How did Earth's continents form? Leading theory may be in doubt

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New research ultimately poses more questions than it answers.

steam rises above a bubbling, iron-rich geyser
The El Tatio Geyser in Chile's Atacama desert. (Image credit: Tatsiana Volskaya/Getty Images)

Scientists have eliminated one possible origin for Earth's continents.

Despite the importance of Earth's continents, the huge pieces of the planet's crust that divide its oceans, very little is known about what gave rise to these large landmasses that make our planet unique in the solar system and play a key role in allowing it to host life.

For years, scientists have theorized that the crystallization of garnet in magma beneath volcanoes was responsible for removing iron from Earth's crust, allowing the crust to remain buoyant in the planet's seas. Now, new research is challenging that theory, forcing geologists and planetary scientists to rethink how this iron may have been removed from the material that would go on to form the continents we see today on Earth.

Related: Planet Earth: Everything you need to know

The crust of Earth, the planet's outer shell, is divided into two rough categories: The older, thicker continental crust; and the younger, denser oceanic crust. New continental crust forms when its building blocks are passed to Earth's surface from continental arc volcanoes. These are found in parts of the globe where oceanic plates sink beneath continental plates, regions called subduction zones.

The distinction between dry continental crusts and oceanic deep-sea crusts is the lack of iron in the continental crust. This means continental crusts are buoyant and rise above sea level to form the dry land masses that make terrestrial life possible.

The low levels of iron found in continental crust has been hypothesized to be the result of the crystallization of garnet in the magmas beneath these continental arc volcanoes. This process removes non-oxidized iron from the terrestrial plates, while also depleting iron from molten magma thus leaving it more oxidized as it forms continental crust.

A team of researchers led by Cornell University assistant Professor Meghan Holycross and Smithsonian National Museum of Natural History geologist Elizabeth Cottrell improved the understanding of the continents by setting about testing and eventually eliminating this hypothesis first formulated in 2018.

"You need high pressures to make garnet stable, and you find this low-iron magma at places where the crust isn't that thick and so the pressure isn't super high," Cottrell said in a release, adding that the team was skeptical of the crystallization of garnet as an explanation for the buoyancy of continental crust.

Creating the intense conditions of Earth's interior in the lab

To test the garnet theory, the team recreated the massive pressure and heat found below continental arc volcanoes using piston-cylinder presses located at the Smithsonian Museum's High-Pressure Laboratory and at Cornell University. These mini-fridge-sized pistons composed of steel and tungsten carbide can induce massive pressures on tiny rock samples while they are simultaneously heated by a surrounding cylindrical furnace. 

The pressures induced were equivalent to 15,000 to 30,000 times that created by Earth's atmosphere and temperatures generated were between around 1,740 and 2,250 degrees Fahrenheit (950 to 1,230 degrees Celsius), hot enough to melt rock. 

In a series of 13 different lab tests performed by the team, Cottrell and Holycross grew samples of garnet from molten rock under pressures and temperatures mimicking conditions inside magma chambers deep in Earth's crust.

A cross section of Earth showing the various layers that make up the planet. (Image credit: Science Photo Library/Getty Images)

These lab-grown garnets were analyzed using X-ray absorption spectroscopy which can reveal the composition of objects based on how they absorb X-rays. The results were compared to garnets with known concentrations of oxidized and unoxidized iron. 

This revealed that the garnets grown from rocks in conditions resembling the interior of Earth didn't take up enough unoxidized iron to explain the levels of iron depletion and oxidation seen in the magmas that form continental crust.

"These results make the garnet crystallization model an extremely unlikely explanation for why magmas from continental arc volcanoes are oxidized and iron-depleted," Cottrell said. "It's more likely that conditions in Earth's mantle below continental crust are setting these oxidized conditions."

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The geologist added that what the team's results can't currently do is provide an alternative hypothesis to explain the creation of continental crust, meaning the findings ultimately pose more questions than they answer. 

