- Two huge patches of dense rock hundreds of miles long sit deep within the earth.
- New research suggests that these spots are remnants of a planet that hit Earth 4.5 billion years ago.
- The collision of the earth and this ancient planet, called Theia, may have helped create the moon.
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If you peered deep under the earth’s crust, you would see two giant rock rocks that filled the core of the planet like a pair of hands.
The source of these mysterious continent-sized formations – one under the Pacific Ocean, the other under Africa – has stunned geologists for four decades. Some experts have suggested that the massive rocks are fragments of tectonic plates trapped beneath their counterparts.
But according to new research, its origins may be different.
A group of scientists from Arizona State University have suggested that the spots are remnants of a ‘Mars-sized planetary embryo’ named Theia, which struck Earth 4.5 billion years ago in its infancy. The impact presumably turned the Earth’s surface into a sea of fiery magma, causing it to shoot out enough planetary debris to create the moon.
Qian Yuan, the lead researcher behind the findings, is studying geodynamics at ASU. He thinks that parts of Theia may have sunk after the ancient collision and been preserved deep in the mantle of our planet – the semi-solid layer between the earth’s crust and the core, he said.
The pieces are “millions of times larger than Mount Everest in terms of volume,” Yuan told Insider.
Dense areas up to 621 miles in altitude
Geologists discovered these pieces – their technical name is large provinces with low shear rates – by sending seismic waves to the planet. Under both Africa and the Pacific, the speed of these seismic waves has slowed to a crawl, indicating that an area is 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 warmer.
If the planet Theia were rich in iron and very dense, Yuan’s models showed, any pieces of it that broke off when it hit the earth would have sunk deep into the mantle of our planet. There they could accumulate undisturbed, rather than get confused in the rest of the mantle.
It is also possible that denser pieces of earth crust sank into the mantle and joined it, which contributed to the growth of the stains over time, Yuan said.
An illustration of the Earth’s layers. The mantle is bright red.
Shutterstock
It is difficult to find out what these plates are made of. Their deepest parts are 800 kilometers below our feet, in the part of the mantle that is closest to the earth’s outer core. They are 1000 kilometers high and two to three times wider than they are high.
But scientists have found that the plumes of hot rock and magma from some Icelandic and Samoan volcanoes come from these spots. By analyzing the composition of this magma, researchers can gain insight into the composition of these mysterious buried pieces. According to a study from 2019, some elements in the volcanic plumes date from about 4.5 billion years ago – when Theia allegedly hit the Earth.
When planets collide
The moon, as seen by NASA’s Mariner 10 in 1973.
NASA / JPL / North-West University
The idea that the impact between a small planet and the earth helps shape the moon has been around for over 45 years. But a problem with the hypothesis is that scientists have found no evidence of Theia’s existence.
A study from 2016 suggests that the core of the earth and Theia merge. Yuan said another idea, emerging in 2018, claims that both almost completely evaporated when the planets collided. According to this thinking, the earth became a fast-moving mass of molten and evaporated rock called a synestia and then collapsed again into a molten planet. Part of the rotating mass became the moon, and Theia was no longer there.
A third theory – according to Yuan called the ‘hit-and-run’ – is that Theia just looked from Earth, and combined pieces of one planet, or pieces of both, to form the moon. But the composition of the moon is almost identical to that of the earth, indicating that it contains very little of Theia.
Yuan’s new findings, soon to be published in the journal Geophysical Research Letters, may well prove that Theia was in our solar system billions of years ago.