May 20, 2021 - Science

The hunt for the start of Earth's plate tectonics

Animated illustration of a wristwatch with the Earth on the face, and the clock is ticking backwards.
Illustration: Aïda Amer/Axios

At some point in Earth's history, the planet's crust began to move, eventually giving rise to continents, mountains and volcanoes and supplying the surface with life-sustaining nutrients and elements. New research points to that movement starting in at least some places more than 3 billion years ago.

Why it matters: Earth is the only planet known so far to show plate tectonics. One of the biggest questions in geoscience is when and how tectonic activity began and changed, and answers could also guide the search for signs of similar processes — and potentially life — on far-away worlds.

  • Estimates of the onset of plate tectonics range from 800 million to more than 4 billion years ago, but recent evidence is converging on that movement being underway globally around 3 billion years ago.

How it works: Our planet began an estimated 4.5 billion years ago as a ball of liquid metal and rock coalescing in the early solar system — what geologist Michael Ackerson of the Smithsonian National Museum of Natural History calls “gooey Earth.”

  • The planet then started cooling and, as it did, plates formed on the outermost layer that, at some point — possibly in fits and starts and here and there — began to move under one another.
  • That subduction process operates like a "conveyor belt," recycling and exchanging material and volatile chemicals between the surface of Earth and deep within it, says Ann Bauer, a geochemist at the University of Wisconsin-Madison.

The big picture: What constitutes plate tectonics and its onset is an open debate among scientists. Earth's modern plate tectonics is characterized by how much large blocks move each year, subduction occurring and mid-ocean spreading that creates new crust.

  • "They occur in unison on the modern Earth, but that doesn’t mean they started together or that they always occurred together," Harvard University geologist Roger Fu tells Axios in an email.
  • "There is no doubt there are some geochemical indicators of subduction" in the Archean eon spanning 4 billion to 2.5 billion years ago, says Michael Brown, a geologist at the University of Maryland. But the key question is what that means for when and how plate-like behavior became stable, continuous and propagated across the globe.

Brown poses a different approach to nailing down the onset of plate tectonics: Rather than looking at early rocks for signs of the different aspects of plate tectonics emerging, he suggests working backward from today's plate tectonics and looking at when those indicators disappear.

  • "If we do that, we get agreement back to 750 million years ago. But once we go back before 750 million years ago, we get disagreement."

Background: Minerals like zircon that crystallize in ancient magma can contain tiny quantities of elements that serve as clues about the chemistry — and indirectly the geology — that formed them.

What's new: In a study of zircons from the Jack Hills of Western Australia — some of which, at 4.3 billion years old, are the oldest known material on Earth — Ackerson and his colleagues found an increase in aluminum in the minerals about 3.6 billion years ago.

  • There are two possible ways the aluminum got into the zircons, Ackerson says. Sediments at the surface with high amounts of aluminum might have been brought deep into the Earth and melted to create magma that then crystallized into zircons, which were then pushed back to the surface. Or the Earth cooled to the point where the crust thickened and magma with high aluminum content came to the surface.
  • "Both require some process akin to plate tectonics," Ackerson says.

Another study of zircons from the Acasta Gneiss in northern Canada, led by Bauer and geologist Jesse Reimink of Penn State University, also suggested some kind of crustal motion formed the minerals between 3.6 billion and 3.8 billion years ago based on a shift in the ratios of isotopes of hafnium in the zircons during that time.

  • The researchers then looked at the records of zircons from six other places on the globe and found that the same shift occurred elsewhere at roughly the same time, Bauer says, adding they don't know if those samples represent the entire globe and the different types of crust at the surface at that time.

Yes, but: Just because these geochemical shifts can and typically do occur due to plate tectonics today "does not conclusively show that these conditions must have existed on the early Earth at the times they discuss," Fu says.

  • But, he adds, "something significant did seem to happen on the Earth at around that time" and evolution toward plate tectonics is "one clear possibility."
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