"What is doing the oxidizing or iron depleting?" Cottrell asked. "If it's not garnet crystallization in the crust and it's something about how the magmas arrive from the mantle, then what is happening in the mantle? How did their compositions get modified?"

These questions are difficult to answer, but Cottrell is currently mentoring researchers at the Smithsonian that are investigating the idea that oxidized sulfur is causing the oxidation of iron beneath the Earth's surface. 

The team's research was published Thursday (May 4) in the journal Science. 

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Robert Lea
Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

21 Comments Comment from the forums
  • Lara
    No no. Skip all of this. Why is the ocean floor all the same age if subduction is happening which is like necessary for the current model that they're building off of? If these major events are not happening then play tectonics doesn't work how we think it does and you have to start considering stuff like the Earth expands and contracts, stuff that sounds crazy but there's evidence for it here and there's evidence for it on several moons in our own solar system.
    Reply
  • billslugg
    The ocean floor is not all the same age. It is youngest at the mid-ocean ridges, where new ocean floor is being created, and oldest where it is being subducted.
    Reply
  • magnoflux
    Grand Canyon Formation in a Coronation Revelation.
    Not only the seabed but what about the grand canyon
    Just visited the south side and was amazed to see the layers of our planet’s historic topology staring at me. See pictures courtesy of US national parks. https://www.nps.gov/grca/learn/nature/grca-geology.htm
    Geologists believe that this 250 km long 25 km wide canyon was formed around 2 to 4 million years ago but cannot explain how. Well, neither could I as if a huge asteroid collided with earth in the east west direction then the crater would be huge with smashed out rocky matter blasted forward of which there is no evidence.

    But if the meteor was made of positron enclosed anti-matter then what we see makes sense as a 25 km wide chunk of anti-matter coming in at a low angle would hit the surface of the planet and bouncing slightly as it moved forward leaving a ragged path as it annihilated kilogram for kilogram of planetary electron enclosed matter magnoflux stuff could leave behind a canyon deep enough to expose the earth’s mantle that formed over 2 thousand million years ago.

    Asteroids or meteors can attain velocities of 50 km/second which means the Grand Canyon could have been formed in just 5 seconds but the annihilation of the adjacent atmosphere would have certainly sucked all trees and vegetation out of the ground tens of kilometres around the site and the Colorado river later forced to find a new way inside the canyon.

    If the asteroid were a 25 km cube then around 5x10^14 tons of planetary matter would have been annihilated by the star sourced anti-matter meteor resulting in what we see now.
    Reply
  • billslugg
    Except if you look at the Grand Canyon from high above, it has that same shape as every canyon in the world, a dendritic tree shape. Can't have been made by any form of energetic impactor. That dirt went to the ocean. In the 60 years since I first visited the Grand Canyon, it has eroded an additional half inch.

    How about all the energy created by annihiliation of 5x10^14 tons of matter? There would be 5x10^17 kilograms converted to energy at the rate of 9x10^16 joules per kilogram. This is 4.5X10^34 joules.
    The Earth has a mass of 6x10^24 kilograms, made mostly (99%) by silicates, atomic weight 60. Heat of fusion 50 joules per mole. This is 833 joules per kilogram. Earth total is 5x10^27 joules.

    The "antimatter option" would melt the Earth 9 million times over.
    Reply
  • Unclear Engineer
    If a mass of 5 x 10^14 tons of antimatter hit the Earth, it would annihilate an equivalet amount of normal matter, so twice Bill's estimate of energy released (unless he already doubled that in his calc).

    Never mind melting the planet. Rapidly releasing that much energy on one side of the planet would shatter it, and propel the fragments well out of the current orbit.

    So, no, it was clearly not created by an antimatter impact.

