The layered internal structure of the Earth, inferred chiefly from the behaviour of earthquake (seismic) waves, comprising the crust, the mantle, and the core.
- The crust is the thin outermost solid shell: thicker continental crust (sial, granitic, silica-and-aluminium rich) and thinner oceanic crust (sima, basaltic, silica-and-magnesium rich).
- The mantle lies below the crust to about 2,900 km; its upper part includes the rigid lithosphere and the plastic, semi-fluid asthenosphere on which plates move.
- The core extends to the centre at about 6,371 km depth: a liquid outer core and a solid inner core, made mainly of iron and nickel (the NiFe layer, also called the barysphere).
- Boundaries (discontinuities): the Mohorovicic (Moho) separates crust from mantle, and the Gutenberg discontinuity separates mantle from core.
- The Earth's magnetic field is generated by motions in the liquid outer core; temperature and pressure both increase with depth.
The crust, mantle, and core sequence, the sial versus sima distinction, the iron-nickel core, and the Moho and Gutenberg discontinuities are direct objective facts.
Sial (continental, lighter) versus sima (oceanic, denser); lithosphere (rigid) versus asthenosphere (plastic, in the upper mantle); liquid outer core (allows convection and the magnetic field) versus solid inner core; Moho (crust to mantle) versus Gutenberg (mantle to core).
Crust (sial and sima), mantle (with the plastic asthenosphere), and an iron-nickel core (liquid outer, solid inner), revealed by seismic waves.