How does our planet work from the inside out?
Earth isn't a uniform ball. It has layers — like a hard-boiled egg, but with a liquid part in the middle. Click each layer in the diagram to explore.
Each layer has different properties — different temperature, thickness, and what it's made of. The deeper you go, the hotter and denser it gets.
We've never drilled deeper than 12 km (the Kola Superdeep Borehole in Russia — and Earth's radius is 6,371 km!). So how do we know? Seismic waves from earthquakes. Different types of waves travel at different speeds through solid vs liquid material, and they bend and reflect at layer boundaries. By studying thousands of earthquakes from stations around the world, scientists mapped the interior — like giving the planet an ultrasound.
Cut a hard-boiled egg in half: the shell is the crust, the white is the mantle, and the yolk is the core. The proportions are surprisingly similar! But notice how thin the shell is compared to everything else — Earth's crust is proportionally even thinner than that.
Earth's crust isn't one solid piece. It's cracked into about 15 major plates that float on the slow-moving mantle — drifting a few centimetres per year, about as fast as your fingernails grow.
Tectonic plates are enormous slabs of crust (and upper mantle) that fit together like a jigsaw puzzle. They're constantly moving, driven by heat currents in the mantle below — hot rock rises, spreads sideways, cools, and sinks back down. This slow circulation is called convection.
Click each to learn what happens
Magma rises from below to fill the gap, creating new crust. This is happening right now in the middle of the Atlantic Ocean, at the Mid-Atlantic Ridge — the longest mountain range on Earth, running 16,000 km along the ocean floor.
When two plates push together, one slides underneath the other (subduction) or they crumple upward. This creates the most violent geology on Earth: deep ocean trenches, explosive volcanoes, and the tallest mountains.
No crust is created or destroyed — the plates just scrape past each other. This builds up enormous stress that gets released suddenly as earthquakes. The San Andreas Fault in California is the most famous example.
Earthquakes and volcanoes aren't random — they happen at plate boundaries. Plot every earthquake and volcano on a map and you'll see the outlines of the tectonic plates appear, clear as day.
Tectonic plates move slowly but they don't slide smoothly past each other. They lock up. Stress builds for years, decades, or centuries. Then suddenly — SNAP — the rocks break and lurch, sending shockwaves through the ground. That's an earthquake.
Each step is 10× more ground movement and ~32× more energy!
Volcanoes form when magma (molten rock from the mantle) reaches the surface. This mostly happens at plate boundaries — either where plates pull apart (magma fills the gap) or where one plate dives under another (the sinking plate melts, and the magma rises).
Seismologists use earthquake waves the same way doctors use X-rays — to see inside something you can't cut open. By tracking how P-waves and S-waves travel through Earth (where they speed up, slow down, bend, or disappear), scientists have mapped every layer of Earth's interior. The discovery that the outer core is liquid came from noticing that S-waves can't pass through it — they create a "shadow zone" on the opposite side of Earth from an earthquake.
Deep inside Earth, rivers of liquid iron flow in the outer core. This flowing metal generates a magnetic field that extends far into space — and it protects everything alive on the surface.
Earth's magnetic field is generated by the flow of liquid iron in the outer core — a process called the geodynamo. The iron is kept liquid by the immense heat from the inner core. As it flows, it creates electric currents, which produce the magnetic field. It's the same basic principle as an electromagnet, just on a planetary scale.
Earth's magnetic field isn't permanent — it flips! On average every 200,000–300,000 years, the north and south magnetic poles switch places. The last flip was about 780,000 years ago, so we're actually overdue.
The aurora borealis (north) and aurora australis (south) are caused by charged particles from the solar wind that slip past the magnetosphere near the poles and collide with gas molecules in the upper atmosphere, making them glow.
Place a bar magnet under a sheet of paper. Sprinkle iron filings on top and gently tap. The filings arrange themselves along the magnetic field lines, showing the invisible field. Earth's field looks like a much larger version of this — field lines emerging from the south, looping through space, and entering at the north.
Every rocky planet and moon has its own geophysics story. Comparing them to Earth helps us understand why our planet is so special — and what we might find elsewhere.
Mars once had a magnetic field, a thick atmosphere, rivers, and possibly oceans. Then its core cooled, the dynamo stopped, and the solar wind stripped away the atmosphere. It's a cautionary tale about what happens when a planet's geophysics shuts down.
Venus is almost Earth's twin in size, but it has no plate tectonics. Without plates recycling carbon, CO₂ built up in the atmosphere, creating a runaway greenhouse effect (465°C surface temperature). It may periodically "resurface" itself with massive global volcanic eruptions instead.
Io has over 400 active volcanoes — more than any other object we know of. Its interior is kept molten not by radioactive decay (like Earth) but by tidal heating: Jupiter's immense gravity flexes Io like a stress ball, generating heat through friction.
For a planet to support life (as we know it), it needs: a magnetic field (to protect the atmosphere), plate tectonics (to regulate temperature and recycle chemicals), and internal heat (to drive both). Earth has all three. Mars lost its heat. Venus lost its plates. Understanding geophysics is understanding what makes a planet habitable.