What's actually down there if you dig straight through the Earth?
The deepest hole ever drilled stops at about 12 kilometres, on a planet whose centre is 6,371 kilometres down, yet scientists can describe its insides layer by layer, because earthquakes do the digging for them.
▶ Start the storyA stack of nested layers, found not by digging but by listening to earthquakes. The centre is 6,371 kilometres down. The deepest hole ever drilled, in Russia, stopped at about 12. So almost everything we know comes from indirect clues: seismic waves, gravity and magnetic measurements, and rocks that volcanoes carry up from below.
By what it is made of, Earth has three main layers. A thin rocky crust, a thick mantle rich in iron and magnesium, and an iron-nickel core. By how the rock behaves, the slicing is different. A rigid outer shell, the lithosphere, rides on a soft, slowly flowing layer, the asthenosphere. Deeper down sit a stiffer lower mantle, a liquid outer core and a solid inner core. The crust itself is thin and uneven: 5 to 10 kilometres under the oceans, up to 70 under the continents.
By chemistry
- Silicate crust
- Iron-magnesium mantle
- Iron-nickel core
By physical behaviour
- Rigid lithosphere
- Flowing asthenosphere
- Liquid outer core, solid inner core
The first boundary was found in 1909. Croatian seismologist Andrija Mohorovičić studied an earthquake near Zagreb. His records showed two sets of waves. One had travelled a direct path. The other had been bent by faster rock deeper down. From the timing, he put that boundary about 54 kilometres down. We still call it the Moho, after him.
Below it, the mantle is the thickest layer by far, reaching 2,890 kilometres down. It is solid rock, yet it flows very slowly, heated by radioactive decay and heat left from Earth's birth. That slow churning drags the tectonic plates around. Under the mantle lies a liquid outer core of iron and nickel. At the very centre sits a solid ball about 1,220 kilometres in radius. Danish seismologist Inge Lehmann spotted it in 1936. Some earthquake waves seemed to reflect off something inside a core everyone thought was all liquid. Within a few years, the leading seismologists agreed with her.
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Recap
Crust, mantle, outer core, inner core: four layers found entirely by listening to how earthquakes shake the planet, not by digging.
Surprising fact · The solid inner core, discovered only in 1936, sits inside a liquid outer core and was found purely from the behaviour of seismic waves, not from any physical sample.
Sources (5)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.