A recent study by researchers at Utrecht University has revealed the existence of two massive structures deep within Earth’s mantle, referred to as "islands" or Large Low Seismic Velocity Provinces (LLSVPs). These subterranean regions are not only vast, spanning a distance of over 1,200 miles (2,000 km) beneath the Earth’s surface, but also ancient, dating back at least half a billion years.

Hidden Islands Inside Earth

These underground "islands" are colossal in size, towering 620 miles (1,000 km) high. This height makes them dramatically larger than any mountain peak on Earth or any other planet in our solar system. They were initially detected in the late 20th century using seismic analysis techniques.

When powerful earthquakes occur, they create seismic waves that bounce through the Earth’s interior. Scientists analyze these oscillations to uncover hidden structures beneath the planet’s surface. The LLSVPs, found beneath Africa and the Pacific Ocean, were first identified through this method.

Large Low Seismic Velocity Provinces (LLSVPs)

The regions, known scientifically as Large Low Seismic Velocity Provinces, were initially confusing to researchers. Arwen Deuss, a seismologist at Utrecht University and one of the study’s senior authors, explains, "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." These mysterious structures have baffled scientists for decades.

Around these "islands" lies a graveyard of cold, sunken tectonic plates that have been subducted into the Earth’s mantle. The peculiar behavior of seismic waves in LLSVPs is an important clue to their composition and age. The waves move slower and with less energy loss in these regions compared to elsewhere in the mantle due to their elevated temperatures.

Earth’s Mantle Is Still a Mystery

To gain deeper insights, the researchers used a novel approach to analyze seismic waves by measuring both the slowing of waves and their damping.

According to co-author Sujania Talavera-Soza, "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." This unexpected finding suggested that the LLSVPs had distinct material properties.

Grain Size Plays a Role in LLSVPs

Further mineralogical analysis revealed that grain size is a critical factor. In the cold slab graveyard, the sinking tectonic plates recrystallize into small grains, increasing energy loss as waves pass through numerous grain boundaries. In contrast, the LLSVPs are characterized by much larger grains, allowing waves to pass with minimal damping.

Because mineral grains in LLSVPs do not grow quickly, they must be much older than the surrounding graveyard of tectonic plates. This indicates that LLSVPs are resilient structures that have remained largely unchanged through the dynamic processes of the mantle.

Why Does This Matter?

The discovery challenges the traditional view of the Earth’s mantle as a uniformly mixed system. Understanding these ancient, persistent structures is crucial for interpreting surface phenomena such as volcanic activity and mountain building.

Mantle plumes, which are thought to originate at the edges of LLSVPs, are responsible for volcanic eruptions like those seen in Hawaii. These plumes are vast columns of hot material rising from deep within the Earth’s interior.

Studying the Hidden Islands

To study these deep regions, seismologists rely on data from massive earthquakes, particularly those that occur at great depths. One such earthquake, which took place in Bolivia in 1994, did not cause any damage but provided invaluable data for researchers.

Seismometers have been recording high-quality data since 1975, allowing scientists to reanalyze past events and gain new insights into the LLSVPs.

LLSVPs and Earth’s Mantle

This research offers a transformative view of Earth’s mantle. It suggests that instead of a uniform system, the mantle contains ancient, stable structures.

Understanding these regions is essential for deciphering the engine driving Earth’s surface processes. As Deuss notes, "The Earth’s mantle is the engine that drives all these phenomena." Mantle plumes originating at the edges of the LLSVPs play a significant role in shaping the Earth’s surface.

The discovery of the LLSVPs’ unique properties and their impact on Earth’s dynamic processes marks a significant step forward in earth sciences.

The study was published in the prestigious journal Nature, highlighting its importance.