Interior of
the Earth
The deepest hole humans have ever drilled is just 12 km. The Earth's centre is 6 378 km away. We've never been there — and we never will. So how do we know there's a solid iron-and-nickel core spinning inside our planet? Two clues: earthquake waves, and a 60-tonne meteorite sitting in a paddock in Namibia.
In Russia there's a rusty steel cap bolted into a remote patch of tundra. It seals the Kola Superdeep Borehole — the deepest hole humans have ever drilled. After 25 years of drilling, the project gave up at 12 262 m. The interior of the Earth was still 6 366 km further down. So how do scientists describe the layers below — when nobody has been there?
By being detectives. We can't see inside the Earth, but we can listen. Every earthquake sends out shockwaves that travel through the planet's interior. Seismometers all around the world record those waves — and the way they speed up, slow down, bend, and stop tells us exactly where one layer ends and another begins.
Meteorites help too: their iron-nickel chemistry matches what we think the Earth's core is made of. Volcanic lava brings up bits of the upper mantle. Earth's gravity, magnetic field and rotation all give clues. eight different lines of evidence, all pointing to the same picture: a planet built like an onion, with a solid iron core, a liquid metal layer, a plastic-flowing mantle, and a thin, broken crust on top.
An Onion-Shaped Planet
Earth is a series of concentric layers, sorted by density. We know this without having been there.
When the early Earth heated up enough to become molten (covered in Topic 9), heavy elements like iron and nickel sank to the centre under gravity. Lighter materials — silicon, aluminium, oxygen — floated up. As the planet cooled, the layers froze in place. Geologists call this density stratification. The result is a planet built in concentric layers, each one denser than the one above it.
The layers, from the outside in: lithosphere (crust + rigid upper mantle), asthenosphere (hot, plastic-flowing rock), mantle (semi-fluid silicate rock), outer core (liquid iron-nickel) and inner core (solid iron-nickel). Each one has a different density, temperature, state and chemistry.
Why a marine scientist studies the inside of the planet
This is the one topic in the sequence that looks like it belongs in a geology course. It does not, and the reason is in your booklet's own introduction: most of the Earth's volcanic and earthquake activity happens under the sea, not on land.
The convection currents in the mantle below are the force that moves tectonic plates. Where plates pull apart, molten rock rises and builds a mid-ocean ridge — the longest mountain chain on the planet, and almost entirely underwater. Where they collide, one dives beneath the other and opens an ocean trench. Undersea earthquakes on those boundaries generate tsunamis.
So the interior of the Earth is what produced the sea floor you mapped in Topic 3, and it is the engine behind everything in Topic 11. Understanding the layers is how you explain the shape of the ocean basin.
Inside the Planet
Below: a cross-section of Earth showing all five layers. Tap each one to see what it's made of, how thick it is, what state it's in, and how scientists figured it out.
This diagram is deliberately not to scale — and that is worth understanding, because it is the single biggest thing a cross-section misleads you about. Earth's radius is 6 371 km. The crust is at most 100 km of that, which is 1.6%. Drawn honestly at the size of the picture below, the crust would be a line about one pixel wide — thinner than the outline around it, and impossible to label. So it is drawn thick enough to read. Keep the real proportion in your head: if Earth were a hard-boiled egg, the crust would be thinner than the shell.
Local connection — Hoba meteorite, Namibia
About 80 km west of Grootfontein in Namibia sits the Hoba meteorite — the largest known intact meteorite on Earth. It weighs about 60 tonnes, measures 2.7 m on a side, and is mostly iron and nickel. Its chemistry is one of the most direct lines of evidence for what Earth's core is made of: meteorites like Hoba formed in the cores of larger asteroids that had gone through their own density stratification, just like Earth did. They're chunks of "core" we can actually walk up and touch. Closer to home, the Pilanesberg in North-West Province is the eroded remnant of an extinct volcano — its concentric ring structure, visible from satellites, gives us a direct window into what once sat in the upper mantle.
How We Know What's Down There
Eight different lines of evidence, all pointing to the same picture. Scientists are detectives — they use every clue they can get. Only the first is direct; everything else is inference, and that distinction is worth holding on to.
Drill a hole
Direct sampling. The Kola Superdeep Borehole (Russia) reached 12 262 m between 1970 and 1995 before giving up — the rock was over 180 °C, hotter than expected, and behaved more like plastic than rock. That is 0.2% of the way to the centre.
Study lava
Indirect sampling. Volcanoes erupt material from the upper mantle. Cooled lava lets us read the chemistry of layers we can't reach.
Analyse meteorites
Cosmic comparison. Iron-nickel meteorites like Hoba match what scientists expect Earth's core to be — chemical fingerprints align.
Compare rocks
Surface clues. Olivine and pyroxene found in volcanic basalt are the same minerals expected in the lithosphere.
Measure the planet
Bulk properties. Earth's mass, density, gravity, rotation and magnetic field can only be explained by a specific layered interior.
Listen to earthquakes
Seismic waves. The biggest tool of all — see the next section.
Read a dead volcano
Indirect. A dormant or extinct volcano exposes rock that was once deep in the upper mantle. The Pilanesberg complex is a direct window into material we could never drill to.
