Earth’s Internal Layers: Core, Mantle, and Crust

Earth’s interior is divided into layers of different composition and properties: the thin rocky crust, the thick mantle, the liquid outer core and the solid inner core; the crust and the uppermost, rigid part of the mantle together form the lithosphere, which floats on the weaker, slowly flowing asthenosphere. In brief: the crust is 5–10 km thick under the oceans (dense basalt) and about 30–50 km thick under the continents (lighter granitic rock). The mantle extends to a depth of about 2,900 km. The outer core, mostly liquid iron and nickel, reaches a depth of about 5,150 km, and the inner core, a solid iron–nickel ball with a radius of about 1,220 km, lies at the center. Temperature and pressure increase with depth.

The layered structure explains plate tectonics, earthquakes, volcanoes and Earth’s magnetic field. This guide describes each layer, how scientists know the interior from seismic waves, compares the layers in a table, works through thickness and volume calculations, outlines the history of the discovery and corrects common mistakes.

What is the crust?

The crust is the thin outer layer of solid rock on which we live. There are two types. Continental crust is thick (about 30–50 km on average, and up to about 70 km under high mountains), relatively light (density about 2.7 g/cm³), composed mainly of granitic rocks rich in silica and aluminum, and old (parts are over 4 billion years old). Oceanic crust is thin (5–10 km), denser (about 3.0 g/cm³), mainly basalt and gabbro (rich in iron and magnesium), and young (not older than about 180 million years, because it is continually created at mid-ocean ridges and destroyed at subduction zones). The crust is only about 1 percent of Earth’s volume. The boundary between the crust and the mantle is called the Mohorovičić discontinuity (the “Moho”).

What is the mantle?

The mantle lies between the crust and the core and is about 2,900 km thick, making up about 83 percent of Earth’s volume. It consists of hot, dense silicate rock rich in iron and magnesium (peridotite). It is solid, but over millions of years it flows very slowly, like extremely thick putty (plastic deformation), carrying heat by convection: hotter rock rises and cooler rock sinks. These convection currents are one of the forces that move the tectonic plates. The temperature increases from about 1,000 °C near the top to over 3,000 °C near the core boundary (approximate values). Molten rock (magma) is formed locally where the pressure drops or water is added, not throughout the mantle.

What is the core?

The core is mainly iron with some nickel and lighter elements. The outer core (from about 2,900 to 5,150 km depth, 2,250 km thick) is liquid, and its motion generates Earth’s magnetic field by the dynamo effect. The inner core (radius about 1,220 km) is solid, even though it is hotter (about 5,000–6,000 °C, roughly similar to the surface of the Sun), because the enormous pressure (over 3 million times atmospheric pressure) keeps the iron solid. The inner core is slowly growing as the outer core cools and freezes onto it.

What are the lithosphere and asthenosphere?

Another way of dividing the outer Earth is by strength rather than composition. The lithosphere (“rock sphere”) is the cool, rigid outer shell made up of the crust and the uppermost mantle, around 100 km thick on average (thinner under oceans, thicker under old continents). It is broken into tectonic plates. Beneath it is the asthenosphere (“weak sphere”), a hot, partly molten and ductile part of the upper mantle (roughly between 100 and 400 km depth) on which the plates move.

Layers compared

Worked example: thickness and volume

1. Thickness of the layers: the outer core is 5,150 − 2,900 = 2,250 km thick. The inner core has a radius of 6,371 − 5,150 = 1,221 km. The mantle is 2,900 − 35 = 2,865 km thick if the continental crust is 35 km thick.
2. Core radius: the whole core has a radius of 6,371 − 2,900 = 3,471 km, about 3,480 km (the commonly quoted value).
3. Volume of the core: the volume of a sphere is proportional to the cube of the radius, so the core makes up (3,480 ÷ 6,371)³ = 0.5462³ = 0.163, or 16.3 percent of Earth’s volume.
4. Volume of the inner core: (1,220 ÷ 6,371)³ = 0.1915³ = 0.0070, or 0.7 percent, tiny compared with its fame.
5. Crust and mantle: for an average crust thickness of about 17 km (between the oceanic and continental values), the crust takes 1 − (6,354 ÷ 6,371)³ = 0.8 percent of the volume, so the mantle takes the rest: 100 − 16.3 − 0.8 = 82.9 percent.
6. How far have we drilled? The Kola Superdeep Borehole in Russia reached a depth of 12.262 km, which is 12.262 ÷ 6,371 = 0.19 percent of the way to the center. Our knowledge of the deeper layers comes from seismic waves.

How did we discover Earth’s interior?

The layers were found from the study of earthquake waves. Andrija Mohorovičić discovered in 1909 a sudden change in the speed of seismic waves at a depth of a few tens of kilometers: the crust–mantle boundary. Beno Gutenberg identified the core–mantle boundary in 1913 at about 2,900 km, from the “shadow zone” where S-waves (which cannot pass through liquids) do not arrive. Inge Lehmann, a Danish seismologist, showed in 1936 that the core has a smaller solid inner part, from the way P-waves are reflected. Later, models of the Preliminary Reference Earth Model (1981) set out the density and wave speed at each depth, and today seismic tomography gives 3D images of the interior.

Which mistakes are common?

Mistake 1: Imagining the mantle as liquid magma

It is solid rock that flows very slowly. Magma is formed only in certain places.

Mistake 2: Thinking that the inner core is liquid because it is hot

The very high pressure keeps it solid. The liquid layer is the outer core.

Mistake 3: Mixing up the crust with the lithosphere

The lithosphere includes the crust and the rigid top of the mantle. The plates are lithosphere, not just crust.

Mistake 4: Showing the crust as thick in diagrams

The crust is very thin compared with the radius: 35 km out of 6,371 km is about half of one percent in thickness.