The Earth is made of layer upon layer of rock and metal – yet no human has ever seen more than the very outermost part of it. We have sent space probes past Pluto, but the deepest hole ever drilled into our own planet reaches only about 12 kilometres down. Compared to the whole Earth, that is like pricking an apple with a needle.
So what is actually down there? On the outside is the crust, the layer of solid rock we live on. It is between 5 and 70 kilometres thick, which sounds like a lot – but compared to the whole planet, it is as thin as the shell of an egg. Everything you have ever seen of the Earth is just that shell.
Beneath the crust lies the mantle: nearly 3,000 kilometres of hot rock. And here comes the surprise – the inside of the Earth is not a sea of lava. Almost everything down there is solid. The mantle glows with heat, but the weight of everything above squeezes the rock so hard that it stays firm, creeping along slower than your fingernails grow.
At the very centre sits a ball of iron and nickel. The outer part is so hot that the metal flows, and that flowing iron is what gives the Earth its magnetic field – the force that makes a compass needle point north. The inner core is even hotter, roughly as hot as the surface of the sun, yet it is solid, because the pressure down there will not let the iron melt.
You can hold a model of all this in one hand. Hard-boil an egg, let it cool, and ask an adult to cut it in half lengthways. The shell is the crust, the white is the mantle, and the yolk is the core. Tap the shell gently first so it cracks – those cracks look remarkably like the plates of the Earth's crust. What happens if you peel the egg and try to fit the shell pieces back together like a puzzle?
Three questions to wonder about together: How far down could you dig before it got too hot? What would happen to compasses if the flowing iron in the core stood still? And how can we know what a place is made of when no one has ever been there?
Every child is made of good atoms. At Good Atoms, we help them discover it. Explore free content at goodatoms.com.