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The North Pole of Venus. (Credit: SSV, MIPL, Magellan Team, NASA)

planets

Was Earth like Venus before plate tectonics?

Earth’s rigid upper layer called lithosphere is composed of moving plates. But just what mechanism first set plate tectonics into motion remains a mystery.

“Knowing what a chicken looks like and what all the chickens before it looked like doesn’t help us to understand the egg,” says Taras Gerya, a geophysics professor at ETH Zurich.

Earth’s lithosphere is divided into several plates that are in constant motion, and today’s geologists have a good understanding of what drives these plate movements: heavier ocean plates are submerged beneath lighter continental plates along what are known as subduction zones. Once the movement has begun, it is perpetuated due to the weight of the dense subducting plate.

Scientists still do not understand what triggered plate tectonics in the first place, or how the first subduction zone formed.

A weak spot in Earth’s lithosphere was necessary in order for parts of the crust to start to descend into Earth’s mantle. Was this weak spot caused by a gigantic meteorite that effectively smashed a hole in Earth’s lithosphere? Or did mantle convection forces shatter the lithosphere into moving parts?

Venus as a model

Gerya is not satisfied with any of these potential explanations.

“It’s not trivial to draw conclusions about what set the tectonic movements in motion,” he says.

Gerya began looking more closely at studies about the surface of Venus, which has never had plate tectonics. He observed (and modeled) huge, crater-like circles (coronae) on Venus that may also have existed on Earth’s surface in the early period (Precambrian) of Earth’s history, before plate tectonics began.

These structures could indicate that mantle plumes once rose from Venus’ iron core to the outer layer, thus softening and weakening the planet’s surface.

Plumes form in the deep interior of the planet. They rise up to the lithosphere, bringing with them partially molten mantle material that causes the lithosphere to weaken and deform. Halted by the resistance of the hard lithosphere, the material begins to spread, taking on a mushroom-like shape.

Such plumes also likely existed in Earth’s interior and could have created the weaknesses in Earth’s lithosphere needed to initiate plate tectonics.

Mantle plumes create weaknesses

Gerya and his team developed new computer models that he then used to investigate this idea for the first time in high resolution and in 3D. They published their results in Nature.

The simulations show that mantle plumes and the weaknesses they create could have initiated the first subduction zones.

[How mantle plumes crack continents]

In the simulations, the plume weakens the overlying lithosphere and forms a circular, thinning weak point with a diameter of several dozen to hundreds of kilometers. Over time this is stretched by the supply of hot material from the deep mantle.

“In order to make a ring larger, you have to break it,” he explains. This also applies to Earth’s surface: The ring-shaped weaknesses can (in the model) only be enlarged and subducted if the margins are torn.

Why water is an ‘absolute necessity’

The tears spread throughout the lithosphere, large slabs of the heavier rigid lithosphere plunge into the soft mantle, and the first plate margins emerge. The tension created by the plunging slabs ultimately sets the plates in motion.

They plunge, well lubricated by the buried seawater of the ocean above. Subduction has begun—and with it, plate tectonics.

“Water acts as a lubricant and is an absolute necessity in the initiation of a self-sustaining subduction,” says Gerya.

[Oceans of water may be locked deep inside Earth]

In their simulations, the researchers compare different temperature conditions and lithosphere states. They came to the conclusion that plume-induced plate tectonics could plausibly develop under the conditions that prevailed in the Precambrian around three billion years ago—when Earth’s lithosphere was already thick and cool, but the mantle was still very hot, providing enough energy to significantly weaken the lithosphere above the plumes.

Had the lithosphere instead being thin and warm, and therefore soft, the simulations show that a ring-shaped, rapidly descending structure called drip would simply have formed around the plume head. While this would have steadily sunk into the mantle, it would not have caused the soft lithosphere to subduct and tear and therefore would not have produced plate margins.

Likewise, the computer simulations show that under today’s conditions, where there is less temperature difference between lithosphere and plume material, plume-induced subduction is hard to initiate because the lithosphere is already too rigid and the plumes are barely able to weaken it sufficiently.

“Our new models explain how plate tectonics came about,” he says. Plume activity was enough to give rise to today’s plate mosaic. He calls the power of the plumes the dominant trigger for global plate tectonics.

Source: ETH Zurich

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