Earth's Tectonic Plates Began Moving Around 2.5 Billion Years Ago

Researchers have produced a new estimate for the origin of Earth's plate tectonics—the movement of large chunks of the planet's outer layer, or crust.

Although there is broad consensus that plate tectonics have played a significant role in our planet's geology during the last billion years or so, when exactly this process emerged and how it has evolved through time are two of the most significant and hotly debated questions in Earth sciences today, according to a study published in the journal Nature.

Now a team, led by Robert Holder from Baltimore's Johns Hopkins University, has estimated that plate tectonics began to develop gradually around 2.5 billion years ago in a finding that could have significant implications for how we understand our planet's geological history.

Some previous studies have suggested that plate tectonics started much earlier in Earth's history, whereas others indicate that such processes began only about 0.7 billion years ago.

"As geologists, we are interested in describing our planet, how the planet works, why it looks the way it does today, and how it has changed through time," Holder told Newsweek. "To study the planet, one of our best tools is looking at Earth's rocks."

"To a geologist, not all rocks are the same; they form in very specific ways such that they provide a record of what has happened to the planet through its history," he said. "The goal of the study was to ask whether plate tectonics was different in Earth's past: Did Earth operate the same way one billion years ago as it does today? Two billion years ago? Three billion years ago?"

The theory of plate tectonics can explain the creation and break-up of supercontinents, how mountain ranges and major mineral deposits form, and the existence of volcanoes and earthquakes.

There is even a school of thought that suggests plate tectonics were crucial to the evolution of life because the process helps regulate a planet's temperature over long time periods—although it should be noted that this view has been challenged by some scientists.

"Plate tectonics is the term we use to describe how Earth's rigid outer shell—the lithosphere—is broken into large fragments (plates) that slide past each other, collide, and pull apart as they slowly move through time," Holder said. "It is often described as the 'unifying theory' of Earth Science, because it explains why rocks, sediment, continents, rivers, lakes, oceans, mountains, faults, volcanoes, mineral resources, etc., are distributed around the world in predictable and organized patterns."

For the latest study, the scientists investigated so-called "metamorphic rocks" from 564 sites around the world that have formed over the past three billion years.

According to the United States Geological Survey, metamorphic rocks are those that have been substantially changed over time from their original form by being subjected to high heat, high pressure, hot mineral-rich fluids, or often, a combination of these factors.

These kinds of conditions are found deep within the Earth at the meeting points between tectonic plates. Studying metamorphic rocks can help to shed light on the history of Earth's plate tectonics.

"Metamorphic rocks form as other rocks are transformed when Earth's plates collide together, grind past each other, and pull apart," Holder said. "Because metamorphic rocks record these changes, they are a particularly useful for studying plate tectonics and how Earth's crust changed through time. The two parameters we evaluated were (1) how deep were the metamorphic rocks when they formed and (2) how hot was it at that depth in Earth."

"These two parameters tell us something about the geothermal gradient at a place and time in Earth's history," he said. "Because different plate-tectonic scenarios produce different geothermal gradients, we can use this information to understand how a particular metamorphic rock formed and, by looking at data from around the world, to get snapshots of how plate tectonics was different in Earth's past."

This investigation led the researchers to the conclusion that plate tectonics began to develop around 2.5 billion years ago, although they note, importantly, that this was a gradual process.

"Much of the academic discussion about plate tectonics in Earth's ancient history has focused on when it began," Holder said. "When did Earth start working the way that it works today? This discussion has been largely binary, like a light switch: either it is on or it is off. 'Yes, Plate Tectonics operated at this time' or, 'No, it did not.'"

"Our findings suggest that this binary perspective might not be the correct way to think about it," he said. "The systematic and continuous changes we observed in Earth's metamorphic rocks, from 3 billion years ago to today, suggest that a light switch might not be the best analogy, but rather a dimmer switch. To use a different analogy: rather than black or white, a grey scale is more appropriate."

The latest results could have significant implications for our understanding of the Earth and its geological processes, according to the researchers.

"Plate tectonics is the framework we use to read Earth's rock record," Holder said. "Plate tectonics is intimately linked to climate, the evolution of life, the growth of mountain belts, and where natural resources occur. Our findings suggest that Earth's plate tectonic framework has changed slowly through Earth's history."

"Understanding how this framework on ancient Earth was different to the modern Earth is key to accurately interpreting how Earth's rocks formed and why they are distributed across the continents in the patterns that we see," he said.

This article was updated to include additional comments from Robert Holder.

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Stock photo: The Earth's tectonic plates. iStock
Earth's Tectonic Plates Began Moving Around 2.5 Billion Years Ago | Tech & Science