How do we know what's in the planet?

09 April 2019

EARTH-SPACE

Earth from space

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How do we know what's inside the Earth? Chris Smith asked John Underhill to fill us in...

John - So one way of knowing is actually to drill into the Earth and take rock samples. And this was attempted a place called Kola in the Baltic Shield. The drilling of a nine inch wide borehole took place in 1989 but it only reached 12000 meters.

Chris - 12 km!? That’s a long hole.

John - Yeah. 7.6 miles deep.

Chris - How far into the crust is that though? How thick is the crust?

John - So the crust is different in different places but it can be from 10 kilometers to 100 kilometers. But they were in one of these cratonic shield areas we've been talking about before. So in the center of one of the continents where it tends to be thickest.

Chris -  So a good hundred kilometers, so they're literally through 10 percent of the earth.

John - So that is the deepest artificial point on Earth. So we actually, to understand the composition of the Earth, of the planet, we have to rely on remote sensing. So that's using techniques like gravity like magnetics. But by far and away the most effective method has been seismology, and it's to use natural earthquakes that are set off on the Earth's surface or in the subsurface and to see the way in which those sound waves that have been generated by the natural earthquakes, how they pass through the planet and what we're looking at is variations in velocity. We're looking at how changes are recorded as that sound wave passes through the Earth. What that shows us is changes in rock properties, particularly because rocks change under temperature and under pressure. So we have seismograms all the way around the Earth to actually record earthquakes. And that allows us to build up a seismic velocity map of the internal parts of the Earth. And so we can split that out into several different pieces. Basically we see the Earth is layered in sort of spherical shells for want of a better word. We've got the outer part which is the crust as you say Chris. That's actually less than 1 percent of the Earth's volume.

Secondly as we go deeper we go into the mantle and we have a sudden increase in seismic velocity and then inside that we have the core. So the centre of the earth is the core, the mantle makes up a mass of about two thirds of the Earth.

Chris - So that's the lion's share. I was very lucky actually I was in Croatia in Zagreb and I went to the lab of Mohorovičić, who is the guy who did all the amazing calculations to about five or six decimal places by hand in the eighteen hundreds to work out a lot of these sound wave propagations. He was using earthquakes to actually try to do this.

John - That's right. And that Moho is actually the boundary between the mantle and the crust.

Chris - They've got all his old notebooks so it was a real privilege to see that.

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