Sunken Continents Could Unbalance Earth’s Magnetic Field

Structures the size of continents, protruding from the lower mantle towards Earth’s outer core, may be contributing to an instability in our planet’s magnetic field.

Initial Assumptions About Similarity

These two formations, one beneath the Pacific and the other under Africa, have similar characteristics in terms of seismic waves, leading scientists to initially believe they had the same composition.

However, James Panton, a geodynamicist at Cardiff University, and his team came to a different conclusion, discovering that the two regions are made up of distinct materials and have different geological histories. If confirmed, this could affect heat flow and convection deep within the Earth in ways that could influence how the planet generates its magnetosphere.

Up to 900 kilometers in height and thousands of kilometers wide, these “large low-velocity provinces” have puzzled scientists since they were identified by seismic data in the 1980s. Subsequent research suggested that they are, in part, composed of ancient oceanic crust.

The Fascinating Link Between Plate Movements and Deep Structures

“It is fascinating to see the connection between the movements of plates on the Earth’s surface and structures 3000 kilometers deep,” says Paula Koelemeijer, a seismologist at the University of Oxford.

Over millions of years, the natural cycling of crust has mixed what was once Earth’s surface deep into the mantle. The resulting composition now covers up to 30% of the core, slowing down the seismic waves used by geologists to study Earth’s inner structure.

“Our models of mantle circulation over the past billion years show that large low-velocity provinces can naturally develop as a result of recycling oceanic crust,” write Panton and his team, countering competing theories that the anomalies arose from the collision with Earth around 4.5 billion years ago, which led to the Moon’s formation.

The structure in the Pacific appears to contain 50% more fresh oceanic crust mixed in than the African province, resulting in a greater compositional difference between the Pacific province and the surrounding mantle, as well as a notable difference in its density.

Impact of Subduction on the Pacific Province

Panton says, “We find that subducted oceanic crust enriches the Pacific large low-velocity province, suggesting that Earth’s recent subduction history drives this difference.”

The geologically active Pacific Ring of Fire has consistently replenished crust material, the researchers suspect.

In contrast, the geological activity around the African structure is lower, so the older crust there has mixed more thoroughly, making this structure less dense.

“The fact that these two large low-velocity provinces differ in composition but not in temperature is key to the story and explains why they appear similar seismically,” explains Koelemeijer.

The different temperatures of these two structures, compared to their surrounding regions, impact how heat dissipates from Earth’s core, which in turn affects convection in the core that drives the planet’s magnetic field.

The researchers suspect that, since these two mantle structures are not allowing the core’s heat to escape evenly on both sides of the planet, they may be contributing to an imbalance in the magnetic field that supports life in our atmosphere.

Africa’s Role in Magnetic Field Weakening

The weakening of the nearby magnetic field has already been linked to Africa’s large low-velocity province.

To better understand the impacts of this deep-Earth asymmetry, the researchers need more data, such as observations of Earth’s gravitational field.

Read the original article on: Science Alert

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