Interior of the Earth

The One Thing

Earth has a layered internal structure consisting of the crust, mantle, liquid outer core, and solid inner core. Because direct access extends only a very small distance into Earth, scientists rely mainly on seismic waves and other indirect evidence to determine the composition, density, physical state, and boundaries of these layers.

Who and What

• Earth’s average radius: Approximately 6,371 km.
• Crust: The thin, outermost solid layer. Oceanic crust is mainly basaltic and approximately 5–10 km thick; continental crust is mainly granitic and approximately 30–70 km thick.
• Mantle: The layer below the crust, extending to approximately 2,900 km depth. It consists mainly of dense silicate minerals rich in magnesium and iron.
• Core: The innermost region, extending from approximately 2,900 km depth to Earth’s centre. It is composed mainly of iron and nickel.
• Outer Core: The liquid part of the core, located approximately between 2,900 km and 5,150 km depth. Its movement of electrically conducting molten material is believed to generate Earth’s magnetic field.
• Inner Core: The solid central part of the core, extending approximately from 5,150 km depth to Earth’s centre. Extremely high pressure helps keep it solid despite its very high temperature.
• Lithosphere: The rigid outer shell consisting of the crust and uppermost solid mantle. It is divided into tectonic plates.
• Asthenosphere: A relatively weak and plastic zone below the lithosphere, generally associated with the upper mantle, on which tectonic plates move.
• Seismic Waves: Waves of energy released during an earthquake. Their speed, direction, and behaviour provide evidence about materials and boundaries inside Earth.
• P-Waves: Primary or compressional waves. They travel fastest through solids, liquids, and gases, pass through the entire Earth, and change speed and direction at layer boundaries.
• S-Waves: Secondary or shear waves. They travel only through solids. Their inability to pass through the liquid outer core demonstrates that the outer core is liquid.
• Shadow Zone: An area in which particular seismic waves are not detected because they are blocked, refracted, or redirected by internal layers. P-wave and S-wave shadow zones reveal changes in composition and physical state.
• Mohorovičić Discontinuity: The boundary between the crust and mantle, commonly called the Moho, identified by a sudden increase in seismic-wave velocity.
• Gutenberg Discontinuity: The boundary between the mantle and outer core at approximately 2,900 km depth, marked by changes in seismic-wave behaviour.
• Lehmann Discontinuity: The boundary between the liquid outer core and solid inner core at approximately 5,150 km depth.
• Direct Sources: Evidence obtained by directly examining material from inside Earth, including mining, drilling, volcanic rocks, and materials brought up by volcanic eruptions. Such samples represent only a very small part of Earth’s total radius.
• Indirect Sources: Evidence inferred from seismic waves, gravity, magnetic fields, meteorites, and the behaviour of temperature, pressure, and density.
• Geothermal Gradient: The rate at which temperature increases with depth. The increase is rapid near the surface but is not uniform throughout Earth’s interior.
• Magnetic Field: The protective field around Earth, believed to be produced by movement within the electrically conducting molten outer core.
• Average density: Expressed approximately as

density = mass ÷ volume.

• Pressure: Expressed conceptually as

pressure = force ÷ area.

• Seismic-wave velocity: Dependent on material density, elasticity, temperature, pressure, and physical state.
• Convection currents: Movements in the mantle that transfer internal heat and are associated with plate movement and several types of tectonic activity.

Causes and Consequences

• Limited direct access leads to reliance on indirect evidence. Mining, drilling, volcanic rocks, and materials brought up by volcanic eruptions provide direct evidence, but they reach only shallow depths relative to Earth’s approximately 6,371 km radius. Consequently, scientists use seismic waves, gravity, magnetic fields, meteorites, and patterns of temperature, pressure, and density to construct models of the interior.

• Differences in material properties alter seismic-wave behaviour. Seismic-wave velocity depends on density, elasticity, temperature, pressure, and physical state. Changes in speed, direction, and transmission therefore show that Earth is not uniform but consists of layers with different compositions and physical states.

• The Mohorovičić Discontinuity identifies the crust–mantle boundary. A sudden increase in seismic-wave velocity marks the Moho, separating the relatively thin crust from the mantle, which extends to approximately 2,900 km depth.

• P-wave and S-wave behaviour demonstrates that the outer core is liquid. P-waves can pass through solids, liquids, and gases, whereas S-waves travel only through solids. The absence of S-waves beyond the outer-core region indicates that they are blocked by a liquid outer core. The refraction and altered paths of P-waves at the Gutenberg Discontinuity further identify the mantle–outer-core boundary and produce seismic shadow zones.

• Changes in P-wave behaviour support the existence of a solid inner core. At approximately 5,150 km depth, the Lehmann Discontinuity separates the liquid outer core from the solid inner core. The reappearance and altered behaviour of P-waves provide evidence for this solid central region.

• Increasing pressure and temperature produce contrasting core states. Temperature, pressure, and density generally increase toward Earth’s centre. Although the inner core is extremely hot, the very high pressure raises its density and helps maintain its solid state, whereas the outer core remains liquid because of differences in temperature and pressure conditions.

• Earth’s internal heat drives movement and surface processes. Internal heat comes mainly from leftover heat of formation, radioactive decay, and heat released during the separation of dense materials toward the core. Convection currents in the mantle transfer this heat and are associated with plate movement, earthquakes, volcanic eruptions, and mountain building.

• Movement in the outer core generates the magnetic field. The movement of electrically conducting molten material in the liquid outer core is believed to produce Earth’s protective magnetic field.

• Meteorites provide comparative evidence about Earth’s composition. Meteorites are studied because they formed from materials similar to those that helped form Earth. They therefore provide indirect evidence about the materials from which Earth developed, supplementing seismic and direct-source evidence.

What Gets Asked

• Compare the composition, thickness, depth, and physical state of the crust, mantle, outer core, and inner core.
• Explain why indirect sources are essential for investigating Earth’s interior, and evaluate the limitations of direct sources such as mining, drilling, volcanic rocks, and materials brought up by volcanic eruptions.
• Explain how P-waves, S-waves, and seismic shadow zones demonstrate that the outer core is liquid and the inner core is solid.
• Describe the significance of the Mohorovičić Discontinuity, Gutenberg Discontinuity, and Lehmann Discontinuity.
• Explain how increasing temperature, pressure, and density toward the centre affect the physical state of the core, including why the inner core remains solid.
• Link internal heat, mantle convection currents, outer-core movement, and Earth’s magnetic field to surface processes including plate movement, earthquakes, volcanic eruptions, and mountain building.