Strange “Weather” Observed Near the Center of the Earth

December 18, 2015 | Joanne Kennell

Cross section of Earth
Photo credit: Argonne National Laboratory/Flickr (CC BY-BC-SA 2.0)

Temperatures 3,000 kilometers below the surface plays an important role in Earth’s magnetic field.

Earth’s magnetic field is a little bit of a mystery — although it is believed that it is generated by the Earth’s core, nobody has ever been able journey to the center of the Earth to find out for certain.  What scientists use instead to determine the structure of the Earth is how shockwaves from earthquakes travel through the planet.

If you didn’t already know this, Earth is made up of several layers — like an onion.  The surface of the Earth, what we live on, is called the crust.  It has a thickness ranging from 3 to 44 miles (5 to 71 kilometers) and makes up a mere one percent of the entire volume of the planet.  

Below the crust, is the mantle, which is separated into upper and lower parts.  The upper mantle makes up 84 percent of Earth’s volume, and it is predominantly solid but it acts as a viscous fluid on a geological timescale — meaning that it moves, just very slowly.  This is the layer responsible for the movement of tectonic plates.

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Next is the lower mantle consisting mainly of silicon and magnesium, followed by the outer core which is a liquid layer of iron, nickel and small quantities of other metals.  Finally we have the inner core, a region of pure, solid iron.  It is the flow of liquid iron in the outer core that generates electric currents and in turn Earth’s magnetic field.

Not much is known about the lower mantle, however, it was previously thought that the temperatures within the lower mantle region were fairly uniform.  As it turns out, temperatures 1,865 miles (3,000 kilometres) below the surface of the Earth are really diverse according to a new study out of the Australian National University (ANU).  In fact, they are three times greater than expected, and this is important for the production of Earth’s magnetic field.

"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 Hrvoje Tkalcic, a geophysicist in the ANU Research School of Earth Sciences.  These strong variations in temperature and other properties such as density and chemical composition affect the speed of waves traveling through the planet, which help scientists understand the structure of Earth and how it formed.

Measuring speeds of molten metal in Earth's hot outer core
Photo credit: Hrvoje Tkalcic

The team made the discovery by examining more than 4,000 seismometer measurements of earthquakes from around the planet and used mathematics to build a detailed map of the lower mantle (seen in the above image).  Blue are regions of high velocity and the red regions show the low velocity.  The map surprisingly showed that seismic speeds varied more than expected, driven by the temperature differences between the core and mantle.

“These images will help us understand how convection connects the Earth's surface with the bottom of the mantle,” said Tkalcic.  “These thermal variations also have profound implications for the geodynamo in the core, which creates the Earth's magnetic field.”

So what does that mean for Earth’s magnetic field?  Currently, scientists are not sure, but don’t worry — the magnetic field going to be sticking around for a very long time.

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