Earth is commonly described as having several major internal layers based on differences in composition and physical properties. The broadest compositional divisions are the crust, mantle and core. The core itself is divided into the liquid outer core and solid inner core. Scientists also describe mechanical layers based on how materials deform, including the lithosphere, asthenosphere, deeper mantle and core. These classifications are not competing descriptions but different ways of organising information about the same planet. The compositional model emphasises chemical differences, whereas the mechanical model emphasises physical behaviour. Earth's layers were inferred largely through seismology, gravity, density measurements, laboratory experiments and knowledge of planetary formation.
The crust is Earth's thin outer rocky layer. There are two principal types, continental crust and oceanic crust. Continental crust is generally thicker and less dense, with compositions that include abundant felsic and intermediate rocks. Oceanic crust is generally thinner and denser and consists largely of basaltic rocks formed at mid ocean ridges. The crust and the uppermost rigid mantle together form the lithosphere. The lithosphere is broken into tectonic plates that move relative to one another. Beneath much of the lithosphere lies the asthenosphere, a mechanically weaker region of the upper mantle that can deform over geological timescales and assists the movement of tectonic plates.
The mantle lies between the crust and the core and constitutes most of Earth's volume. It is composed predominantly of silicate minerals rich in magnesium and iron. Although the mantle is solid overall, it can deform extremely slowly over geological timescales because temperatures and pressures allow minerals to undergo solid state flow. The mantle is divided broadly into upper and lower regions, with additional transitions associated with changes in mineral structures at particular depths. Convection and other forms of heat transport within the mantle are fundamental to plate tectonics and the long term thermal evolution of Earth. Mantle rocks can partially melt under suitable conditions, producing magma that may reach the surface as lava.
The core begins at a depth of roughly 2900 kilometres below Earth's surface and is predominantly metallic, consisting mainly of iron with significant amounts of nickel and lighter elements. The outer core is liquid, while the inner core is solid. This difference is caused by the combined effects of temperature and pressure. The core is extremely hot, but pressure at the centre is so great that the inner core remains solid under present conditions. Convection within the electrically conducting liquid outer core generates Earth's global magnetic field through the geodynamo process. The core is therefore important not only as a physical layer but also as the source of a major planetary field.
Scientists cannot directly drill anywhere near Earth's deepest layers, so much of what is known about the interior comes from indirect evidence. Earthquakes generate seismic waves that travel through the planet. Primary waves, or P waves, can travel through solids and liquids, while secondary waves, or S waves, cannot propagate through liquids. The absence of direct S wave transmission through the outer core provided crucial evidence that the outer core is liquid. Changes in seismic wave speed, direction and reflection reveal boundaries and variations in material properties. Seismology therefore provides one of the most important tools for investigating Earth's inaccessible interior.
Mechanical layers provide another important classification. The lithosphere is relatively rigid and includes the crust and uppermost mantle. Beneath it, the asthenosphere is comparatively weak and can flow slowly. Deeper mantle regions become stronger under increasing pressure but still undergo solid state deformation over geological time. The outer core behaves as a liquid, whereas the inner core behaves as a solid. These mechanical differences strongly influence plate movement, mantle convection, seismic wave propagation and Earth's magnetic field. The distinction between compositional and mechanical layers is important because the boundaries do not always coincide exactly.
Temperature and pressure generally increase with depth, but the relationship is complex. Heat comes from the planet's formation, radioactive decay and other sources, and Earth's interior continues to lose heat into space. Pressure increases because deeper materials support the weight of the overlying planet. Increasing pressure changes the physical properties and crystal structures of minerals. Temperature and pressure together determine whether a material is solid, liquid or capable of slow deformation. The internal layers are consequently defined not merely by depth but by composition, temperature, pressure, phase and mechanical behaviour.
Earth's layered structure developed during its early history as the young planet heated and underwent extensive differentiation. Dense metallic material migrated toward the centre, forming the core, while less dense silicate materials became concentrated above it. Later geological processes continued to modify the outer layers through melting, crystallisation, erosion, sedimentation and plate tectonics. The layered structure is therefore both a consequence of planetary formation and an active system that continues to evolve. Studying the layers helps explain earthquakes, volcanism, plate tectonics, mountain building, magnetic fields, planetary heat loss and many other major features of Earth.
