Last updated August 4, 2025

The Life of Earth: Structure and Layers of the Earth

A differentiated structure since formation

Since its formation about 4.56 billion years ago, the Earth has gradually differentiated under the effect of gravity, residual heat, and meteoritic bombardment. This differentiation produced an organization into concentric layers, each with distinct physical, thermal, and mechanical properties. The internal dynamics of these layers are the origin of phenomena such as continental drift, volcanism, or terrestrial magnetism.

The four major layers of the Earth

The Earth is composed of four main layers: the crust, the mantle, the outer core, and the inner core. These layers are separated by distinct physical discontinuities (Mohorovičić, Gutenberg, Lehmann), detected by the analysis of seismic wave propagation.

Characteristics of the Earth's internal layers
Layer	Depth (km)	Physical state	Dominant composition	Estimated temperature
Crust	0 – 35	Solid	Silicates (granite, basalt)	200 – 1,000 °C
Upper Mantle	35 – 670	Ductile solid (partially molten asthenosphere)	Magnesium silicates (olivine, pyroxene)	1,000 – 3,000 °C
Lower Mantle	670 – 2,890	Rigid solid	Peridotite, dense oxides	3,000 – 3,700 °C
Outer Core	2,890 – 5,150	Liquid	Iron, nickel, sulfur	4,000 – 5,000 °C
Inner Core	5,150 – 6,371	Solid (dense metallic alloy)	Iron, nickel	5,000 – 6,000 °C

Sources: USGS – Earth Structure, Inside the Earth, Nature, 2012 – Inner core rotation.

The internal discontinuities of the Earth

The internal layers of the Earth are not separated progressively but by marked physical transitions called seismic discontinuities. These correspond to abrupt jumps in density, chemical composition, or mechanical behavior detected by the analysis of seismic waves.

A. Mohorovičić Discontinuity (Moho)

Discovered in 1909 by Croatian seismologist Andrija Mohorovičić, this discontinuity marks the boundary between the Earth's crust (oceanic or continental) and the upper mantle. It is characterized by a sudden increase in the speed of seismic waves, due to the transition from crustal rocks (granitic or basaltic) to denser rocks (peridotites) of the mantle. The depth of the Moho varies between 5 km under the oceans and 70 km under continental mountain ranges.

B. Gutenberg Discontinuity

Located at about 2,900 km deep, the Gutenberg discontinuity separates the solid lower mantle from the liquid outer core. At this boundary, S-type seismic waves (shear waves) abruptly stop, as they do not propagate in liquids, while P-type waves (compression waves) undergo a significant decrease in speed and notable refraction. This behavior indicates a radical change in the phase of matter (solid → liquid) and composition (silicates → iron and nickel alloy).

C. Lehmann Discontinuity

Discovered in 1936 by Danish seismologist Inge Lehmann, this discontinuity is located around 5,100 km deep and separates the liquid outer core from the solid inner core. It is deduced from the reappearance of P waves refracted in the inner core, implying a change in the physical state of iron (liquid → solid) under the effect of very high pressures. This inner core, although hotter, remains solid due to extreme pressures that favor the crystallization of iron.

When the four major layers of the Earth stacked up

The Earth formed through the accretion of solid and liquid materials within the protoplanetary disk. Originally, this body was largely homogeneous. The stacking of the internal layers (crust, mantle, outer core, and inner core) is the result of a process of gravitational and thermal differentiation that occurred mainly during the first hundreds of millions of years following its formation.

Physical mechanisms behind differentiation

Several sources of energy caused the partial melting of the primitive Earth, facilitating the separation of materials according to their density:

Accretion heat: Repeated meteorite impacts converted kinetic energy into intense heat, partially melting the planet.
Radioactive decay: The decay of radioactive isotopes (such as \(^{26}\)Al, \(^{60}\)Fe) generated continuous internal heat.
Gravitational segregation: Under the effect of gravity, the densest materials (iron, nickel) migrated to the center, forming the core.
Gravitational settling: The density difference between silicates (~3.3 g/cm\(^3\)) and metals (~7.8 g/cm\(^3\)) allowed the formation of distinct layers, analogous to a separation of liquid phases.

Timeline of layer formation

Approximate timeline of the formation of the Earth's internal layers
Age (Ga)	Event	Physical description
~4.56	Initial formation	Accretion of solid and liquid materials in the protoplanetary disk
4.5 – 4.4	Peak of accretion and intense heating	Global partial melting due to meteoritic impacts and internal heat
~4.45	Formation of the metallic core	Gravitational migration of liquid iron and nickel to the center
4.4 – 4.0	Segregation of the mantle and formation of the crust	Differentiation of silicates, crystallization of the primordial crust on the surface
Since 4.0	Stabilization of the internal structure	Gradual cooling, establishment of tectonics and the magnetic field

Why is this stacking stable?

The stacking into concentric layers is explained by the minimization of the gravitational potential energy of the system. Heavier and denser materials migrate to the center, while lighter materials form the outer envelopes. The increasing pressure at depth modifies the physical and thermal properties, allowing the existence of a solid inner core despite very high temperatures.

Transitions detected by seismology

Seismic waves (\(P\), \(S\), \(L\), \(R\)) inform us about the internal nature of the Earth. For example, \(S\) waves do not pass through the liquid outer core, while \(P\) waves slow down considerably there. The existence of the solid inner core was postulated as early as 1936 by Inge Lehmann (1888-1993), based on a reflected seismic echo.

The layers and terrestrial life

The outer core and the Earth's magnetic field

The liquid outer core, mainly composed of molten iron and nickel, generates a terrestrial magnetic field through the dynamo mechanism. This field forms a protective barrier against the solar wind, composed of energetic charged particles that, without this protection, would erode the atmosphere and expose the surface to deadly radiation. The preservation of the atmosphere is thus ensured, an indispensable condition for life.

Plate tectonics and the cycle of elements

Plate tectonics, resulting from the plasticity and convective movements in the upper mantle, ensures the continuous renewal of the Earth's crust. This process recycles essential chemical elements, such as carbon, nitrogen, and phosphorus, through subduction and magmatic upwellings, regulating the global climate and providing the nutrients necessary for ecosystems.

Volcanic activity and the maintenance of the atmosphere

The gradual cooling of the Earth causes a transfer of heat to the surface, fueling volcanic activity. Volcanic emissions, particularly of carbon dioxide and water vapor, contribute to the formation and maintenance of the atmosphere and hydrosphere, elements indispensable to life.

The continental crust: a stable support for complex life

The increasing rigidity and solidity of the continental crust provide a stable substrate for the biosphere. The continents influence oceanic and atmospheric cycles, creating environmental conditions conducive to biodiversity and the evolution of complex life forms.

An internal structure indispensable to life

The internal structure of the Earth, from the core to the crust, is both the engine and regulator of the physicochemical conditions necessary for the emergence, maintenance, and evolution of terrestrial life.