Earth’s ever-shifting, underground network of tectonic plates was firmly in place more than 4 billion years ago—at least a billion years earlier than scientists generally thought, according to a new study.
Tectonic plates are large slabs of rock embedded in the Earth’s crust and upper mantle, the next layer down. The interactions of these plates shape all modern land masses and influence the major features of planetary geology—including earthquakes, volcanoes, and the emergence of continents.
“Understanding when plate tectonics started on Earth has long been a fundamentally difficult problem,” says Jun Korenaga, a professor of earth and planetary sciences at Yale University and senior author of the new study in Science Advances. “As we go back deeper in time, we have fewer geological records.”
The growth of continents serves as one promising proxy for determining if tectonic plates were operational, Korenaga says. That’s because the only way to build up a continent-sized chunk of land is for surrounding surface rock to keep sinking deeply over a long period—a process called subduction, possible only through plate tectonics.
For the new study, the researchers found evidence of continental growth starting as early as 4.4 billion years ago. They devised a geochemical simulation of the early Earth based on the element argon—an inert gas that land masses emit into the atmosphere. Argon is too heavy to escape Earth’s gravity, so it remains in the atmosphere like a geochemical ledger.
“Because of the peculiar characteristics of argon, we can deduce what has happened to the solid Earth by studying this atmospheric argon,” Korenaga says. “This makes it an excellent bookkeeper of ancient events.”
Most of the argon in Earth’s atmosphere is 40Ar—a product of the radioactive decay of 40K (potassium), found in the crust and mantle of continents. The researchers’ model looked at the atmospheric argon that has gradually accumulated over the history of the planet to determine the age of continental growth.
Part of the challenge in creating their simulation, the researchers say, was incorporating the effects of a geological process called “crustal recycling.” This refers to the cycle by which continental crust builds up, then is eroded into sediments, and eventually carried back underground by tectonic plate movements—until the cycle renews itself.
The simulation therefore, had to account for argon gas emissions not part of continental growth. “The making of continental crust is not a one-way process,” Korenaga says.
The National Science Foundation supported the research.
Source: Yale University