Unveiling the Hidden Link Between Earth's Core and Plate Tectonics

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An ancient protective shield: The early Earth's magnetic field

The oldest Earth minerals are 3.4 billions years old, and they recorded a paleo-magnetisation, meaning that Earth had a magnetic field for at least this age. This field is generated deep within the planet by the convection-driven movement of liquid iron in the outer core. But to keep this magnetic field going for billions of years, the core needs to cool efficiently. Core cooling mainly depends on the cooling of the solid mantle above.

Today, the mantle cools mainly through plate tectonics, the movement of large plates on Earth’s surface. But when did plate tectonics actually begin? Did it start soon after Earth formed, around 4.5 billion years ago? Did it appear later, between 4 and 3 billion years ago? Or is it a more recent process, less than a billion years old?

Modelling the interconnected system of core and mantle

Valentin Bonnet together with Nicola Tosi explored how different styles of mantle cooling would have influenced Earth’s thermal and magnetic history. They explored either a mobile surface like modern plate tectonics or a less efficient, like a stagnant lid (where the surface doesn’t move). This is important because how Earth’s mantle cooled over time is closely tied to its ability to keep generating a magnetic field.

To investigate this, Valentin built a global model for the Earth coupling two models: one that calculate how the core evolves, including the formation of the solid inner core, and another that simulates how the mantle behaves under different cooling regimes.

Peering into the past: Viscosity, cooling, and thermal conductivity scenarios

He tested different scenarios: how viscous the mantle is, how effective each cooling style is, when plate tectonics might have started, and how hot the mantle and core were at the beginning. Additionally, he considered two possible values for how well iron conducts heat since that’s still a matter of debate. Then he compared our model’s predictions with different observations like the size of the inner core, the ancient magnetic record, and how mantle temperature has changed over time.

By bringing all this together, Valentin's study helps explain how Earth’s deep interior and surface processes have worked in tandem to sustain its magnetic field for billions of years. The result shows an interesting co-variance between the age of the onset of plate tectonic and the thermal conductivity of the core.