with new chapters on how plate fragments wander and transform in ways not previously documented. These discoveries remind us that geoscience, like any science, is never settled.

The search continues

In the future, improved supercomputers may crunch even larger datasets to produce clearer images of Earth’s lower mantle and its plate structures. That could resolve debates about the true nature of these mysterious anomalies. 

By integrating various approaches – seismic, chemical, and computational – Earth scientists will keep uncovering pieces of our planet’s hidden complexity.

As Schouten’s team demonstrated, sometimes seeing the entire picture means looking beyond what we think we already know.

The study is published in Scientific Reports.

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