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Earth's interior is divided into concentric layers defined by compositional and rheological boundaries, inferred primarily from how seismic waves refract and reflect at depth. The crust (5–70 km thick) is compositionally distinct from the silicate mantle beneath; the Mohorovičić discontinuity (Moho) marks this boundary. The mantle is solid but convects over geological timescales; the outer core (~2,900–5,150 km depth) is liquid iron-nickel (explaining the S-wave shadow zone) and generates Earth's magnetic field through dynamo action; the inner core is solid due to immense pressure. The lithosphere—crust plus uppermost rigid mantle—overrides the ductile asthenosphere, making the lithosphere-asthenosphere boundary the mechanical basis for plate tectonics.
Correlating each seismic discontinuity (Moho, 410 km, 660 km, CMB, ICB) with a change in seismic velocity or wave type on a velocity-depth profile makes the layered model a reading of actual data rather than a diagram to memorize. Comparing Earth's interior structure to those of the Moon and Mars (both lack liquid outer cores) connects planetary differentiation to magnetic field generation.
When you studied seismic waves, you learned that P-waves (compressional) travel through both solids and liquids, while S-waves (shear) travel only through solids. Earth's interior structure is almost entirely inferred by tracking how these waves travel through the planet — their speed changes at boundaries, they refract, and they reflect. The result is a detailed picture of Earth's layering, assembled entirely from surface observations without ever drilling past about 12 km depth.
The outermost layer is the crust — 5–10 km thick beneath oceans (oceanic crust, mafic in composition) and 30–70 km thick beneath continents (continental crust, more felsic). Below the crust lies the mantle, which extends to about 2,900 km depth. The mantle is solid rock, not magma — this is perhaps the most important misconception to correct. The mantle is so hot that over millions of years it flows by solid-state creep, like extremely viscous putty, driving convection that moves tectonic plates. But on human timescales it behaves as a solid: seismic S-waves pass through it without difficulty.
At 2,900 km the core-mantle boundary (CMB) marks one of the most dramatic discontinuities in the planet. Below it lies the outer core: liquid iron-nickel alloy, about 2,250 km thick. The proof of its liquid nature is the S-wave shadow zone — a region on the far side of an earthquake where S-waves do not arrive because they cannot traverse liquid. P-wave velocities also drop abruptly at the CMB. The outer core convects vigorously because it is cooling from the outside in, and this convection of electrically conducting iron is what generates Earth's magnetic field through dynamo action.
At about 5,150 km depth, a sharp increase in P-wave velocity marks the inner core boundary (ICB): the transition to the solid inner core, roughly 1,200 km in radius. The inner core is solid not because it is cooler than the outer core but because pressure at that depth raises iron's melting point above the actual temperature. The inner core has been growing slowly as Earth cools — its solidification releases latent heat and lighter elements that buoy the overlying liquid, helping to drive the dynamo.
Two mechanical layers cut across these compositional layers and matter for tectonics. The lithosphere — crust plus uppermost rigid mantle, totaling 50–250 km depending on age and location — is the brittle lid that fractures and breaks along faults. Beneath it lies the asthenosphere, a zone in the upper mantle that is close enough to its melting point to be weak and ductile. The lithosphere floats and slides on the asthenosphere — this is the mechanical basis for plate tectonics. The lithosphere-asthenosphere boundary is not a compositional boundary but a rheological one: same rock type, different mechanical behavior due to temperature and pressure.