How does the Earth's crust work?

Where did the Earth's crust go?

Modern plate tectonics probably only began in the last billion years.

Silfra Hall between two tectonic plates in Singvilliers, Iceland.

Like a giant crumbled cookie floating in a sea of boiling milk, the Earth's crust is made up of (not so tasty) rock rafts that constantly bump and crush against each other in a process known as plate tectonics.

So what happens when this lost crust dives into Earth's milky interior?

The new model shows that they become brittle and bent, like a slinky toy, but they don't completely disintegrate. The models also suggest that plate tectonics may have begun only in the past billion years, given modern plate forms.

Plate tectonics drives earthquakes and volcanic eruptions, creating mountains and islands, as well as transforming what was once a single Earth continent into what is now separated by oceans. But there are still a lot of unknowns about how plate tectonics works, such as what happens when one plate slides under another (in an area called a subduction zone) and disappears into the mantle, a process in which hard rock constantly pushes against each other, not like the metaphorical milk.

To figure this out, the researchers used 2D computer models of subduction zones and programmed them using what is known about the behavioral physics of materials, such as how rocks deform under certain forces. They then looked at the model to understand what was happening at the subduction zone and compared their findings with real-life observations.

Their model suggests that when one plate subducts beneath another, the descending plate suddenly bends downward and breaks apart. Bending also results in finer and weaker grains on the underside of the plate. The pressure left the upper plate largely intact, but with many weak spots.

That means plates don't break apart and therefore keep pulling what's left of them, says lead author Taras Gerya, a professor of geophysics at ETH Zurich in Switzerland. One plate can slide over the other for hundreds of millions of years.

Gerya told Live Science that their simulations match observations and deep seismic imaging of weak areas in the Japanese subduction zone.

Kent Condie, a professor emeritus of geochemistry and Earth and environmental sciences at the New Mexico School of Mines and Technology, who was not involved in the study, called their model "robust and meaningful."

When does it start?

The team also simulated what would happen if the Earth's interior warmed by 270 degrees Fahrenheit (150 degrees Celsius), similar to temperatures around a billion years ago.

They found that in these simulations, the plate broke up only a few miles into the mantle because it was unable to maintain its weight in a less viscous mantle due to hot conditions. So unlike modern subductions, which can last for hundreds of millions of years, those would have ended quickly within a few million years, Gerya says.

The discovery suggests that modern plate tectonics probably did not begin until sometime in the last 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, it would have been very different from what Earth is experiencing today, Gerya said. About 1.8 billion to 1 billion years ago, there was a quiet period when the plates were much less active.

But that's just speculation, he says, and there's a lot of controversy right now around when plate tectonics started.

Condie agreed with Gerya. "Modern plate tectonics with all the geological indicators... Probably didn't start until the last billion years, "Condie told Live Science. But "some form of plate tectonics has been with us since at least two billion years ago."

Condie said, though, that because we don't know the exact temperature of the Earth's core over time, it's not yet possible to give an accurate timeline of when the plates will stop breaking apart and begin a more continuous journey into the mantle.

This is indeed when modern plate tectonics began, says Gerya. The researchers now hope to use more advanced 3D models to explore the phenomenon and how it relates to earthquakes.

The relevant knowledge

In geography, the crust is the solid, thin crust of the outermost layer of a planet, which can be chemically distinguished from the mantle. The crust of the Earth, Moon, Mercury, Venus, Mars and other planets is mostly formed from igneous rocks, and the planets' crusts have more incompatible components than their mantles.