Venus as a model: Today this planet looks like the Earth might have looked before the onset of plate tectonics. Credit: NASA/JPL
It is common knowledge that Earth’s rigid upper layer called lithosphere is composed of moving plates. But just what mechanism first set plate tectonics into motion still remains a mystery. Scientists have now come up with one possible answer with simulations. But just as in the past, earth scientists still do not understand what triggered plate tectonics in the first place, nor how the first subduction zone was formed. A weak spot in the Earth’s lithosphere was necessary in order for parts of the Earth’s crust to begin their descent into the Earth’s mantle. Was this weak spot caused by a giganticmeteorite that effectively smashed a hole in the Earth’s lithosphere? Or did mantle convection forces shatter the lithosphere into moving parts?
Gerya is not satisfied with any of these potential explanations. He found inspiration in studies about Venus, which has never had plate tectonics. Gerya observed (and modelled) huge, crater-like circles (coronae) on Venus that may also have existed on the Earth’s surface in the early period (Precambrian) of the Earth’s history before plate tectonics even 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 hot 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.
High res 3D simulations show that mantle plumes and the weaknesses they create could have actually initiated the first subduction zones. 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 kilometres. This is stretched over time by the supply of hot material from the deep mantle. “In order to make a ring larger, you have to break it,” explains the researcher. This also applies to the Earth’s surface: the ring-shaped weaknesses can (in the model) only be enlarged and subducted if the margins are torn.
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.
In simulations, they 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 3 billion years ago. Back then the 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.
“Our new models explain how plate tectonics came about,” says the geophysicist. 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. The simulations can also explain how triple junctions, i.e. zones in which 3 plates come together, are nucleated by multi-directional stretching of the lithosphere induced by plumes. Eg in the Horn of Africa where Ethiopia, Eritrea and Djibouti meet.
A possible plume-weakened zone analogous to a starting point for global plate tectonics likely exists in the modern world: the researchers see such a zone in the Caribbean plate. Its shape, location and spread correspond largely to the new model simulations.
https://www.ethz.ch/en/news-and-events/eth-news/news/2015/11/plate-tectonics-thanks-to-plumes.html
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