Beneath our feet, hundreds of kilometers deep, lies an invisible yet essential world: the Earth's mantle. For a long time, scientists believed they understood its structure and functioning. But a recent discovery has shaken this view, with the finding of an unexpected structure beneath the Pacific Ocean at the very heart of the mantle.
The Earth's mantle, the layer between the crust and the core, is the stage for slow but powerful movements. These movements, called convection, are responsible for plate tectonics. Oceanic plates, being denser, plunge into the mantle at subduction zones, while hot material rises to the surface. This cycle has shaped our planet for billions of years.
Thanks to a new model, researchers highlight areas in the lower Earth's mantle where seismic waves travel more slowly (in red) or faster (in blue). The large blue area in the western Pacific (right above the center of the image) was not known until now.
Graphic: Sebastian Noe / ETH Zurich Plate tectonics: a well-established model
According to this model, submerged tectonic plates should be found near subduction zones, where they plunge into the mantle. These cold, dense plates alter the speed of seismic waves passing through them, allowing scientists to locate them.
However, a team of geophysicists from ETH Zurich has discovered anomalies. These zones, where seismic waves behave differently, suggest the presence of materials with unusual temperature or composition. These findings challenge our understanding of mantle dynamics.
An innovative technique to explore the mantle
To achieve these results, researchers used an innovative method called Full Waveform Inversion. Unlike traditional techniques, which focus on a single type of seismic wave, this approach analyzes all the waves generated by earthquakes.
This method, much more precise, allows for the reconstruction of a detailed image of the Earth's interior. Researchers were thus able to identify anomalies in areas where previous models detected nothing. This technological advancement was made possible thanks to the use of the Piz Daint supercomputer, capable of processing massive amounts of data.
Anomalies where they were not expected
Thanks to this high-resolution modeling, researchers identified abnormal zones beneath the western Pacific Ocean, far from any known subduction zone. These anomalies could be remnants of ancient plates or accumulations of iron- or silica-rich rocks.
These results, published in
Scientific Reports, show that the Earth's mantle is far more complex than expected. Scientists still do not know the exact origin of these anomalies, but they could come from materials dating back to the formation of the Earth four billion years ago.
Towards a new understanding of the Earth
To explain these discoveries, researchers are considering several hypotheses. These anomalies could be remnants of ancient tectonic plates, or areas where iron-rich rocks have accumulated over time.
This work paves the way for new research to refine models of the Earth's mantle. It reminds us that our planet still holds many mysteries, especially in its inaccessible depths.
To go further: What is Full Waveform Inversion?
Full Waveform Inversion is an advanced technique used in geophysics to study the internal structure of the Earth. It works somewhat like a medical ultrasound, but on a planetary scale. While doctors use ultrasound to visualize organs without opening the body, geophysicists analyze seismic waves to map the Earth's depths without drilling.
This method goes further than traditional techniques, which only study a single type of seismic wave. It examines all the waves generated by earthquakes, providing a much more precise and detailed image. Just as an MRI reveals details invisible to standard ultrasound, this approach reveals subtle anomalies in the Earth's mantle.
Thanks to this technique, scientists have discovered unexpected rocky zones. These anomalies, located far from subduction zones, challenge our understanding of the Earth's internal dynamics. By combining computing power and fine wave analysis, this method opens a new window on the mysteries of our planet.
What is the Earth's mantle?
The Earth's mantle is an intermediate layer located between the Earth's crust and the core. It extends about 2,900 kilometers (1,800 miles) thick and represents nearly 84% of our planet's volume. Composed mainly of silicate, iron, and magnesium-rich rocks, it plays a central role in Earth's dynamics.
Although solid, the mantle behaves like a viscous fluid over geological time scales. This phenomenon, called mantle convection, drives slow but powerful movements that influence plate tectonics, earthquakes, and volcanic activity. These movements are fueled by heat from the Earth's core.
The mantle is divided into two parts: the upper mantle and the lower mantle. The upper mantle, closer to the surface, is the site of the formation and recycling of tectonic plates. The lower mantle, denser and hotter, remains poorly understood due to its inaccessibility.
The study of the Earth's mantle relies on indirect methods, such as the analysis of seismic waves. These techniques allow mapping its structure and better understanding its role in the Earth's geological evolution.
What is plate tectonics?
Plate tectonics is a scientific theory that explains the structure and movements of the Earth's surface. According to this theory, the lithosphere (the Earth's rigid outer layer) is divided into several tectonic plates that float and move slowly on the viscous mantle. These movements are responsible for major geological phenomena such as earthquakes, volcanic eruptions, and mountain formation.
Tectonic plates interact with each other in different ways. They can move apart (divergence), come together (convergence), or slide past each other (transformation). These interactions create subduction zones, where one plate plunges beneath another, and oceanic ridges, where new crust forms.
Plate tectonics is fueled by the Earth's internal heat, which causes convection movements in the mantle. These movements push the plates to move a few centimeters per year, continuously reshaping the planet's surface over millions of years.
This theory, formulated in the 1960s, revolutionized our understanding of geology. It explains not only the distribution of continents and oceans but also the evolution of the Earth through geological ages.
Article author: Cédric DEPOND