The outermost shell of the Earth is constantly in motion. Over eons, our planet's tectonic plates have dragged the continents apart and slammed them into one another, forming ocean trenches, mountains, and volcanoes.

Plate tectonics explains continental drift, seafloor spreading, and why California has so many earthquakes. The Earth's shifting plates also created an ideal environment for life itself to evolve and thrive. But exactly how our planet's seven or eight major plates started shifting remains a mystery. Today, in a study published in Nature, researchers model the conditions that may have kick-started plate tectonics billions of years ago.

"Over the past decade, we have kept with the idea that plate tectonics was not something that was born with our planet," says Patrice Rey, a geoscientist at the University of Sydney in Australia and lead author on the study. "The Earth started with a stagnant plate. The question was, how do we move from there to a situation where we have plate tectonics?"

A Hotter, Heavier Earth

Modern geological plates move about in a predictable fashion. The top portion of a plate is relatively cooler and denser than the Earth below it, and this state of "negative buoyancy" causes one plate to slip, or subduct, under another. But scientists have long suspected that during the Archaean Eon, 2.5 billion years ago, geological plates were more static.

"We look at these old Archaean crusts, for instance, and we don't see the features that characterize subduction," Rey says.

This implies that some geological event must have initially set the plates in motion. Rey and his colleagues suspected that early Earth's unique geology might have had something to do with it. Hundreds of millions of years ago, Earth's interior was hotter and its crust was thicker. Rey wondered whether heavier continents might have pushed against nearby plates, initiating subduction.

"Continents have a natural tendency to spread horizontally and push other plates on their sides," Rey says. "When we put this physics into a computer model, we realized that it had the power to force the plate into subduction."

In the movie a continent (made of a depleted lithospheric mantle in green and continental crust in red) is slowly spreading toward the adjacent oceanic lithosphere (blue). A short-lived subduction zone develops following 45 myr of slow spreading. This subduction episode allows the continent to surge toward the ocean, leading to its "boudinage" and allowing the detachment of a continental block. This movie shows an 87 myr long story. (Credit: Patrice F Rey, Nicolas Coltice and Nicolas Flament)

New Models for Ancient Problems

The study presents a compelling story about how plate tectonics began. Continents grew denser, sank, and ultimately shoved their compatriots out of the way. Over time, these shoving matches developed into a familiar, self-sustaining cycle of subduction.

For some experts, though the most exciting aspect of the study is that Rey and his team found a way to use modern techniques to answer very old problems.

"What jumped out was that they could apply the modern tools of mechanical modeling to a problem that really pre-dates plate tectonics," says Larry Brown, a geophysicist at Cornell University who was not involved in the research. "It's tying it all together, with a nice dash of physical quantitativeness."

Nourishing Life with Rocks

There's also a curious link between the onset of plate tectonics and the evolution of biology. By moving landmasses apart and forming mountains and valleys, plate tectonics effectively gave rise to a number of unique ecosystems where new life forms could flourish.

"If you want to have very vibrant viable biosphere with a lot of species, what you need is to provide lots of different ecosystems," Rey says. "There's a link between initiation of plate tectonics on Earth and the development of a strong and vibrant biosphere."

Rey and his colleagues are now preparing a more complete model, based on their initial findings, that would show how the continents themselves came to be. "What our model doesn't cover is the formation of continents," Rey says. "We're working actively on that at the moment to bring that story into our model."

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