Giant pieces of planet Theia are buried in Earth’s mantle, study finds

  • Two giant patches of dense rock hundreds of miles high sit deep within the Earth.
  • New research suggests that these blobs are remnants of a planet that hit Earth 4.5 billion years ago.
  • The collision of Earth and this ancient planet, called Theia, may have helped create the moon.
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If you looked deep beneath the earth’s crust, you would see two giant blobs of rock cupping the core of the planet like a pair of hands.

The source of these mysterious, continent-sized formations, one under the Pacific Ocean and the other under Africa, has puzzled geologists for four decades. Some experts have suggested that the massive rocks are fragments of tectonic plates that got trapped underneath their counterparts.

But according to new research, its origin may be from another world.

A group of scientists at Arizona State University suggested that the spots are remnants of a “planetary embryo the size of Mars” called Theia, which struck Earth in its infancy 4.5 billion years ago. The impact is believed to have turned the Earth’s surface into a sea of ​​fiery magma and caused it to spew out enough planetary debris to create the moon.

Qian Yuan, the principal investigator behind the findings, studies geodynamics at ASU. He thinks that after the ancient collision, parts of Theia may have sunk and conserved deep into our planet’s mantle, the semi-solid layer between Earth’s crust and core, he said.

Those pieces are “millions of times bigger than Mount Everest in terms of volume,” Yuan told Insider.

Dense areas up to 621 miles high

Geologists discovered these fragments, their technical name is large provinces of low shear speed, sending seismic waves to the planet. In both Africa and the Pacific Ocean, the speed of these seismic waves slowed to a slow advance, suggesting an area of ​​rock denser than its surroundings. The animation below, based on a 2016 analysis, shows the size of these areas.


According to Yuan, these spots are between 1.5 and 3.5% denser than the rest of the Earth’s mantle and hotter.

If the planet Theia were rich in iron and very dense, Yuan’s models showed, any chunk of it that fell off when it hit Earth would have sunk deep into our planet’s mantle. There, they could have accumulated undisturbed, rather than blending into the rest of the mantle.

It’s also possible that denser chunks of the Earth’s crust sank into the mantle and became attached to it, contributing to the growth of the patches over time, Yuan said.

layers of the crust mantle of the earth's core shutterstock

An illustration of the layers of the Earth. The coat is bright red.


Finding out what these slabs are made of is a challenge. Its deepest parts are 1,800 miles below our feet, in the part of the mantle closest to the outer core of the Earth. They are 621 miles (1,000 kilometers) high and two to three times as wide as they are tall.

But scientists have discovered that hot rock columns and magma in some volcanoes in Iceland and Samoa come from these patches. By analyzing the composition of this magma, researchers can gain insight into the composition of these mysterious buried chunks. According to a 2019 study, some elements of the volcanic columns date back to about 4.5 billion years ago, when Theia supposedly struck Earth.

When the planets collide

the surface of the moon

The moon, as seen by NASA’s Mariner 10 in 1973.

NASA / JPL / Northwestern University

The idea that the impact between a tiny planet and Earth helped form the Moon has been around for more than 45 years. But one problem with that hypothesis is that scientists haven’t found any evidence for Theia’s existence.

A 2016 study suggested that’s because the cores of Earth and Theia merged. Another idea, presented in 2018, posits that when the planets collided, they were both “almost completely vaporized,” Yuan said. According to that thinking, the Earth became a rapidly spinning, vaporized mass of molten rock called synesthesia, and then collapsed back into a molten planet. Part of that spinning mass became the moon, and Theia was no more.

A third theory, called a “hit and run,” according to Yuan, is that Theia simply bounced off Earth and pieces of a planet, or pieces of both, combined to form the moon. But the composition of the moon almost exactly matches that of Earth, suggesting that it contains very little Theia.

Yuan’s new findings, which will soon be published in the journal Geophysical Research Letters, may finally offer proof that Theia was in our solar system billions of years ago.

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