The Depths of the Earth: From the Lithosphere to the Core

Earth's internal structure — Image description: The structure of the Earth placed in an aquarium. The successive layers: the atmosphere, the Earth's crust (continental and oceanic crust), the mantle (upper and lower), and the core (inner and outer). If each layer is converted to a thickness of identical density (for example, that of water), this representation shows that the Earth's crust is insignificant (continental crust in light blue and oceanic crust in white) and the atmosphere is non-existent. Image source: astronoo.com (new window)

A Layered Internal Structure

The Earth has a layered internal structure resulting from billions of years of gravitational differentiation and planetary cooling. Each layer, from the crust to the core, has distinct physical and chemical properties. These characteristics determine tectonics, volcanism, the magnetic field, and the planet's internal movements.

The Earth's Crust

The Earth's crust is the rigid outer layer. It is divided into continental crust, which is granitic and thicker, and oceanic crust, which is basaltic and thinner. The crust contains essential minerals for the geochemical cycle, including silicon, aluminum, and iron. It plays a crucial role in supporting continents and the dynamics of tectonic plates.

N.B.:
The continental crust can reach 70 km beneath mountain ranges, while the oceanic crust generally does not exceed 10 km. Differences in density and composition influence subduction and the formation of oceanic ridges.

The Mantle

The mantle lies beneath the crust and extends to the outer core. It consists of silicate rocks rich in magnesium and iron. The upper mantle, partially ductile, allows for convection (movement of hot matter upwards and cold matter downwards), the driving force behind plate tectonics and hotspot volcanism. The lower mantle is more rigid but can deform slowly under high pressure.

N.B.:
The upper mantle can reach 660 km and the lower mantle ~2,200 km. Convection speeds in the upper mantle are a few centimeters per year, sufficient to cause continental drift and the formation of reliefs and mountains.

The Core

The Earth's core includes the outer core (earth's liquid outer core) and the inner core (earth's solid inner core). It is mainly composed of iron and nickel. Convection movements in the outer core generate the Earth's magnetic field, which protects the planet from solar winds and cosmic radiation.

N.B.:
The outer core can reach ~2,200 km and the inner core can reach ~1,220 km. The temperature of the inner core exceeds 5,000 K, comparable to that of the Sun's surface. The pressure there reaches about 360 GPa, which solidifies iron despite the high temperature.

Table of Main Characteristics

Characteristics of Earth's Layers
Layer	Thickness (km)	Main Composition	Comment
Continental Crust	30 to 70	Granite	Solid and rigid, supports continents and terrestrial biodiversity
Oceanic Crust	5 to 10	Basalt	Thin, forms ocean floors and is recycled by subduction
Upper Mantle	~660	Mg- and Fe-rich silicates	Partially ductile, drives tectonics and volcanism
Lower Mantle	~2,200	Dense silicates	High rigidity, transmits seismic waves and supports the core
Outer Core	~2,200	Iron and Nickel (liquid)	Liquid, generates the magnetic field and influences the geodynamo
Inner Core	~1,220	Iron and Nickel (solid)	Solid, high pressure, central core and thermal memory of the planet

Why Is the Outer Core Liquid and the Inner Core Solid?

The Earth's outer core is liquid, while its inner core is solid. This may seem paradoxical since the inner core is hotter. The key lies in pressure: the deeper you go, the greater the pressure. This pressure crushes the metal, forcing it to remain solid despite the extreme heat. In the outer core, the pressure is less overwhelming, so iron and nickel can behave like a liquid.

This difference in state is crucial: the liquid in the outer core creates convection movements that generate the Earth's magnetic field, protecting the planet from solar particles and enabling life as we know it.