Earth's Interior Cooling Faster Than Expected, Say Scientists

The evolution of our Earth is the story of its cooling: 4.5 billion years ago, extreme temperatures prevailed on the surface of the young Earth, and it was covered by a deep ocean of magma.

Photo Insert: An optical absorption measurement system in a diamond unit heated with a pulsed laser is used for the measurements.

Over millions of years, the planet's surface cooled to form a brittle crust. However, the enormous thermal energy emanating from the Earth's interior set dynamic processes in motion, such as mantle convection, plate tectonics, and volcanism.

Still unanswered, though, are the questions of how fast the Earth cooled and how long it might take for this ongoing cooling to bring the aforementioned heat-driven processes to a halt. One possible answer may lie in the thermal conductivity of the minerals that form the boundary between the Earth's core and mantle, ScienceDaily reported.

This boundary layer is relevant because it is here that the viscous rock of the Earth's mantle is in direct contact with the hot iron-nickel melt of the planet's outer core. The temperature gradient between the two layers is very steep, so there is potentially a lot of heat flowing here.

The boundary layer is formed mainly of the mineral bridgmanite. However, researchers have a hard time estimating how much heat this mineral conducts from the Earth's core to the mantle because experimental verification is very difficult.

Now, ETH Zurich Professor Motohiko Murakami and his colleagues from Carnegie Institution for Science have developed a sophisticated measuring system that enables them to measure the thermal conductivity of bridgmanite in the laboratory under the pressure and temperature conditions that prevail inside the Earth.

For the measurements, they used a recently developed optical absorption measurement system in a diamond unit heated with a pulsed laser. "This measurement system let us show that the thermal conductivity of bridgmanite is about 1.5 times higher than assumed," Murakami says.

This suggests that the heat flow from the core into the mantle is also higher than previously thought. Greater heat flow, in turn, increases mantle convection and accelerates the cooling of the Earth. This may cause plate tectonics, which is kept going by the convective motions of the mantle, to decelerate faster than researchers were expecting based on previous heat conduction values.

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