Early in its history the Earth was hot enough to be partly molten. Heavy iron and nickel sank to the centre and lighter, silica-rich rock rose. That left four main layers, sorted by density.

The inner core is hotter than the outer core, yet it is solid. The reason is pressure. At the centre of the Earth the pressure is over three million times that at the surface, which squeezes the iron atoms together so tightly that they lock into a solid even at more than 5,000 °C. Whether something is molten depends on two numbers, not one: its temperature, and its melting point at that pressure. Pressure pushes the melting point up.

Both kinds of crust are lighter than the mantle beneath them (~3.3 g/cm³), so at the surface neither would sink on its own. What sinks is old ocean plate: the thin crust plus a thick layer of mantle rock under it that has cooled and shrunk for tens of millions of years. Cold rock is denser than hot rock, so old oceanic plate ends up slightly denser than the hot mantle below. Two separate questions, then. When two plates meet, the difference in crust decides which one goes under: the oceanic one. The age and temperature of that plate decide whether it keeps sinking. Once a slab is deep, its basalt crust also turns into a denser rock, eclogite, which helps it keep going.

Geologists also divide the outer Earth by how it behaves rather than what it's made of. The lithosphere is the crust plus the uppermost, rigid part of the mantle: about 100 km thick under the oceans, and 150–200 km or more under old continents. It is cold and brittle and breaks into pieces called plates. Below it is the asthenosphere, mantle rock hot enough to flow very slowly, like warm toffee. The plates ride on top of it.

The two ways of slicing don't line up, and that is the classic confusion. The boundary between crust and mantle, called the Moho, is a change in rock type. The base of the lithosphere is a change in temperature, at about 1,300 °C, with the same rock above and below.

The deepest hole ever drilled, on the Kola Peninsula in Russia, reached about 12 km. That is 0.2% of the way to the centre. Everything we know about the deep Earth comes from indirect evidence, and the most powerful kind is earthquake waves.

A large earthquake sends seismic waves through the whole planet, and thousands of instruments called seismometers record when they arrive. Two kinds of wave travel through the interior:

Waves travel faster in deeper, more compressed rock, so their paths bend (refract) and curve back up to the surface. In 1914 Beno Gutenberg used a gap in where P waves arrive to work out how deep the core begins: about 2,900 km. Seismometers more than about 103° around the globe from an earthquake also never record S waves. That region is the S-wave shadow zone. In 1926 Harold Jeffreys combined this with other evidence to show that the core must be liquid: S waves cannot cross it. And the size of the core can be worked out from the size of the shadow.

Several other lines of evidence point the same way:

• Density. Weighing the Earth (from the strength of its gravity) gives an average density of about 5.5 g/cm³. Surface rocks are only about 2.7–3.0 g/cm³, so the interior must be much denser.
• Spin. How a spinning object wobbles depends on where its mass sits. Earth's moment of inertia, measured from the way it responds to the pull of the Sun and Moon, shows the mass is concentrated towards the centre.
• Meteorites. Most meteorites are fragments of small bodies that formed alongside the Earth. The most primitive stony ones match the composition we expect for the whole Earth, mantle and core together. Iron-nickel ones are thought to be the cores of small planets that were smashed apart.

The inside of the Earth is hot, and it is slowly cooling. Two sources supply the heat:

• Primordial heat, left over from the planet's formation. Countless collisions as the Earth assembled, and the energy released as iron sank to form the core, heated it enormously. It is still leaking out.
• Radiogenic heat, from the decay of radioactive uranium, thorium and potassium spread through the rocks. Each decay releases a tiny amount of energy, and across a whole planet it adds up.

Roughly half of today's heat flow is thought to come from each source, though the exact split is still debated. Heat flows out through the surface at about 47 terawatts in total. That is more than twice humanity's total energy use, but spread so thinly over the planet that you could never feel it through the ground.

Going down into the crust, temperature rises by about 25–30 °C per kilometre on average. This is the geothermal gradient. It's why the deepest mines, including the Kolar Gold Fields in Karnataka at over 3 km, needed heavy cooling to be workable.

Heat can escape by conduction, passing from atom to atom, but rock conducts heat poorly. Over millions of years, though, the hot, solid mantle can flow, creeping a few centimetres a year. So it carries heat out the way a pan of soup does: hot material rises, cools near the top, becomes denser and sinks again. This is convection.

The cold, rigid plates are the top of this system. Where an old oceanic plate dives back into the mantle at a subduction zone, the sinking slab is colder and denser than the mantle around it, and its weight drags the rest of the plate behind it. This slab pull is thought to be the main force moving the plates. Ridge push, where newly made plate slides off the high, warm ridge, helps but is smaller. Separately, narrow columns of unusually hot rock called mantle plumes may rise from deep in the mantle and feed hotspot volcanoes such as Hawaii.

Earthquake waves suggest that many slabs sink right through the mantle. Their remains are thought to collect on the core in slab graveyards. Beneath Africa and the Pacific, earthquake waves travel unusually slowly through two continent-sized regions sitting on the core. These are thought to be hot, dense piles: hot, but made of denser rock, so they stay at the bottom. Plumes may rise from their edges.

The liquid outer core convects too, much faster than the mantle. Moving molten iron is an electrical conductor, and a conductor moving through a magnetic field generates electric currents, which in turn create a magnetic field. Earth's spin organises the swirling flow so that the fields add up rather than cancel. The result is a self-sustaining geodynamo that produces Earth's magnetic field, the one that points your compass north and shields the surface from charged particles streaming from the Sun.

Every so often, at irregular intervals of tens of thousands to tens of millions of years, the field reverses: magnetic north and south swap places. The last reversal was about 780,000 years ago.

Basalt lava contains small grains of the iron mineral magnetite. As the lava cools below about 580 °C, the magnetisation of these grains locks in the direction of the magnetic field of the day, like tiny frozen compass needles. At a mid-ocean ridge, new basalt forms continuously and is carried away on both sides. So the seafloor becomes a tape recording of the field's flips, in stripes that are mirror images either side of the ridge. Ships towing a magnetometer measure the stripes as a wiggly signal, stronger over normal stripes and weaker over reversed ones.

In the 1960s this barcode, identical on both sides of the ridge, was the decisive evidence that the seafloor spreads apart at ridges, which led directly to the theory of plate tectonics.