Continent-sized 'islands' found inside Earth are 100 times taller than Mount Everest

Scientists have discovered two continent-sized regions hidden deep within Earth’s mantle, referred to in a recent study as “islands.” Research from Utrecht University reveals that these regions are not only hotter than their surroundings, but are also ancient, dating back at least half a billion years or longer.

The findings challenge conventional theories about the Earth’s mantle being a well-mixed and rapidly flowing system. Instead, the research suggests there is much less flow within the mantle than previously believed.

Hidden islands inside Earth

These subterranean islands are buried 1,200 miles (2,000km) beneath our feet, and reach heights of roughly 620 miles (1,000km), dwarfing every mountain peak found on Earth’s surface and every other planet in our solar system.

The regions in question, located beneath Africa and the Pacific Ocean, were first identified in the late 20th century through seismic analysis. 

When large earthquakes strike, they cause the Earth to resonate like a bell, producing oscillations that seismologists can analyze to reveal subsurface anomalies.

Large Low Seismic Velocity Provinces (LLSVPs)

The studies showed the presence of what scientists now call Large Low Seismic Velocity Provinces (LLSVPs).

“Nobody knows what they are, and whether they are only a temporary phenomenon or if they have been sitting there for millions or perhaps even billions of years,” explained study senior author Arwen Deuss, a seismologist at Utrecht University.

Surrounding these “islands” is a graveyard of cold, sunken tectonic plates that have descended through Earth’s mantle via subduction – the process by which one plate sinks beneath another.

Unlike the surrounding regions, seismic waves slow down significantly in the LLSVPs due to their elevated temperatures.

Earth’s mantle is still a mystery

To delve deeper into these structures, Deuss and her colleague Sujania Talavera-Soza incorporated a novel approach to studying seismic waves. 

Beyond analyzing how much waves slow down, they also measured “damping,” or how much energy the waves lose as they travel through the Earth.

“Against our expectations, we found little damping in the LLSVPs, which made the tones sound very loud there. But we did find a lot of damping in the cold slab graveyard, where the tones sounded very soft,” Talavera-Soza said.

The findings revealed that the LLSVPs, despite being hot, do not significantly dampen seismic waves. This contradicted expectations based on the behavior of waves in the hotter upper mantle, where both slowing and damping are observed.

Grain size plays a role in LLSVPs

The unexpected results prompted the team to investigate the material properties of the LLSVPs. According to mineralogical analyses suggested by study co-author Laura Cobden, grain size plays a pivotal role. 

In the cold slab graveyard, tectonic plates recrystallize into small grains as they sink into the mantle, leading to significant energy loss as seismic waves cross numerous grain boundaries.

In contrast, the LLSVPs appear to consist of much larger grains, which allow waves to pass with minimal damping.

Since those mineral grains do not grow fast, it means that LLSVPs are much older than the surrounding slab graveyards.

This insight suggests the LLSVPs are not part of the mantle’s convection processes and have remained largely unchanged for vast periods – which is in stark contrast to the constantly recycling material around them.

Why does any of this matter?

The rigidity and age of the LLSVPs challenge the notion of the mantle as a uniformly mixed system. “After all, the LLSVPs must be able to survive mantle convection one way or another,” Talavera-Soza said.

This has profound implications for understanding Earth’s evolution. The mantle drives surface phenomena such as volcanism and mountain building. 

For instance, mantle plumes – columns of hot material rising from deep within the Earth – are believed to originate at the edges of LLSVPs. These plumes eventually cause volcanic activity, such as that seen in Hawaii.

Studying the hidden islands

To study these deep regions, seismologists rely on oscillations caused by massive earthquakes, particularly those occurring at great depths, such as the 1994 Bolivia earthquake.

“It never made it into the newspapers because it took place at a large depth of 650 km and luckily did not result in any damage or casualties at the Earth’s surface,” Deuss said.

Seismometers have been recording high-quality data since 1975, allowing researchers to revisit past earthquakes to gather valuable insights.

LLSVPs and Earth’s mantle

This research provides a transformative view of Earth’s mantle. Instead of a uniform, well-mixed system, the mantle contains ancient, stable structures that defy conventional models. 

Understanding these regions is crucial for interpreting surface phenomena and offers a clearer picture of Earth’s complex inner workings.

“The Earth’s mantle is the engine that drives all these phenomena. Take, for example, mantle plumes, which are large bubbles of hot material that rise from the Earth’s deep interior as in a lava lamp. And we think that those mantle plumes originate at the edges of the LLSVPs,” Deuss said.

The discovery of the LLSVPs’ unique properties and their role in shaping Earth’s surface processes marks a significant step forward in the study of our planet’s dynamic interior.

The study is published in the journal Nature.

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