    As Bill said, it looks like a regular erosion product, just much bigger. It cuts through a highland because the land bulged upward at a slow enough rate that the river was able to keep cutting its path through the area at a faster rate, instead of getting dammed up behind the bulge and needing to flow around it.
    Reply
  • bolide
    magnoflux said:
    Grand Canyon Formation in a Coronation Revelation.
    Not only the seabed but what about the grand canyon
    Just visited the south side and was amazed to see the layers of our planet’s historic topology staring at me. See pictures courtesy of US national parks. https://www.nps.gov/grca/learn/nature/grca-geology.htm
    Geologists believe that this 250 km long 25 km wide canyon was formed around 2 to 4 million years ago but cannot explain how. Well, neither could I as if a huge asteroid collided with earth in the east west direction then the crater would be huge with smashed out rocky matter blasted forward of which there is no evidence.

    But if the meteor was made of positron enclosed anti-matter then what we see makes sense as a 25 km wide chunk of anti-matter coming in at a low angle would hit the surface of the planet and bouncing slightly as it moved forward leaving a ragged path as it annihilated kilogram for kilogram of planetary electron enclosed matter magnoflux stuff could leave behind a canyon deep enough to expose the earth’s mantle that formed over 2 thousand million years ago.

    Asteroids or meteors can attain velocities of 50 km/second which means the Grand Canyon could have been formed in just 5 seconds but the annihilation of the adjacent atmosphere would have certainly sucked all trees and vegetation out of the ground tens of kilometres around the site and the Colorado river later forced to find a new way inside the canyon.

    If the asteroid were a 25 km cube then around 5x10^14 tons of planetary matter would have been annihilated by the star sourced anti-matter meteor resulting in what we see now.
    The Grand Canyon was cut by the Colorado River. Whatever gave you the idea that "Geologists ... cannot explain how" it formed?
    Reply
  • magnoflux
    When matter is annihilated the kg for kg's deletion leaves nothing behind. The only energy released is the magnoflux spin inertia which is similar to the electromagnetic energy of a light beam which can travel through space with no external energy loss as it tumbles forward. It is totally magnetically and charge balanced and nothing remains just a vacuum.
    Reply
  • Unclear Engineer
    magnoflux said:
    When matter is annihilated the kg for kg's deletion leaves nothing behind. The only energy released is the magnoflux spin inertia which is similar to the electromagnetic energy of a light beam which can travel through space with no external energy loss as it tumbles forward. It is totally magnetically and charge balanced and nothing remains just a vacuum.
    That is not correct. When a positron and an electron annihilate each other, energy is released in the form of 2 photons, which I have used to detect the positrons. Similarly, when protons and anti-protons annihilate, they also release their mass-equivalent energy as photons. Those photons get absorbed by surrounding matter, heating that matter.

    If a large amout of anti-matter was annihilated, it would make an "atomic bomb" seem like a fire cracker by comparison. In atomic bombs and nuclear reactors, whether fission or fusion, only a small fraction of the rest mass of the fuel is liberated by the fission and fusion processes, leaving a lot of matter at a much higher temperature due to the energy released.
    Reply
  • billslugg
    Yes, Unclear is correct. When a positron and an electron annihiliate, two photons at .511 MV are emitted. However, most of the energy in an antimatter explosion comes from protons and antiprotons, of which about 75% is emitted as neutrinos, not gamma rays.

    "Magnoflux spin inertia" is not a recognized term in the standard model. I only study the standard model. Once I understand it, and find it lacking, then I'll look elsewhere. Until then, I'll just ignore the term.
    Reply
  • Unclear Engineer
    Bill, Do you have a reference for the neutrino energy derived from proton-anitproton annihilation?

    Because protons are composed fo 3 quarks, it isn't a simple two-particle annihilation. See https://en.wikipedia.org/wiki/Annihilation . That link says that a lot of mesons are produced that are at significant fractions of the speed of light, and either interact with other matter or decay on their own. That suggests to me a lof of energy transfer to regular matter. On the other hand, neutrinos do not interact much with regular matter, so I would not expect them to locally heat the site of the annihilation.

    Still, 25% of the rest-mass energy is a lot of energy for local heating when the rest mass is double 5 x 10^14 tons!
    Reply