Core

1. Position, dimensions and composition

Earth’s interior can be classified by composition into crust, mantle and core. The core begins beneath the mantle at roughly 2,900 km depth and continues to the centre, approximately 6,371 km below the surface. Its radius is therefore about 3,480 km. Although it occupies only around one-sixth of Earth’s volume, its high density means that it contains roughly one-third of the planet’s mass. These approximate proportions help distinguish volume from mass in examination questions.

The core consists predominantly of iron, together with nickel and a smaller proportion of lighter elements. School geography sometimes calls it NIFE, derived from nickel and ferrum, the Latin word for iron. This is a useful mnemonic, not a complete chemical description. Seismic density estimates indicate that the core cannot be pure iron–nickel; proposed lighter constituents include oxygen, sulphur, silicon, carbon and hydrogen, with their proportions still debated.

The liquid outer core extends from approximately 2,900 to 5,150 km depth. The solid inner core extends from this boundary to the centre. Density generally increases inward, with discontinuous increases at major boundaries. The core–mantle boundary, traditionally called the Gutenberg discontinuity, separates metallic liquid from the overlying predominantly solid silicate mantle.

• Compositional division: crust, mantle and core.
• Mechanical classification distinguishes the liquid outer core from the solid inner core.
• The Mohorovičić discontinuity separates crust and mantle; it is not the core’s upper boundary.

2. How seismic waves reveal the core

Direct sampling of the core is impossible with existing technology. Even the deepest boreholes penetrate only a small part of the crust. Knowledge of the core therefore comes mainly from earthquake waves, supported by Earth’s mass, moment of inertia, magnetic field, meteorite studies and laboratory experiments on materials under extreme pressure. Meteorites provide compositional analogues and evidence about planetary formation, but they are not samples of Earth’s core.

Primary or P waves are compressional waves that travel through solids and liquids. Secondary or S waves are shear waves and cannot propagate through liquids because liquids lack the required shear rigidity. At the core–mantle boundary, P-wave velocity falls sharply and their paths are strongly refracted. Direct S waves do not pass through the liquid outer core. Together, these observations establish that the outer core is liquid.

In the standard textbook model, the P-wave shadow zone lies approximately between 103° and 142° angular distance from an earthquake’s epicentre. Direct S waves are absent beyond approximately 103°. These angles describe distances measured at Earth’s centre, not kilometres along a straight line. Special converted or diffracted phases may still be detected, so a shadow zone should not be understood as the absence of every possible seismic signal.

Reflected and transmitted seismic phases reveal a distinct inner core. Their travel times, combined with other seismic observations, support its solid state. The discovery also demonstrates that an apparent absence or unexpected arrival of waves can reveal an inaccessible internal boundary.

3. Formation, temperature and physical state

Earth formed approximately 4.54 billion years ago through accretion. Impacts, gravitational compression and radioactive decay contributed to heating the young planet. During differentiation, dense metallic material separated from silicate material and moved inward to form the core. Lighter silicates formed the mantle and crust. Core formation released additional gravitational energy and occurred early in Earth’s history rather than through the slow settling of metals in the present-day solid mantle.

Temperatures near the inner-core boundary and centre are estimated to be broadly around 5,000–6,000°C, depending on the model and location. These are indirect estimates, not direct thermometer measurements. Pressure reaches approximately 330 gigapascals at the inner-core boundary and about 360 gigapascals at the centre. Consequently, the physical state of core material depends on pressure, temperature and composition together, rather than temperature alone.

The outer core is liquid because local temperatures exceed the melting temperature of its alloy. Farther inward, enormous pressure raises the melting temperature sufficiently for the inner core to remain solid despite greater heat. As Earth cools, iron-rich material crystallises at the inner-core boundary, allowing the inner core to grow. Crystallisation releases latent heat and expels some lighter elements into the liquid outer core, helping sustain buoyancy-driven motion.

• Higher temperature does not automatically imply a liquid state under extreme pressure.
• The inner core formed later than the initial metallic core; its precise age remains debated.
• Radioactive heating is important to Earth’s overall thermal budget, but its contribution within the core is uncertain.

4. The geodynamo and geomagnetic field

Earth’s main magnetic field is generated by the geodynamo: the movement of electrically conducting liquid metal in the outer core maintains electric currents and a magnetic field. Thermal and compositional buoyancy drive fluid motion, while Earth’s rotation organises much of that motion through the Coriolis effect. Rotation alone is insufficient; a conducting fluid and a continuing energy supply are also required.

The core is not simply a permanent bar magnet. Temperatures there are far above those at which iron retains ordinary permanent magnetism. The magnetic field is approximately dipolar at the planetary scale, but its detailed structure is more complex and changes with time. Magnetic poles therefore do not exactly coincide with geographic poles and can migrate. Magnetic declination is the angle between magnetic north and true north at a location.

The magnetic field helps deflect charged particles from the solar wind and shapes the magnetosphere. Its polarity has reversed repeatedly during geological history. Magnetic minerals in cooling rocks can preserve earlier field directions, providing evidence used in palaeomagnetism and plate-tectonic reconstruction. Reversals are irregular and do not mean that Earth’s direction of rotation reverses.

• Liquid outer core: principal region of magnetic-field generation.
• Growing inner core: contributes energy and compositional buoyancy to the dynamo system.
• Magnetic-field generation and permanent magnetism are different physical processes.

5. Relevance to geomorphology and examination interpretation

The core affects surface processes indirectly rather than supplying material directly to ordinary landforms. Heat transferred across the core–mantle boundary contributes to mantle dynamics. Along with heat generated within the mantle and cooling of the planet, it supports the long-term internal energy system associated with tectonics and volcanism. Mantle circulation is also strongly influenced by sinking cold slabs; plate movement should not be attributed to core heat alone.

The mantle is predominantly solid but deforms and flows over geological timescales. The asthenosphere is therefore not equivalent to the liquid outer core. Likewise, magma feeding volcanoes generally forms by partial melting in the mantle or crust, not by rising from the core. Some mantle plumes may originate near the core–mantle boundary, but their material is mantle rock rather than liquid core metal.

For Prelims, connect each observation to its inference: the absence of direct S waves through the core indicates a liquid layer; strong P-wave refraction marks a major change in material properties; and a continuing geomagnetic field indicates motion in conducting fluid. Keep established features separate from active research topics, including the inner core’s age, detailed structure and differential rotation.

Previous year questions

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Practice questions