Listen to sound in rock
Indirect. Sound travels at different speeds through different materials, and faster through denser rock. Timing how sound moves through the Earth is how the layers were sized.
Three kinds of seismic wave
Earthquakes send out three types of wave. Each behaves differently in solids, liquids and gases — and that's how scientists figured out the outer core is liquid and the inner core is solid without anyone going there.
Primary waves
Fastest. First to arrive at any seismograph. Pressure waves — they push and pull. Travel through solids, liquids AND gases.
Secondary waves
Slower than P. Shear waves — they shake side-to-side. Travel through solids only. Stopped completely by liquids and gases.
Longitudinal/surface waves
Slowest. Travel along Earth's surface, not through the interior. Cause most of the visible damage in earthquakes.
How S-waves proved the outer core is liquid
When a big earthquake happens, every seismograph in the world records it. Scientists noticed a pattern: S-waves never reach the far side of the Earth. There's a "shadow zone" past about 105° from the epicentre where S-waves disappear completely. That can only mean one thing — they hit a liquid layer along the way and stopped. That's how we know the outer core is liquid metal. P-waves still come through, but they bend (refract) at the boundary, and they slow down too. Decades of patiently mapping these arrival patterns is how we know the size and depth of every layer.
In the Lab — the CAPS Investigations
CAPS sets one investigation for this block and it is completely explicit: "Draw a diagram to illustrate the interior of the earth. Label its parts inner core, mantle, and lithosphere." The Resources column adds "sample rocks (eg igneous rock and granite)". Both are on the practical sheet.
Reading this page is not the same as being able to draw it
The Depth column says the content is for "reproduction in diagrams or a paragraph description". Reproduction is the word that matters — you are examined on producing the diagram, not on recognising it. Tapping five layers on a screen will not get you there. Draw it, from memory, on blank paper, and then check.
- 🟢 desk Draw and label the interior of the Earth — from memory, onto a blank circle, with depths, states and composition. Then a second attempt against the clock.
- 🟢 desk Eight methods, one table — sort the evidence into direct and indirect, and work out why only one of the eight is direct.
- 🟢 kitchen 🌊 shore Sample rocks — find and identify igneous rock and granite, and work out what each tells you about where it came from.
- 🟢 desk The shadow zone — use supplied seismic arrival data to prove for yourself that the outer core must be liquid.
→ Open the Topic 10 practical sheet — printable, with supplied data for every task.
Key Vocabulary
Tap each card to flip it. These eight terms are examinable.
The rigid outer layer of Earth — the crust plus the uppermost solid mantle. About 60–100 km thick. Contains the 15 major tectonic plates that drift around on the asthenosphere.
A zone of the upper mantle just below the lithosphere where rock is so hot it flows like plastic, even though it's still technically solid. About 80–200 km deep. The "lubricated" layer that lets tectonic plates move.
The thick semi-fluid layer between the crust and the core, reaching ~2 900 km deep. Made of silicate rocks rich in magnesium and iron. Convection currents here drive plate tectonics.
Earth's iron-and-nickel centre. Two parts: a liquid outer core (~2 260 km thick — your CAPS notes give 2 180 km; either is accepted) that generates Earth's magnetic field, and a solid inner core (~1 200 km radius), kept solid by crushing pressure despite being as hot as the Sun's surface. The inner core also spins slightly faster than the surface.
The process by which Earth's interior settled into layers by density when it was still molten. Heavy elements (iron, nickel) sank; lighter ones (silicon, oxygen) floated up. Why each deeper layer is denser than the one above.
An instrument that measures and records ground motion caused by earthquakes or explosions. Connected to a seismometer (the sensor) and produces a seismogram showing wave intensity and timing.
The shockwaves released by earthquakes (or human-made explosions). Three types: P, S, and L. Their reflection, refraction and speed through different materials give scientists their best evidence for Earth's layering — and, because wave speed depends on what the rock is like, for the density and temperature variations inside it too.
An insoluble mineral group containing silicon and oxygen. Includes quartz, feldspar, mica, olivine, pyroxene. Silicates make up most of the mantle and crust — they're the building blocks of Earth's solid layers.
The study of shock waves travelling through the Earth — earthquakes, and deliberate human-made explosions. Almost everything we know about the interior came from it.
A wave reflects when it bounces off a boundary, and refracts when it bends on crossing into a different material. CAPS names both: the angles, plus the speed, are what reveal the layering.
The thin outermost rocky shell. ⚠️ CAPS uses "lithosphere or crust" as if they are the same thing, while strictly the lithosphere is the crust plus the rigid uppermost mantle. Answer the question as asked; know the difference.
Rock formed from cooled molten material — so it is a sample of the interior that came to you. Granite cooled slowly deep down, giving big visible crystals; basalt cooled fast at the surface, giving fine ones. The crystal size tells you where it cooled.
An underwater mountain chain where plates pull apart and mantle material rises to fill the gap. It is mantle convection made visible — and it is why this topic belongs in a marine sciences course. See Topics 3 and 11.
Quick Check
Question 1
Well done!
Move on to Topic 11 when you're ready.