Scientists Discover Huge 'Islands' Deep in Earth's Mantle

Scientists find two huge 'islands' hidden beneath Earth's surface

Deep within Earth’s mantle, more than 2,000 kilometers below the surface, lie two massive, continent-sized structures. These enigmatic formations are not only significantly hotter than their surroundings but also far older than previously thought—dating back at least 500 million years, if not more.

New research from Utrecht University challenges the long-standing belief that Earth's mantle is well-mixed and rapidly flowing. Instead, the study suggests these ancient "islands" have remained largely undisturbed for eons. Published in the journal Nature, the findings reveal that these regions are thermally distinct from the cooler, subducted tectonic plates nearby. This breakthrough reshapes our understanding of mantle dynamics, indicating that Earth's interior is far less mixed and mobile than previously assumed.

The location and shape of two continent-sized LLSVPs in the mantle, highlighting their hotter, older, and more rigid nature.

A Deep and Hidden World

First identified through seismic studies in the late 20th century, the Large Low-Shear-Velocity Provinces (LLSVPs) lie more than 2,000 kilometers below the surface, beneath Africa and the Pacific Ocean. Seismologists study Earth's interior by analyzing vibrations caused by large earthquakes, much like doctors use X-rays to examine the human body. These underground "super-continents" were discovered by observing how seismic waves slow down or change direction as they travel through different materials.

"Nobody knew whether these structures were transient features or if they had been sitting there for millions—or even billions—of years," explains Arwen Deuss, a professor at Utrecht University specializing in Earth's deep interior. "Our findings show they are ancient and likely to be a permanent part of the planet’s mantle."

Heat and Grain Size Hold the Key

The researchers discovered that the LLSVPs are significantly hotter than their surroundings, which explains why seismic waves slow down in these regions. However, one unexpected finding was that the LLSVPs do not exhibit the expected damping effect—where seismic waves lose energy as they pass through hot material. Instead, damping was strongest in the surrounding "graveyard" of sunken tectonic plates.

To explain this, the team examined the grain size of the minerals within the LLSVPs. According to geophysicist Ulrich Faul, temperature alone could not account for the absence of damping. Instead, the research points to large mineral grains within the LLSVPs, which allow seismic waves to pass through with minimal energy loss. These large grains indicate extreme age, as they take millions or even billions of years to grow.

Cross-section diagram of a subduction zone, depicting a mantle plume rising from an LLSVP. Labels indicate the relative sizes of mineral grains in the subducted plate versus the plume.

A More Rigid Mantle Than Expected

This discovery challenges the widely accepted idea that Earth's mantle is in constant motion, churning and mixing over geological time. The study suggests that the LLSVPs are too rigid to participate in mantle convection, meaning they have remained largely unchanged for hundreds of millions of years. "Contrary to what we’ve long assumed, the mantle is not as well-mixed as we thought," says Deuss. "These structures must have survived mantle convection in some way."

Implications for Earth's Surface and Evolution

The existence and stability of these deep-mantle structures have profound implications for understanding Earth's geological processes. The edges of LLSVPs are thought to be the origin points for mantle plumes—columns of hot material rising toward the surface that can drive volcanic activity, such as the eruptions that formed the Hawaiian Islands.

"The Earth's mantle is the engine driving volcanism, mountain building, and even plate tectonics," explains Deuss. "Understanding these deep structures helps us understand surface processes."

Using Earthquakes as a Window into the Deep Earth

To reach these conclusions, the researchers analyzed seismic data from large earthquakes dating back to 1975. One particularly useful event was the 1994 Bolivia earthquake, which, despite occurring at a depth of 650 kilometers, caused no surface damage but provided crucial data about the deep Earth.

Cross-section of Earth illustrating seismic wave speed and deceleration through low-seismic velocity provinces (LLSVPs). Top left and right show LLSVPs, while bottom left depicts slower wave travel due to warmth in these regions. Bottom right highlights reduced deceleration within LLSVPs compared to surrounding areas. Image credit: Utrecht University.

"These deep earthquakes don’t make headlines, but they are goldmines for understanding our planet’s interior," says Deuss. "They allow us to mathematically separate different properties of seismic waves, such as damping and wave speed, providing us with a clearer picture of the structures below."

The study represents a major leap forward in understanding Earth's internal structure and raises new questions about the origins and evolution of these ancient mantle structures. As research continues, scientists hope to uncover even more about the hidden forces shaping our planet from within.

The discovery of these ancient, continent-sized islands deep within Earth’s mantle opens a new chapter in our understanding of the planet’s inner workings. As scientists continue to unravel the secrets of these hidden giants, one thing is clear: Earth’s interior is far more complex—and fascinating—than we ever imagined. So the next time you feel the ground shake, remember: it’s not just an earthquake—it’s a clue to the mysteries buried beneath our feet.