The structure of the Earth is arranged in concentric layers that differ in composition, temperature and mechanical behavior. From the surface inward these layers are commonly described as the crust, the mantle, the liquid core (outer core) and the solid inner core. The planet's overall shape is best approximated by an oblate spheroid, slightly flattened at the poles and bulging at the equator, a result of rotation and the distribution of mass.

Layers and their key characteristics

  • Crust: The thin outermost shell made of solid rock. Continental crust is generally thicker and richer in silica and aluminium (silicon, aluminium) and is often described as felsic. Oceanic crust is thinner and denser.
  • Upper mantle and lithosphere: The uppermost mantle plus the crust form the rigid lithosphere, which is broken into tectonic plates. The very top of the mantle is composed largely of peridotite (peridotite).
  • Asthenosphere: Beneath the lithosphere lies a zone that behaves plastically; partial melting and the presence of melt and hot solid rock (commonly associated with magma) allow plates to move.
  • Lower mantle: A more viscous, higher-pressure region that extends to the core–mantle boundary; it is rich in silicate minerals that include magnesium (magnesium) and silicon and is often termed mafic in bulk composition.
  • Core: Composed primarily of iron and nickel. The core has a liquid outer layer and a solid inner center; the motion of the liquid outer core sustains Earth's magnetic field.

These layers are distinguished on both chemical and physical grounds: chemical layering refers to differences in elemental or mineral composition, while physical layering refers to solid, partially molten or fluid behavior. Scientific understanding of these distinctions relies heavily on the study of seismic waves produced by earthquakes and artificial sources. Seismographs detect reflections and refractions at discontinuities such as the Mohorovičić discontinuity (the "Moho") between crust and mantle and the core–mantle boundary; these seismic signatures reveal changes in density and rigidity deep inside the planet.

Boundaries, detection and important discontinuities

Key boundaries are identified when seismic waves change speed or direction. The Moho marks the crust–mantle transition, while deeper boundaries indicate changes in mineral phases and melting behavior. At great depths, pressure and temperature induce mineral transformations and changes in crystallization and density, which affect the propagation of seismic waves. The boundary separating the liquid outer core from the solid inner core was identified by how seismic waves are blocked or transmitted through the central region.

Dynamics, heat and evolution

Earth's internal heat, supplied by residual heat from planetary formation, radioactive decay and core crystallization, drives convection in the mantle and circulation in the outer core. Mantle convection is the engine behind plate tectonics—lithospheric plates move, collide and slide past one another, producing earthquakes, mountain building and volcanic activity. In the outer core, convective motions of electrically conducting iron–nickel fluid produce the geodynamo that maintains the magnetic field, which protects the surface from charged particles.

Over geological time the planet has differentiated: heavier elements sank toward the center while lighter silicate materials rose, forming the layered arrangement seen today. The inner core is believed to be slowly solidifying as the planet cools, releasing latent heat and possibly light elements that contribute to outer-core convection.

Why Earth's structure matters

Understanding Earth's internal structure is essential for multiple reasons: it explains the distribution of earthquakes and volcanoes, guides exploration for mineral and energy resources, informs models of the planet's thermal history, and helps predict changes in the magnetic field. Seismology, mineral physics, high-pressure experiments and geodynamo models together build a coherent picture, yet many details—such as fine-scale heterogeneity in the deep mantle or the precise composition of the core—remain active areas of research.

For additional general reference, see introductory treatments of the layers of Earth and resources on planetary shapes and interiors such as those describing the relationship between rotation and the photosphere of stars by analogy. Together these topics frame why Earth's internal structure is a central subject in geology, geophysics and planetary science.

poles | equator | oxygen | mantle | lithosphere

Structure of the Earth, crust, silicon, aluminium, felsic, magnesium, mafic, peridotite, magma, core, iron, nickel, crystallization