Scientists Date Start of Plate Tectonics Using Ancient Minerals

A group of researchers studied zircons from Australia that show signs of large-scale shifting of Earth’s outermost layer over 3 billion years ago.

The Jack Hills of Western Australia, where the zircons studied were sampled from 15 grapefruit-sized rocks collected by the research team. (Credit: Dustin Trail, University of Rochester)

(CN) — A new study of Earth’s oldest minerals suggests that plate tectonics began over 3 billion years ago.

Published Friday in the journal Geochemical Perspectives Letters, the study from the Smithsonian’s National Museum of National History dates the start of the important geologic process to 3.6 billion years ago. Earth is estimated to be about 4.5 billion years old.

Earth is the only planet know to have both life and plate tectonics, the process in which the hardened outer crust is split into plates that slowly move, fracture and collide atop the molten inside of the globe. It’s “a defining feature of Earth and its unique ability to support life,” the researchers said in a press release announcing the study results.

“Plate tectonics afford a connection between the chemical reactor of Earth’s interior and its surface that has engineered the habitable planet people enjoy today, from the oxygen in the atmosphere to the concentrations of climate-regulating carbon dioxide,” the release said.

Yet scientists are just now learning when this life-enabling process may have started on Earth.

Zircons studied by the research team, photographed using cathodoluminescence, a technique that allowed the team to visualize the interiors of the crystals using a specialized scanning electron microscope. Dark circles on the zircons are the cavities left by the laser that was used to analyze the age and chemistry of the zircons. (Credit: Michael Ackerson, Smithsonian)

A group of scientists led by Michael Ackerson, a research geologist at the Smithsonian, gathered 3,500 zircons from the Jack Hills in Western Australia that were between 3 billion and 4.3 billion years old, the oldest known minerals in the world. About 200 of the minerals were intact enough to use for the study.

These zircons provide the best chemical record of the planet, existing through almost its entire history, minus about 200 million years.

The researchers used Uranium radioactive decay to date them and a mass spectrometer to find their chemical composition. They saw a marked increase in aluminum present around the 3.6 billion year mark.

According to the study, extreme geologic conditions would be needed to allow aluminum to penetrate the zircon crystals.

“For sediment melting, weathered continental material is required to be brought to depths great enough to be melted,” the study added.

This means that hardened rocks and other geologic materials would be shifting beneath the outer crust and being melted before rising again to crystalize into the zircons found today in Australia.

The release says that prior research in Canada has yielded a similar time frame for the start of plate tectonics.

“We are reconstructing how the Earth changed from a molten ball of rock and metal to what we have today,” Ackerson said. “None of the other planets have continents or liquid oceans or life. In a way, we are trying to answer the question of why Earth is unique, and we can answer that to an extent with these zircons.”

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