Hidden depths: Earth’s ancient magma ocean and its seismic legacy

The early history of Earth conceals many mysteries, one being the existence of a deep ocean of magma beneath the planet's surface. A new study, published in the scientific journal "Nature," has shown that not only could a magma ocean have existed, but its presence was virtually inevitable. Regardless of the exact point at which the newly formed molten planet began crystallising into a solid body, a base ocean still developed. This phenomenon could account for contemporary seismic activity.

Charles-Édouard Boukaré, the study’s lead author, suggested that this may have affected the way heat is transferred between the Earth’s core and mantle. This discovery might also explain the presence of large areas in the mantle where seismic waves propagate more slowly.

Research indicates that the magma ocean could have formed at the boundary between Earth's core and mantle during the first hundreds of millions of years of its existence. Models suggest that even if the planet crystallised from the bottom up, the formation of the magma ocean was unavoidable. Boukaré told "Live Science" that no matter the point at which crystallization began, the formation of the magma ocean was already underway.

Remnants of this hidden sea of magma may persist today as LLVPs or "mantle blobs." LLVP stands for "Large Low-Shear-Velocity Provinces," which are extensive anomalies in Earth's lower mantle where seismic waves move more slowly than usual. Scientists have debated whether these anomalies are remnants of oceanic crust that was pushed deep into the mantle, dating back hundreds of millions of years, or if they are relics from Earth's primordial magma ocean, making them 4.4 billion years old.

The new study argues for the latter option, and its findings could significantly affect how scientists understand Earth's history, said lead author Charles-Édouard Boukaré, a planetary physicist at York University in Toronto. "It would affect thermal communication between the core and the mantle," Boukaré told "Live Science." He believes it might also affect the positioning of tectonic plates.

To support this, scientists developed a new model of Earth's formation, incorporating both geochemical and seismic data—two primary methods for delving into Earth's deep history. In particular, certain trace elements prefer to remain in magma chemically, while other minerals crystallise into rock. The concentration of these trace elements in rock can indicate when and in what order the mantle's solidification occurred.

Most studies of this era of Earth's formation focus on the initial solidification of the mantle and its dynamics when it was still predominantly liquid. Boukaré and his team found that, regardless of where solidification initially began—in the middle of the mantle or at the boundary with the core—a base magma ocean formed.

This discovery could have major implications for understanding Earth's history. "We can then predict most of its behavior on long timescales," Boukaré adds. The research suggests that the planet's structure was established very early, with ancient structures still influencing its evolution.

Scientists plan further research to gain a better understanding of how these primordial magma oceans could have affected other planets, such as Mars. "Maybe this basal ocean thing is not something that is unique to the Earth," Boukaré wonders.