- Two gigantic blobs of dense rock hundreds of miles tall sit deep inside the Earth.
- New research suggests 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 were to peer deep under Earth’s crust, you’d spot two giant blobs of rock cupping the planet’s core like a pair of hands.
The source of these mysterious, continent-sized formations — one under the Pacific Ocean, the other under Africa —has baffled geologists for four decades. Some experts have suggested that the massive rocks are fragments of tectonic plates that got trapped under their counterparts.
But according to new research, their origin may be otherworldly.
A group of scientists from Arizona State University suggests the blobs are remnants of a “Mars-sized planetary embryo” named Theia, which struck the Earth in its infancy 4.5 billion years ago. The impact is thought to have turned Earth’s surface into a sea of fiery magma and caused it to shoot out enough planetary debris to create the moon.
Qian Yuan, the lead researcher behind the findings, studies geodynamics at ASU. He thinks that following the ancient collision, parts of Theia may have sunk down and gotten preserved deep in our planet’s mantle — the semi-solid layer between Earth’s crust and core.
Those pieces are “millions of times larger than Mount Everest in terms of volume,” Yuan told Insider.
Dense areas up to 621 miles in height
Geologists discovered these chunks — their technical name is large low-shear-velocity provinces — by sending seismic waves down into the Earth. Under both Africa and the Pacific, the speed of these seismic waves slowed to a crawl, 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 blobs are between 1.5% and 3.5% more dense than the rest of Earth’s mantle, and also hotter.
If the planet Theia was rich in iron and highly dense, Yuan’s models show, any pieces of it that broke off when it hit Earth would have sunk deep into our planet’s mantle. There, they could have accumulated undisturbed, rather than getting mixed into the rest of the mantle.
It’s also possible denser chunks of Earth’s crust sank down into the mantle and joined them, contributing to the blobs’ growth over time, Yuan said.
An illustration of Earth’s layers. The mantle is in bright red.Shutterstock
Figuring out what these slabs are made of is challenging. Their deepest parts are 1,800 miles under our feet, in the part of the mantle closest to Earth’s outer core. They’re 621 miles (1,000 kilometers) high, and two to three times wider than they are tall.
But scientists have figured out that plumes of hot rock and magma from some Icelandic and Samoan volcanoes came from these blobs. By analyzing this magma’s make-up, researchers can glean insight into composition of these mysterious buried chunks. According to a 2019 study, some elements in the volcanic plumes date back to about 4.5 billion years ago — when Theia supposedly hit Earth.
When planets collide
The moon as viewed by NASA’s Mariner 10 in 1973.NASA/JPL/Northwestern University
The idea that the impact between a tiny planet and Earth helped to form the moon has been around for more than 45 years. But a problem with that hypothesis is that scientists haven’t found any evidence of Theia’s existence.
A 2016 study suggested that’s because Earth’s and Theia’s cores fused together. Another idea, put forward in 2018, posits that when the planets collided, both were “almost completely vaporized,” Yuan said. According to that thinking, Earth became a rapidly spinning mass of molten and vaporized rock called a synestia, then collapsed back into a molten planet. Part of that spinning mass became the moon, and Theia was no more.
A third theory — called the “hit-and-run,” according to Yuan — is that Theia just glanced off of Earth, and chunks of one planet, or pieces from both, combined to form the moon. But the moon’s composition matches Earth’s almost exactly, which suggests it contains very little of 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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