What would half the Earth look like?

02 August 2016


If we could slice the Earth in half and leave the inner or outer core intact (like cutting an avocado), what would it look like from space?
Miami, Florida, USA.


Lucka Bibic asked Professor Marian Holness from University of Cambridge to find an answer to Robert's question...

If we can slice the Earth in half and leave the inner and outer core intact like cutting the avocado, what would it look like from space?

Lucka - For a glimpse of what Earth would look like from space, we turned to Earth expert Dr Marian Holness from the University of Cambridge.

Marian - Well the Earth is about 6,400 kilometres in radius. The crust, which is the bit we walk on and know most about ranges from about 10 kilometers thick to about 70 kilometres thick, so that's absolutely nothing really. So, if we go further down, we come to the mantle, which goes all the way down to 3,000 kilometres depth below the surface, and that's predominantly made of silicate minerals (magnesium, iron silicates), In the shallowest part of the mantle, that is dominated by a mineral called olivine and perhaps, paradoxically, if you look at all the text book pictures with that slice through the Earth, the mantle was always shown as red, presumably because it's hot, but actually the mineral olivine is green. So if it had cooled down, we'd see from space this great mass of green rock.

Lucka - Life underneath the Earth's mantle is actually green then. What can we see if we dig even deeper into the Earth's core?

Marian - If we go further down still, we get to the core and the core is made predominantly of iron and nickel. And since it's been cooling down ever since the Earth was formed, the centre of the core is actually solid now - it's been crystallised. So we've got this inner core that's solid and the outer core that's convecting.

Lucka - Alright then. But how did you figure out what is actually going on over 6,000 kilometres underground?

Marian - We can find out how heavy the Earth is and we know how big it is. So we can look at the rocks on the surface and we know what their density is, and we know that there must be something much, much denser in the centre of the Earth to make the Earth as heavy as it actually is for its size.

The next thing we do is look at meteorites that are coming to the Earth's surface and a lot of those meteorites that come are made purely of iron. So we can say, ah yes, we've got all these bits of iron coming down from the sky so it's likely that what we've got in the centre of our Earth is something very much like this.

Now how big is it? The question can be answered by listening to the Earth. So when the Earth experiences an earthquake, it rings like a bell and it sends seismic waves all the way through the Earth. And we can pick up those seismic waves using seismometers scattered all the way through the Earth, and we can tell how fast the waves are moving through the Earth and what type they are. So we can just piece it together from observations of meteorites and by listening to the Earth.

Lucka - So if we slice the Earth as we slice an avocado and see it from outer space, what would we see then?

Marian - We'd see this thinnest of thin skin, which is the crust, and then we'd see this mass of green mantle and then, in the centre, we'd get this beautiful, shiny, metallic core.

Lucka - Thanks Marian for explaining to us how Earth shows us its true colours!

Next week we will be answering Robert's question:

Is it possible to unlearn what we've already learnt?

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