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A recent study from Utrecht University challenges the long-held notion that the Earth’s mantle is fast-flowing and uniformly mixed. This research uncovers new insights from gigantic geological formations, described as “out of tune” islands, located 1,800 feet beneath the Earth’s surface in a region characterized by tectonic plate subduction.
Published on January 22nd in the journal Nature, the study examines seismic waves created during significant earthquakes, which cause the Earth to vibrate similarly to a massive bell. This analysis enables scientists to delve into the planet’s interior by interpreting the acoustic signatures of these oscillations. Disturbances in these tones indicate anomalies, suggesting whether certain regions are out of tune or experiencing diminished volume.
Research dating back over two decades revealed evidence for two subterranean “super-continents” situated hundreds of miles below Africa and the Pacific Ocean. At that time, there was uncertainty about whether these geological formations, located at the boundary of the Earth’s mantle and core, were transient or had existed for millions or even billions of years.
Schematic representation of the process of subduction of tectonic plates and of a mantle plume rising from an LLSVP. In the latter, the mineral grains are larger than those in the subducted plates. Credit: Utrecht University
According to Arwen Deuss, a seismologist who co-authored the study, these substantial islands are encircled by remnants of tectonic plates that have been redirected to this area through subduction—a process where one tectonic plate shifts below another, transporting it deep underground.
Known as Large Low Seismic Velocity Provinces (LLSVPs), these sub-continental regions appear to slow down seismic waves due to their comparatively higher temperatures. The research focused on how acoustic waves are affected by these LLSVPs, specifically looking into the energy loss or “damping” that occurs as seismic waves travel through various geological structures.
Surprisingly, the findings indicated minimal damping within the LLSVPs, which resulted in louder tones, contrasting sharply with the weaker tones produced by the cooler, denser areas surrounding them. Study co-author Sujania Talavera-Soza compared this phenomenon to a runner’s performance—lower temperatures help maintain higher speed and stamina compared to a hot environment.
Further investigation led the researchers to consider the mineral composition of LLSVPs, particularly the sizes of individual grains. Deuss noted that grain size plays a critical role in the behavior of seismic waves in these provinces.
Location of the LLSVPs and a schematic representation of the Earth’s cross-section for speed and damping of the seismic waves. Credit: Utrecht University
The LLSVPs were found to consist of larger mineral grains, which indicates their ancient origin—estimated to be at least 500 million years old, and potentially extending beyond 1 billion years. These larger grains possess higher rigidity, allowing them to withstand the dynamic processes of mantle convection.
Talavera-Soza emphasized the resilience of LLSVPs to mantle convection, suggesting these formations play a significant role in the geological activity seen on the Earth’s surface.
These findings could significantly alter the conventional understanding of the mantle as being homogeneous and fluid. Such a seismic shift in perspectives extends beyond just the characteristics of LLSVPs. Understanding how these geological features evolve and their interactions with surrounding materials can enhance our comprehension of Earth’s geological history, as well as its current volcanic and mountain processes.
Deuss pointed to mantle plumes—large masses of molten rock that rise from deep within the Earth—as an example of this interplay. Such plumes are linked to volcanic activity, and the researchers propose that these phenomena likely originate at the edges of the LLSVPs, showcasing the dynamic nature of Earth’s internal processes.
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