Continent-Sized ‘Islands’ Discovered Deep Inside Earth’s Mantle

Deep within Earth, two continent-sized regions, dubbed “islands” by researchers, have been discovered lurking nearly 1,200 miles beneath the surface. These massive structures, located under Africa and the Pacific Ocean, aren’t composed of rock as we typically understand it, but represent areas of significantly different seismic activity – and they’re far older than previously imagined. A recent study from Utrecht University challenges long-held beliefs about the Earth’s mantle, suggesting it’s less fluid and more structured than scientists once thought.

Unearthing the Subterranean Structures

These “islands,” formally known as Large Low Seismic Velocity Provinces (LLSVPs), were initially identified through seismic analysis in the late 20th century. When major earthquakes occur, they generate oscillations that travel through the Earth, much like a bell ringing. Seismologists analyze these vibrations to map the planet’s interior, revealing anomalies like the LLSVPs. The recent research, published in Nature, focuses not just on the speed of these seismic waves, but also on how much energy they lose as they travel – a property called “damping.”

The LLSVPs are roughly 620 miles (1,000km) high, dwarfing Mount Everest and any other feature on the Earth’s surface. Arwen Deuss, a seismologist at Utrecht University and senior author of the study, explained that “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.” This uncertainty highlights the ongoing mystery surrounding these deep-Earth structures.

How Seismic Waves Reveal Hidden Details

The team’s approach involved analyzing the “ringing” of the Earth after large earthquakes, specifically looking at how seismic waves slow down and how much energy they lose. Surprisingly, they found very little damping within the LLSVPs themselves. Instead, the most significant damping occurred in the surrounding areas – the “graveyard” of cold, sunken tectonic plates that have descended through subduction. This finding was unexpected, as hotter materials typically cause both slowing and damping of seismic waves.

This difference in damping led researchers to consider the material properties of the LLSVPs. Mineralogical analyses, as suggested by study co-author Laura Cobden, pointed to grain size as a key factor. In the cold, sinking tectonic plates, the material recrystallizes into small grains, causing significant energy loss as seismic waves encounter numerous boundaries. However, the LLSVPs appear to be composed of much larger grains, allowing waves to pass through with minimal damping. Because these larger grains don’t grow quickly, the researchers conclude that the LLSVPs are ancient – potentially half a billion years old or even older.

Implications for Mantle Dynamics

The discovery challenges the traditional view of the Earth’s mantle as a constantly churning, well-mixed system. The rigidity and age of the LLSVPs suggest they’ve remained relatively stable for vast periods, resisting the forces of convection. This has significant implications for understanding how the mantle drives processes on the Earth’s surface, such as volcanism and mountain building. Earth’s mantle is a critical component in shaping the topography we see today.

Mantle plumes – columns of hot material rising from deep within the Earth – are believed to originate at the edges of LLSVPs. These plumes are responsible for volcanic hotspots like Hawaii. Understanding the nature of the LLSVPs could therefore provide insights into the origins and behavior of these plumes, and the distribution of volcanic activity around the globe.

Beyond Seismic Analysis: Uncovering the Mantle’s Secrets

Studying these deep-Earth structures relies heavily on analyzing the oscillations generated by large earthquakes. The 1994 Bolivia earthquake, despite occurring at a depth of 650 km and causing no surface damage, proved particularly valuable due to the high-quality data it generated. Seismometers have been continuously recording data since 1975, allowing researchers to revisit past earthquakes and extract new information about the Earth’s interior.

The Utrecht University team’s novel approach of analyzing both wave speed and damping provides a more complete picture of the LLSVPs than previous studies. This technique allows scientists to differentiate between temperature variations and material properties, offering a more nuanced understanding of these enigmatic structures. Further research will likely involve combining seismic data with other geophysical techniques, such as analyzing variations in Earth’s gravitational field, to build a more comprehensive model of the deep Earth.

What’s Next for LLSVP Research?

The findings from Utrecht University open new avenues for research into the Earth’s mantle. Future studies will focus on refining the models of LLSVP composition and structure, and investigating their long-term stability. Researchers will also continue to analyze seismic data from large earthquakes, seeking to identify additional clues about the origin and evolution of these mysterious “islands.” The team plans to incorporate more advanced computational models to simulate mantle convection and assess the impact of LLSVPs on global geological processes. Mantle dynamics is a complex field, and this research represents a significant step forward in unraveling its mysteries.

The discovery also highlights the importance of continued investment in seismological monitoring networks. High-quality seismic data is essential for probing the Earth’s interior and understanding the processes that shape our planet. As technology advances, we can expect even more detailed and accurate images of the deep Earth, revealing further secrets hidden beneath our feet.