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Earth’s heart is like solid gold

By David, 25 October 2018 News

Image of half a planet with three central glowing layers. The Earth's core being the brightest.

Earth is made of many layers of increasingly hot and liquid rock, with a metal core.Image: ©iStock.com\forplayday

You might not think about it much, but there’s an amazing planet under your feet. Dig down about 50 kilometres and the rocky crust begins to give way to the hot, soft mantle. Deeper down, the rocks get hotter and runnier, reaching temperatures of thousands of degrees. At the very centre, there’s a solid core of ultra-hot metal.

At least, that’s the textbook explanation. But there’s no way to go and visit the layers of Earth – the deepest hole ever bored was only 12 kilometres down. Which means if you want to learn more, you need a different approach.

You don’t always need to dig or cut to see inside something. A gentle way to look inside a human body is to use an ultrasound scan. Vibrations are passed through the body, and a trained expert can see inside by looking at the echoes on a computer. Geologists also use vibrations to peer deep into the Earth.

There are vibration detectors placed all over our planet, known as seismometers. These detectors are constantly listening in, producing records known as seismograms for geologists to analyse. Sometimes seismometers detect thunder or a nearby bouncing kangaroo. Occasionally, they detect a different vibration, something a lot larger: an earthquake.

Associate Professor Hrvoje Tkalčić, a geophysicist at the Australian National University, is an expert at analysing Earth’s vibrations. Hrvoje and his team analysed echoes inside our planet to take a look at Earth’s core.

a slice of the Earth with layers labelled: crust 0-35 km, lithosphere 35-100 km, asthenosphere 100-700 km, mantle 700-2890 km, outer core 2890-5100 km, inner core 5100-6378 km

Echoes of earthquakes have revealed many layers hiding beneath Earth’s surface.Image: ©iStock.com\vectortatu

The analysis is so sensitive, that big earthquakes can be a bit too loud. “We’re throwing away the first three hours of the seismogram,” says Hrvoje. “What we’re looking at is between three and 10 hours after a large earthquake happens. We want to get rid of the big signals.”

The team detected a type of vibration, known as shear waves, travelling through Earth’s inner core. These waves can only travel through solids, which means the centre of Earth must also be solid. But that wasn’t all they discovered.

“We found the inner core is indeed solid, but we also found that it’s softer than previously thought,” says Hrvoje. “It turns out – if our results are correct – the inner core shares some similar elastic properties with gold and platinum.”

So Earth has a gold-like heart. Surprisingly poetic!

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