The Earth’s surface may appear uniform, but the planet is structured into a series of distinct, concentric layers, much like an onion. This layered arrangement is a result of a process called planetary differentiation, where denser materials sank to the center while lighter materials rose toward the surface early in Earth’s history. Scientists use two primary classification systems to define these subterranean regions: one based on the chemical composition of the material and a second based on its physical and mechanical properties. Understanding these layers provides insight into phenomena from volcanic activity and earthquakes to the generation of the planet’s magnetic field.
Classification by Chemical Composition
The most fundamental way to divide the planet is by its chemical composition, which establishes three primary layers: the crust, the mantle, and the core. These layers are defined by their density and the concentration of elements within them.
The crust is the outermost, thinnest, and least dense layer, primarily composed of silicate minerals rich in oxygen and silicon. It is divided into two types. Oceanic crust is thin (5 to 10 kilometers thick) and made of dense, iron- and magnesium-rich basaltic rock. Continental crust is much thicker (up to 70 kilometers) and composed of less dense, silica- and aluminum-rich granitic rock.
Beneath the crust lies the mantle, a thick layer extending down nearly 2,900 kilometers and making up about 84% of Earth’s total volume. It is composed of silicate rock with a high concentration of iron and magnesium. Although predominantly solid, intense heat and pressure cause the rock to deform and flow very slowly over geologic timescales, a process known as convection.
The innermost layer is the core, composed primarily of a metal alloy of iron and nickel. This dense metallic center is chemically distinct from the mantle and accounts for approximately one-third of the planet’s mass.
Classification by Physical Properties
A second, more detailed classification system defines layers based on their physical state, such as whether the material is rigid, plastic, or liquid, which is governed by temperature and pressure. This system results in five distinct layers that do not perfectly align with the compositional layers.
The lithosphere is the outermost layer in this classification, defined by its rigid, brittle behavior. It is a relatively cool shell that includes the entire crust and the uppermost, solid portion of the mantle. The lithosphere is broken into the large pieces known as tectonic plates, whose movements are responsible for earthquakes and volcanism.
Directly beneath the rigid lithosphere is the asthenosphere, a mechanically weak layer within the upper mantle. While still solid, the rock is ductile and behaves plastically due to high heat. This flowing nature allows the tectonic plates of the lithosphere to move across the planet’s surface.
Below the asthenosphere is the mesosphere, which corresponds to the lower mantle. Here, the material remains solid, but it is more rigid than the asthenosphere because the extreme pressure counteracts the high temperatures. The core is then divided into two distinct layers based on their physical state, despite having a similar chemical composition.
The outer core is a layer of liquid iron and nickel extending to a depth of about 5,150 kilometers. Convective movement of this electrically conductive liquid metal generates Earth’s global magnetic field. The inner core is the planet’s innermost layer, a solid sphere of iron and nickel. Despite temperatures exceeding 5,000 degrees Celsius, immense pressure prevents the metal from melting, forcing it into a solid state.
How Scientists Map Earth’s Interior
Direct observation of Earth’s interior is impossible, as the deepest drilling projects have only penetrated a tiny fraction of the crust. Therefore, scientists rely on the study of seismology to determine the existence, depth, and physical state of these layers.
Seismology involves analyzing seismic waves, which are vibrations generated by earthquakes that travel through the planet. There are two main types of body waves used for this mapping: P-waves (primary waves) and S-waves (secondary waves). P-waves are compressional waves that can travel through solids, liquids, and gases, but their speed changes depending on the density and rigidity of the material.
S-waves, in contrast, are shear waves that can only travel through solid material. By monitoring the travel times and paths of these waves at seismic stations around the globe, scientists can map the internal structure. A sharp change in wave speed indicates a boundary between two different layers, known as a seismic discontinuity.
The most significant evidence for the core’s liquid and solid parts comes from S-waves, which disappear when they encounter the liquid outer core, proving its fluid state. P-waves are slowed down by the liquid outer core but increase speed again in the solid inner core, confirming the change in state. This analysis of wave behavior allows scientists to construct a detailed model of the planet’s hidden structure.