Layers of the Earth: Crust, Mantle and Core

Earth is one of the rocky planets of the solar system, but below the surface we see, the interior splits into layers that differ sharply in composition, temperature and physical state.

Earth’s crust

According to National Geographic Education, Earth’s crust is about 40 km thick on average and makes up only 1% of the planet’s mass. Yet it holds all known life.

The crust is not uniform. According to National Geographic Education, oceanic crust (the sima) runs 5-10 km below the seafloor, while continental crust (the sial) can reach 70 km, far thicker than oceanic crust. The USGS gives a somewhat different picture for continental crust, putting the average closer to 30 km, with peaks up to 100 km under major mountain ranges: the two sources measure with different methods and samples, which is why the figures for the same thing diverge. Either way, the composition stays the same: according to National Geographic Education, both oceanic and continental crust consist mainly of silicates, the compounds of silicon and oxygen that make up most common rocks.

Earth’s mantle

Below the crust lies the mantle. According to National Geographic Education, it is about 2,900 km thick and takes up 84% of Earth’s volume, made mostly of silicates such as olivine, garnet and pyroxene, plus magnesium oxide and traces of other elements.

According to the USGS, the mantle is a dense, hot layer of mostly solid rock, holding higher levels of iron, magnesium and calcium than the crust sitting above it. According to National Geographic Education, temperature rises with depth, from about 1,000°C near the crust to about 3,700°C near the core. Only in certain spots — tectonic plate boundaries and mantle plumes, columns of hot rock rising toward the surface, according to National Geographic Education — does the rock become less viscous. It is at those plate boundaries that the mantle’s effects show up most clearly, from volcanoes to earthquakes.

Outer core and inner core

According to National Geographic Education, the core begins at a depth of about 2,900 km and has a radius of about 3,485 km. Unlike the mineral-rich crust and mantle, it is made almost entirely of metal, mostly iron and nickel.

It splits into two parts. According to the USGS, the outer core, about 2,200 km thick, is liquid iron and nickel, and its rotation, combined with Earth’s own rotation, generates the planet’s magnetic field. The inner core, about 1,250 km thick according to the USGS, is solid instead: according to National Geographic Education, the pressure of the entire planet pressing down on it keeps the iron from melting, even though it stays at extreme temperatures. Overall, according to National Geographic Education, the core ranges from about 4,400°C to about 6,000°C.

Practical example: imagine drilling a well down to the deepest point of continental crust, about 70 km according to National Geographic Education. You would have crossed only the planet’s outer skin: below you would still lie about 2,900 km of mantle, and then the core, all the way to Earth’s center.

How we know what’s inside Earth

According to INGV, Earth’s internal structure is reconstructed by studying seismic waves: when an earthquake sends waves through the planet, changes in their speed and “shadow zones” (regions the waves never reach) reveal where the material changes, for instance from the solid mantle to the liquid outer core. No drill has ever reached that deep.

The discovery of the solid inner core, distinct from the liquid outer core, is credited to seismologist Inge Lehmann, according to INGV. Further research has found hints of a structure inside the inner core itself, a so-called “innermost inner core” still under study.

Does Earth have 3 layers or 5?

According to Geosciences LibreTexts, Earth can be divided into layers either by chemical composition or by physical behavior under mechanical stress: two different criteria describing the same planet from two angles. The chemical count — crust, mantle, core — lands on three layers; the physical count instead distinguishes the lithosphere, asthenosphere, mesosphere, outer core and inner core, adding up to five. These are not rival theories, just two ways of counting the same planet. The four-layer model common in school textbooks grows out of exactly this overlap: a core split in two, borrowed from the physical criterion, next to a single mantle, borrowed from the chemical one.