A surprise at Earth’s centre? Image Credit: Kelvinsong via Wikipedia, edited by S. Sutherland
Scott Sutherland
Monday, February 9, 2015, 4:16 PM – Crack the Earth open and you’ll find a surprise inside. It turns out that the classic four part interior – crust, mantle, outer core, inner core – is missing a layer, as scientists discover a core-within-a-core at the heart of our planet.
Earthquakes can range from barely-noticeable tremors to shaking strong enough to wreak havoc across a countryside, but one thing they have in common is that they send waves of vibrations passing through the interior of our planet.
Scientists learned long ago to read these vibrations, and they used the difference in the timing of when these waves reach detectors on the surface to solve a long-standing mystery about what the inside of the Earth looks like. The reason for the this difference in arrival time has to do with two factors, the speed of the waves as they move through materials of different densities (solid rock vs molten rock vs liquid iron vs solid iron), and due to the waves bouncing off the boundaries between the different layers.
 Seismic wave paths. Courtesy: Wikimedia Commons |
These observations led to the development of the four-part model that is now featured in every textbook, which is shown to the right.
However, new research is showing that this model is not quite complete, as a team of geologists led by Xiaodong Song and Tao Wang, working at the University of Illinois, has discovered that the Earth’s solid iron inner core actually has an inner core of its own.
“The fact that we have two regions that are distinctly different may tell us something about how the inner core has been evolving,” Song said in a University of Illinois news release. “For example, over the history of the earth, the inner core might have had a very dramatic change in its deformation regime. It might hold the key to how the planet has evolved. We are right in the center – literally, the center of the Earth.”
An inner-inner core? Image Credit: Kelvinsong via Wikipedia, edited by S. Sutherland
The researchers discovered this by looking past the shockwave generated at the start of earthquakes, to carefully examining the vibrations and resonances that pass through the Earth from the start of the quakes, all the way until they completely die down – the quakes’ coda. Looking over 20 years worth of earthquake data, from 1992 to 2012, the researchers found that the codas revealed a distinctly different inner-inner core, roughly half the diameter of the entire solid core. Not only do the iron crystals that make up this inner-inner core behave differently from those in the outer-inner core, the crystals in both layers are oriented in very different directions. While the crystals in the outer-inner core are oriented in a North-South direction, parallel to Earth’s axis, the iron crystals of the inner core point East-to-West, with the ‘axis’ of this inner-inner core pointing towards Central America and Southeast Asia.
Why is studying the core of the Earth important?
“Even though the inner core is small – smaller than the moon – it has some really interesting features,” Song explained in the press release. “It may tell us about how our planet formed, its history, and other dynamic processes of the Earth. It shapes our understanding of what’s going on deep inside the Earth.”
Sources: University of Illinois
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