Scientists figure out what happens to Earth's disappearing crust

Like a giant broken-up cookie whose pieces float atop a sea of scalding milk, Earth's outer shell is made of (less-tasty) rocky rafts that constantly bump into and dive beneath each other in a process called plate tectonics. 

So what happens to those hunks of disappearing crust as they dive into Earth's milky interior?

It turns out that they get weak and bendy, like a slinky snake toy, but they don't disintegrate completely, new modeling shows. The models also suggested that plate tectonics, at least in its modern form, likely only got going in the past billion years.

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Plate tectonics drives earthquakes and volcanoes, creates mountain ranges and islands, and is the reason Earth's continents, once a supercontinent, are now oceans apart. But there's still much unknown about how plate tectonics works, such as what happens when a plate slides beneath another (in an area called a subduction zone) and disappears into the mantle, the middle layer of the planet, which is, perhaps sadly, not composed of milk but rather of sizzling solid rock. 

To figure this out, the researchers used 2D computer models of subduction zones and programmed them using known physics of how materials behave, such as how rocks deform under certain forces. Then, they observed the model to see what happened at the subduction zone and compared their findings to real-life observations.

Their models suggested that as one plate dove beneath another, the descending piece, known as a slab, abruptly bent downward and cracked; the bending also caused the grains on the underside of the plate to become finer and weaker. The pressures left the plate mostly intact but with many weak points.

That means that the plates don't break apart and thus keep pulling on the parts behind them, "for a very long time," said lead author Taras Gerya, a professor of geophysics at ETH Zurich in Switzerland. Indeed, the plate can keep sliding under the other plate for hundreds of millions of years, he said.

Their simulations matched observations and deep seismic imaging that showed weakened areas of a subduction zone in Japan, Gerya told Live Science.

Kent Condie, a professor emeritus of geochemistry and Earth and environmental science at the New Mexico Institute of Mining and Technology who was not involved in the study, called their models "robust and meaningful."

When did it start?

The team also modeled what would have happened if Earth's interior were 270 degrees Fahrenheit (150 degrees Celsius) hotter, similar to temperatures it would have reached about a billion years ago. 

They found that in these simulations, the slab broke up only a few miles into the mantle, because it was unable to sustain its own weight in a mantle that was less viscous due to the hot conditions. So, unlike modern subduction that can continue for hundreds of millions of years, subduction back then would have ended very quickly,within a few million years, Gerya said. 

This finding suggests that modern plate tectonics may not have begun until sometime in the past billion years, he added. 

While a primitive form of plate tectonics may have existed between 3.5 billion and 2 billion years ago, during the Archean or Proterozoic eras, it was probably very different from what the planet experiences today, Gerya said. And around 1.8 billion to 1 billion years ago, there was a quiet period in which the plates were much less active. 

But this is just speculation, he said, and there is currently a lot of controversy surrounding when plate tectonics started. 

Condie agreed with Gerya. "Modern plate tectonics, with all the geologic indicators … probably did not begin until the last billion years," Condie told Live Science. But "plate tectonics in some form has been with us since at least 2 billion years ago."

Still, because we don't know the exact temperatures of Earth's core through time, it's not yet possible to give a precise timeline of when slabs stopped breaking apart and started a more continuous journey into the mantle, Condie said. 

That's really when modern plate tectonics began, Gerya said. The researchers now hope to explore the phenomenon and its relation to earthquakes, using more advanced 3D models. 

The findings were published Nov. 10 in the journal Nature.

Originally published on Live Science.