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Scientists discover what happens to the Earth's vanishing crust.

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Plate tectonics may have only begun in the last billion years.


Earth's outer shell is formed of (less-tasty) rocky rafts that constantly bump into and dive beneath one other in a process known as plate tectonics, much like a gigantic broken-up cookie whose bits float atop a sea of hot milk.


So, what happens to those hunks of vanishing crust as they plunge into the milky interior of the Earth?


According to new modelling, they become weak and bendy, similar to a slinky snake toy, but they do not entirely dissolve. The simulations also revealed that plate tectonics, at least in its current form, probably only began in the last billion years.


Plate tectonics is responsible for earthquakes and volcanoes, the formation of mountain ranges and islands, and the separation of Earth's continents, which were once supercontinent.


 But much remains unknown about how plate tectonics works, such as what happens when one plate slides beneath another (in a region known as a subduction zone) and vanishes into the mantle, the planet's middle layer, which is, regrettably, not made of milk but rather of searing solid rock.


The researchers used 2D computer models of subduction zones and programmed them using known physics of how materials react, such as how rocks deform under particular forces, to figure this out. The researchers then observed the model see what happened near the subduction zone and compared their findings to real-world data.


According to their models, as one plate dipped beneath another, the descending piece, known as a slab, abruptly bowed downward and fractured; the bending also caused the grains on the plate's bottom to become finer and weaker. The forces left the plate essentially intact, but with numerous weak spots.


This implies that the plates do not break apart and hence continue to draw on the components behind them "for a very long period," according to lead author Taras Gerya, a geophysics professor at ETH Zurich in Switzerland. According to him, the plate can continue to slide beneath the other plate for hundreds of millions of years.


According to Gerya, their models matched observations and deep seismic imaging that revealed weakened portions of a subduction zone in Japan.


Kent Condie, an emeritus professor of geochemistry and Earth and environmental science at the New Mexico Institute of Mining and Technology who was not involved in the study, praised the researchers' models as "strong and useful."


When did it start?

The researchers then imagined what would have happened if Earth's interior had been 270 degrees Fahrenheit (150 degrees Celsius) hotter, as it would have been around a billion years ago.


They discovered that in these simulations, the slab broke up only a few miles below the mantle because it couldn't bear its weight in a less viscous mantle due to the hot circumstances. So, unlike modern subduction, which can last hundreds of millions of years, subduction back then would have finished fast, within a few million years, according to Gerya.


This discovery shows that contemporary plate tectonics did not begin until the last billion years, he noted.


While a primitive version of plate tectonics may have existed between 3.5 billion and 2 billion years ago, during the Archean or Proterozoic eras, Gerya believes it was quite different from what the world is experiencing today. And there was a period when the plates were significantly less active, between 1.8 billion and 1 billion years ago.


But this is simply a theory, he says, and there is now a lot of debate about when plate tectonics began.


Condie concurred with Gerya. Condie told Live Science that "modern plate tectonics, with all the geologic indications... probably did not begin until the last billion years." However, "plate tectonics has existed in some form for at least 2 billion years."


Still, because we don't know the exact temperatures of Earth's core over time, it's impossible to provide a precise timeframe of when slabs ceased breaking apart and began a more continuous journey towards the mantle, according to Condie.


Gerya believes that is when contemporary plate tectonics began. Using more powerful 3D models, the researchers intend to investigate the phenomena and their relationship to earthquakes.

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