Deep within Earth’s mantle lie two giant ‘islands’ comparable to the size of continents. Recent research from Utrecht University indicates that these areas are not only hotter than the nearby cold, sunken tectonic plates but are also ancient, estimated to be at least 500 million years old or possibly older. These findings challenge the notion of a rapidly flowing and well-mixed Earth’s mantle, a theory that is increasingly coming under scrutiny. “The flow in the Earth’s mantle is less than what is widely assumed,” states the research, which is set to be published on January 22nd, 2025, in Nature.
Large earthquakes resonate through the Earth, causing it to vibrate like a bell, producing various tones, just like a musical instrument. Seismologists examine the inner Earth by analyzing the discrepancies in these tones, as Earth’s oscillations will appear ‘out of tune’ or softer when they encounter anomalies. This method allows seismologists to create images of the Earth’s interior, similar to how doctors use X-rays to visualize the human body. At the end of the 20th century, studies of these oscillations revealed the presence of two underground ‘super-continents’: one beneath Africa and another beneath the Pacific Ocean, both situated over 2,000 kilometers below the Earth’s surface. “No one knew what these formations were or if they were temporary or had been there for millions, or potentially even billions, of years,” explains Arwen Deuss, a seismologist and professor of Structure and Composition of the Earth’s Deep Interior at Utrecht University in the Netherlands. “These two vast islands are encircled by a graveyard of tectonic plates that have been pushed there via a process known as ‘subduction,’ where one tectonic plate slips beneath another, descending nearly 3,000 kilometers into the Earth.”
Slow waves
“For many years, we have recognized that these islands are positioned at the boundary between the Earth’s core and mantle, and we notice that seismic waves decelerate there.” Earth scientists refer to these locations as ‘Large Low Seismic Velocity Provinces’ or LLSVPs. “The decrease in wave speed is due to the heat of the LLSVPs; just as you can’t run as swiftly in hot weather as you do in cooler conditions.” Deuss and her colleague Sujania Talavera-Soza were determined to learn more about these areas. “We attempted to gather new information regarding the ‘damping’ of seismic waves, which reflects the energy loss of waves as they traverse through the Earth. Instead of only analyzing how out of tune the tones were, we also examined their loudness.” Talavera-Soza added: “Surprisingly, we discovered minimal damping in the LLSVPs, resulting in loud tones there. In contrast, significant damping was evident in the cold slab graveyard, where tones were soft. As for the upper mantle, it behaved as anticipated: it is hot, and the waves experience damping. Much like running on a hot day, where you not only slow down but also become more fatigued than on a cooler day.”
Grain size
Their colleague Laura Cobden, an expert in deep Earth minerals, proposed an investigation into the grain size of the LLSVPs. According to their American colleague Ulrich Faul, temperature alone cannot account for the low damping present in the LLSVPs. Deuss noted: “Grain size plays a much more crucial role. Subducted tectonic plates that arrive in the slab graveyard consist of smaller grains as they recrystallize during their descent into the Earth. Smaller grains result in a greater number of grains and consequently more boundaries between them. Due to these numerous boundaries in the slab graveyard, there is increased damping as waves lose energy crossing each boundary. The fact that the LLSVPs display minimal damping implies they must contain much larger grains.”
Ancient
These mineral grains do not develop overnight, indicating that LLSVPs are significantly older than the surrounding slab graveyards. Moreover, the LLSVPs, with their substantially larger structural components, are quite rigid. This rigidity means they do not participate in mantle convection (the flow within the Earth’s mantle). Contrary to what traditional geography lessons suggest, the mantle cannot be well-mixed. Talavera-Soza elaborates: “The LLSVPs must be able to endure mantle convection somehow.”
Engine
Understanding the Earth’s mantle is essential for comprehending our planet’s evolution. “It also sheds light on other surface phenomena such as volcanism and mountain formation,” adds Deuss. “The Earth’s mantle is the engine driving all of these activities. Consider mantle plumes, which are massive bubbles of hot material that rise from deep within the Earth like in a lava lamp.” When these plumes reach the surface, they can cause volcanic activity, as seen in Hawaii. “We believe these mantle plumes originate at the edges of the LLSVPs.”
Large earthquakes
Seismologists leverage oscillations from significant earthquakes in their research, especially those occurring at great depths, like the major Bolivia earthquake of 1994. “It didn’t make headlines as it happened 650 km underground and fortunately caused no damage or injuries on the surface,” Deuss explains. The oscillations of the whole Earth, or tones, can be mathematically analyzed to easily ‘read’ the damping (the loudness of the oscillation) attributable to specific structures and separate it from wave speed (how out of tune they are). “This is quite impressive since the damping signal is merely one-tenth of the total information we can derive from these oscillations.” For this research, waiting for another earthquake isn’t necessary; data from past earthquakes is equally valuable. “We can analyze data going back to 1975 when seismometers improved enough to provide high-quality information useful for our studies.”
