What's inside the Earth?
Four layers — a thin rocky crust, a thick mantle, a liquid outer core and a solid inner core. But the names are the easy part. Below is a short interactive powerbook that walks the actual chain of evidence — the way Mohorovičić, Gutenberg and Lehmann worked it out — so by the end you don't just know the answer, you can explain how anyone knows it.
The interactive powerbook above is the lesson — read it section by section with the Next button. What follows is a plain recap and the sources, for when you only need the facts.
The four layers, at a glance
Sliced by composition, surface to centre.
There is a second, equally valid way to divide the Earth — by how it behaves rather than what it is made of. That split gives the rigid lithosphere and the slowly-flowing asthenosphere beneath it, and it is the one that matters for plate tectonics.
How we actually know
No one has sampled the mantle, let alone the core. The deepest borehole ever drilled — the Kola Superdeep Borehole in Russia, stopped at 12,262 m in 1989 — got roughly 0.19% of the way to the centre before the rock grew too hot and plastic to drill. Everything below that is inferred, and almost all of it from earthquakes.
The boundaries were found one by one. In 1909 Andrija Mohorovičić spotted the crust–mantle boundary (the “Moho”) from waves arriving in two batches. Around 1913 Beno Gutenberg used the S-wave shadow zone (roughly 105°–140° from a quake) to place the liquid outer core at ~2,890 km. And in 1936 Inge Lehmann, noticing faint P-waves where there should have been silence, discovered the solid inner core at ~5,150 km.
Two more lines of evidence lock it in. The whole Earth averages about 5.5 g/cm³ while surface rock is only ~2.7, so the interior must be dense metal — and meteorites come in iron-rich and rock-rich types that match a metal core in a rocky shell. Finally, the churning of that liquid-iron outer core acts as a geodynamo, generating the magnetic field that moves a compass and shields the planet.
For HSC Earth & Environmental Science students
This powerbook covers the “Structure of the Earth” thread of Module 1: Earth's Resources in the NSW Stage 6 course: modelling the compositional layers, using seismic-wave and meteorite/density evidence to justify them, and the differentiation of a molten early Earth. The lithosphere–asthenosphere split it ends on is the hinge into Module 2: Plate Tectonics.
It's one book in a course we're building thread by thread. See the full map on the Earth & Environmental Science hub.
Sources & notes
- Deepest borehole: the Kola Superdeep Borehole (SG-3) reached 12,262 m true vertical depth in 1989 and remains the deepest; the centre of the Earth is ~6,371 km down. (Kola Superdeep Borehole, Wikipedia.)
- Crust–mantle boundary (the “Moho”): identified by Andrija Mohorovičić, 1909.
- Core–mantle boundary (Gutenberg discontinuity, ~2,890 km, liquid outer core below): Beno Gutenberg, ~1913, from the S-wave shadow zone of ~105°–140°. (Gutenberg discontinuity, Wikipedia.)
- Solid inner core (Lehmann discontinuity, ~5,150 km): Inge Lehmann, 1936, in a paper titled “P′”. (Inge Lehmann, Encyclopaedia Britannica.)
- Earth's mean density ~5.5 g/cm³ vs ~2.7 for surface rock; the liquid outer core drives Earth's magnetic field (the geodynamo). Central temperature ~5,200 °C and pressure ~3.5 million atmospheres are standard estimates.
- Course framing follows the NSW NESA Stage 6 Earth and Environmental Science Syllabus (2017). Explanations here are original and not a reproduction of the syllabus; this site is not affiliated with NESA.
We will never reach the centre of the Earth — and yet, from a handful of clues that reach the surface, three people across thirty years described it down to the last solid, crushed-together inch.