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Two giant mineral structures located at the boundary between the lower mantle and the outer core may be causing instability in Earth's protective magnetic field.
A new study shows that these continent-sized formations, known as “large low-velocity provinces,” have diverse compositions and origins. This finding challenges previous assumptions of their homogeneity and impacts our understanding of geological processes deep within the planet.
The results open a new chapter in understanding the relationship between surface processes and deep structures of the Earth. They also help explain the features of our magnetic shield, which protects the biosphere from cosmic radiation.
Mysterious continents deep beneath the surface
These mysterious formations – one under the Pacific Ocean and the other under Africa – were discovered using seismic data in the 1980s. Their height reaches 900 kilometers and their width is thousands of kilometers, making them comparable to continents.
Researchers assumed that these structures had the same composition because of the similar characteristics of the seismic waves that pass through them. However, geodynamicist James Panton of Cardiff University and his colleagues have reached the opposite conclusion.
This difference has important consequences for heat flows in the depths of the planet.
Recycling of the Earth's crust in the depths of the planet
The researchers found that both structures are at least partially composed of former oceanic crust. Subduction processes, where one tectonic plate sinks beneath another, push material from the surface deep into the mantle.
“It's fascinating to see the connection between plate movements on the Earth's surface and structures 3000 kilometers deep,” – notes seismologist Paula Kelemeyer from the University of Oxford. These structures cover up to 30 percent of the surface of the core.
Modeling of mantle circulation over the past billion years suggests that such provinces could have formed naturally through the reworking of oceanic crust, a mechanism that refutes competing theories that they were formed by a collision with another celestial body 4,5 billion years ago.
Differences between structures
The Pacific structure contains 50 percent more fresh oceanic crust compared to the African one. This causes a significant difference in density and composition between the Pacific Province and the surrounding mantle.
The constant replenishment of material is due to the activity of the Pacific Ring of Fire, known for its intense volcanic and seismic activity. In contrast, the region around the African structure is geologically less active.
The African structure contains older, more thoroughly mixed crust, making it less dense.
Impact on the Earth's magnetic field
The difference between these two structures affects the dissipation of heat from the Earth's core. This, in turn, affects convection in the core - the process that generates the planet's magnetic field.
Because these mantle structures do not allow heat to dissipate evenly, they can contribute to an imbalance in the magnetic field. This is especially important because the magnetosphere protects us from harmful solar radiation and cosmic rays.
It is already known that the African province is associated with a weakening of the magnetic field nearby. This could have implications for climate and protection from cosmic radiation.
Research perspectives
To better understand the impact of this deep asymmetry in Earth, scientists need more data. Observations of the planet's gravitational field will be of particular value.
This study, published in the journal Scientific Reports, opens new horizons for studying the relationship between geological processes and the Earth's magnetic field. In the future, this may help to better predict changes in the magnetosphere and their impact on our planet.
