Colossal "Anomalies" in Earth's Mantle Turned Out to Be Something Else

They may be unique "lost" fragments of another planet.

Scientists have discovered that the giant structures in the Earth's mantle, known as "low seismic velocity provinces" (LLSVP), have a more complex nature than previously thought. New research has shown that these are not just hot spots, but stable formations with a unique composition that may have existed for billions of years.

Colossal structures, comparable in size to continents, are located about 3,000 km deep beneath the Pacific Ocean and the African continent. They were first discovered in the 1980s through seismic observations: earthquake waves slowed down as they passed through them, indicating their elevated temperature. However, the exact composition of these "anomalies" remained a mystery for a long time.

In a new study, geophysicists from the Netherlands and the USA examined not only the speed of seismic waves but also their attenuation—how much energy they lose while passing through these structures. Analyzing data from 104 powerful earthquakes allowed for the creation of a detailed 3D model of the Earth's mantle.

It turned out that unlike the surrounding "graveyards" of submerged tectonic plates, LLSVPs hardly weaken seismic waves. This indicates their unique structure: they consist of minerals with large "grain" sizes, making them less mobile. According to seismologist Arwen Doyce from Utrecht University, this is an important finding as it changes the understanding of material movement in the mantle—perhaps it is not as active as previously thought.

Previously, there were two main hypotheses regarding the origin of LLSVPs. According to the first, they consist of fragments of ancient lithospheric plates that sank into the mantle during subduction, which is the process of one tectonic plate sliding under another. However, new analysis suggests that their structure is too stable for such formations.

An alternative hypothesis, which has received new confirmation, is that LLSVPs may be remnants of ancient protoplanetary material. This could be material left over from Earth's collision 4.5 billion years ago with the hypothetical planet Theia—this event is believed to have led to the formation of the Moon.

If this theory is correct, then LLSVPs may turn out to be unique "lost" fragments of another planet, preserved in the Earth's depths throughout its history. This discovery radically changes the understanding of how well the parts of the Earth's mantle mix and how stable its deep structures are.