The patchy weather in the centre of the Earth

18 December 2015

The temperature 3,000 kilometres below the surface of the Earth is much more varied than previously thought, scientists have found.

The discovery of the hot and cold regions in the lower mantle where it meets the core, which are up to three times more varied than expected, will help scientists explain the structure of the Earth and how it formed.

"Where the mantle meets the core is a more dramatic boundary than the surface of the Earth," said the lead researcher, Associate Professor Hrvoje Tkalčić, a geophysicist in the ANU Research School of Earth Sciences.

"The contrast between the solid mantle and the liquid core is greater than the contrast between the ground and the air. The core is like a planet within a planet." said Associate Professor Tkalčić.

"The centre of the earth is harder to study than the centre of the sun."

Temperatures in the lower mantle reach 3,000-3,500 degrees Celsius and the barometer reads about 125 gigapascals, about one and a quarter million times atmospheric pressure.

Variations in these temperatures and other material properties such as density and chemical composition affect the speed at which waves travel through the Earth.

The team examined more than 4,000 seismometer measurements of earthquakes from around the world.

In a process similar to a CT scan, the team then ran a complex mathematical process to unravel the data and build the most detailed global map of the lower mantle, showing features ranging from as large as the entire hemisphere down to 400 kilometres across.

The map showed the seismic speeds varied more than expected over these distances. Associate Professor Tkalčić said the variations were probably driven by heat transfer across the core-mantle boundary and radioactivity.

"These images will help us understand how convection connects the Earth's surface with the bottom of the mantle," he said

"These thermal variations also have profound implications for the geodynamo in the core, which creates the Earth's magnetic field."

The research is published in the Nature Journal Scientific Reports.


An animation of the map is in dropbox

Image caption for map: Tomogram of the lowermost mantle (on top of core-mantle boundary, such as in our paper) centred on the equatorial region north of Australia. Green dots are stations and red dots are earthquakes near the Earths surface, the Earths mantle is transparent, and the ray paths through the interior are shown by solid lines. This image was made by our NCI Vizlab facility based on my data and the tomographic model of the lowermost mantle. You can see that the stations and earthquakes used in the tomographic inversion are not uniformly distributed across the surface. The blue regions are the regions of high velocity and the red regions show the low velocity.