A fascinating discovery has been made at the tectonic plate cemetery, prompting a rewrite of textbooks (photo).

The History of Earth spans over 4.5 billion years, during which the planet has undergone numerous visual transformations. For centuries, scientists have endeavored to explore the Earth's depths using every possible technology. The prevailing theory suggests that our planet has a rapidly flowing and well-mixed mantle, but this theory may now be revised, as reported by Popular Science.

In a new study, a team from Utrecht University in the Netherlands examined a pair of submerged "disrupted" geological islands, each the size of a continent, located within the Earth's tectonic plate graveyard at a depth of about 549 meters below the planet's surface.

The researchers' investigation is based on tones generated during significant earthquakes, causing the Earth to vibrate like a giant bell. Scientists study the planet's interior by analyzing the acoustic signatures of these vibrations. Experts can also identify anomalies based on whether regions are in a disordered state or if their volume is diminishing.

More than a quarter-century ago, scientists discovered that some of their deep Earth reverberations indicated the existence of two underground "supercontinents" hundreds of kilometers beneath Africa and the Pacific Ocean. At that time, researchers were uncertain whether these formations, located near the mantle-core boundary, were temporary or had existed for millions or even billions of years.

According to co-author of the study, seismologist Arven Deuss, these two islands are actually surrounded by a graveyard of tectonic plates that have been moved into the planet's interior through subduction. It is noteworthy that during this process, one tectonic plate shifts beneath another, causing it to sink.

The two subcontinents, along with any other areas causing seismic wave slowdowns, are known as Large Low Shear Velocity Provinces, or LLSVPs. Researchers note that one of the primary reasons for the acoustic slowdown is the higher temperature of the LLSVPs compared to the surrounding environment.

In the course of the new study, scientists focused on the ability of LLSVPs to "dampen" seismic waves, causing them to lose energy. The team also paid special attention to where the tones of seismic waves became disrupted, as well as how loud or quiet they became during their journey through the planet's depths.

According to another co-author of the study, Sujania Talavera-Sosa, contrary to their expectations, the researchers found minimal damping in the LLSVPs, making the tones sound very loud in these areas. At the same time, scientists discovered significant damping in the tectonic plate graveyard—where the tones sounded extremely quiet.

The results obtained by the researchers contrasted with data from the Earth's upper mantle. This data appeared as expected: waves dampened due to higher temperatures. Scientists compare the difference to running in hot or cold weather: on hot days, runners tend to slow down, while in cooler conditions, they speed up.

Next, the scientists examined the mineral composition of the LLSVPs, including individual grain sizes. The study's findings indicate that grain size was "far more significant." It is important to note that tectonic plate graveyards consist of fine grains formed after the crystallization of minerals during each downward motion of the formation into the planet's interior.

Smaller grains mean a much larger quantity, as well as more tiny gaps between them. Any acoustic waves passing through these formations lose energy as they traverse the numerous grain boundaries, leading to greater damping. However, since the two LLSVPs exhibited very little damping, their grain sizes must be significantly larger.

The study's authors note that the larger grain size also suggests that these LLSVPs are likely much older than previously thought. The team believes they are at least 500 million years old, and possibly over 1 billion. The results also indicate that these mineral grains are significantly stiffer, allowing them to withstand the flow of the Earth's mantle, known as mantle convection.

Interestingly, these discoveries contradict descriptions of a very fluid and well-mixed mantle found in most textbooks. Such a potentially large seismic revision extends far beyond the composition, age, or movement of LLSVPs. Scientists believe that understanding how these massive formations grow in size and interact with their environment helps shed light on Earth's planetary evolution. This is also important in the context of the internal workings of the planet and mountain formation.