Geologists recently unveiled groundbreaking seismic evidence of massive, ancient “fortresses” located nearly 1,800 miles below the Earth’s surface, in the mantle. These colossal structures are not only older and tougher than the surrounding rock, but they have reshaped our understanding of the planet’s interior. Their discovery was made possible through innovative seismic techniques, and the findings have been published in Nature. These formations, covering vast areas beneath Africa and the Pacific Ocean, offer insights into the complex, dynamic processes happening deep within our planet.
Earth’s Mantle: The Hidden World of Giant ‘Fortresses’
At the core of this discovery are the Large Low Seismic Velocity Provinces (LLSVPs), large, sluggish regions of hot rock located at the core-mantle boundary. These provinces, known colloquially as “fortresses,” span across an area roughly the size of continents. The African and Pacific LLSVPs are the two most prominent, each reaching over 3,000 miles wide, with the African fortress rising about 600 miles above the mantle. What makes these structures so fascinating is that they appear to remain largely unaffected by the usual mantle convection processes, meaning they don’t flow or move as expected.
Seismic tomography, a technique used to map the Earth’s interior, first revealed these “fortresses” in the 1990s. However, it wasn’t until more recent studies that their true nature and role were fully understood. “Large earthquakes make the whole Earth ring like a bell with different tones,” said Arwen Deuss, a seismologist from Utrecht University. Her team, which included scientists from across Europe, Australia, and the United States, investigated seismic waves from major earthquakes and found that these “fortresses” didn’t absorb energy as expected, thus proving their unique properties.
The Science Behind the Seismic Mystery
The LLSVPs are characterized by their ability to slow down seismic waves, which makes them stand out from the surrounding mantle material. Scientists discovered that these areas are not only hotter but also possess different mechanical properties compared to the cooler regions around them. The key difference, however, lies in their grain structure. In the mantle, minerals are formed into microscopic crystals, but in the case of the LLSVPs, the crystals are significantly larger and more aligned.
This phenomenon, which has been observed in laboratory settings using high-pressure experiments, plays a crucial role in why seismic waves travel more easily through these regions. The fewer microscopic grain boundaries within the fortresses mean less energy is lost as seismic waves pass through them. “There is less flow in Earth’s mantle than is commonly thought,” Deuss explained, underscoring how these massive, stationary structures disrupt conventional understandings of mantle convection.
Revisiting Earth’s Interior: The Shift in Our Understanding
The discovery of these massive structures forces scientists to reconsider the entire model of Earth’s internal dynamics. The traditional view of a uniformly convecting mantle, where heat rises and cooler rock sinks, is being challenged. The immovable fortresses disrupt this picture, suggesting a more layered and heterogeneous mantle, where parts of it remain rigid while others are fluid. This could have profound implications not only for our understanding of geological processes but also for phenomena observed at the Earth’s surface, such as volcanic activity and plate tectonics.
These “fortresses” could also explain the formation of mantle plumes, columns of hot rock that rise from the Earth’s interior to the surface. These plumes are responsible for volcanic hotspots such as those that create the Hawaiian Islands. It is believed that the edges of these fortresses act as the launchpads for these plumes, creating volcanic chains and large igneous provinces — which have been linked to mass extinctions in Earth’s history. The stability of these structures could potentially affect the movement of tectonic plates over geologic time, influencing the drift of continents and the formation of mountain belts.
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