What is the Earth made of? Crust, mantle and core, explained

The Earth is made of rock and metal in layers: a thin, rocky crust; a mantle of hot rock nearly 2,900 km thick that flows very slowly; and a core of iron and nickel, liquid on the outside and solid at the center, 6,371 km down. By mass, the whole planet is mostly four elements: iron, oxygen, silicon and magnesium. Nobody has been below the crust, so all of this was worked out from earthquake waves, meteorites and physics.

In short

• The crust is a skin: on a globe 30 cm across, it would be about a millimeter thick or less.
• The heavy iron sank to the middle when the young Earth was molten, so the planet is sorted by density, lightest on top.
• Earthquake waves revealed the layers: some waves can't pass through liquid, and the gap where they vanish showed the outer core is molten.

What are the Earth's layers?

From the surface down, there are four main layers. The figures below come from the U.S. Geological Survey's Inside the Earth and from published estimates; deep temperatures are calculated, not measured, so they are given as ranges.

Layer	Depth from the surface	Thickness	State	Made of	Temperature, approximate
Crust	0 to about 5–70 km	5–10 km under oceans, about 30–40 km under continents, more under mountains	Solid, brittle	Granite-like rock on continents, basalt under oceans	From surface temperature to several hundred °C at its base
Mantle	To about 2,890 km	About 2,900 km	Solid rock that creeps very slowly	Silicate rock rich in magnesium and iron, such as peridotite	From a few hundred °C at the top to roughly 3,500–4,000 °C at the bottom
Outer core	2,890 to about 5,150 km	About 2,260 km	Liquid	Iron and nickel, with some lighter elements	Roughly 4,000 to 5,000 °C or more
Inner core	5,150 km to the center at 6,371 km	Radius about 1,220 km	Solid	Mostly iron, some nickel	Roughly 5,100 to 6,000 °C

The mantle holds about two-thirds of the planet's mass and the core about a third; the crust, everything humans have ever stood on or mined, is well under 1%. The uppermost mantle and the crust together form a rigid shell roughly 100 km thick, the lithosphere, broken into the pieces whose motion is plate tectonics. Below it the mantle is hot enough to flow like very stiff putty over millions of years, though it is solid rock, not a sea of lava. Magma forms only in particular places where rock partly melts, which is how volcanoes form.

The inner core is hotter than the outer core and yet solid. The reason is pressure. At the center the pressure is more than three million times that at the surface, and pressure raises the temperature iron needs to melt. As the planet slowly cools, the inner core grows by freezing iron out of the liquid around it.

What is the whole Earth made of, by element?

Weighing the planet's chemistry as a whole gives a different list from the rocks underfoot. By mass, Earth is about 32% iron, 30% oxygen, 15% silicon, 14% magnesium, 3% sulfur, 2% nickel, and 1.5% each of calcium and aluminum, according to a widely cited 1980 estimate by John Morgan and Edward Anders; later estimates differ by a few percent, most of all for sulfur. The crust alone is very different: mostly oxygen and silicon, with aluminum, iron, calcium, sodium, potassium and magnesium, combined into minerals such as quartz and feldspar. The elements themselves, and why iron is so common, are the subject of the periodic table.

The difference comes from history. About 4.5 billion years ago the young planet was hot enough to melt, and dense iron sank toward the center while lighter rock floated up, a sorting called differentiation. That left most of the iron in the core and most of the light elements near the surface.

How is the inside known, if no one has been there?

The deepest hole ever drilled, the Kola Superdeep Borehole in northern Russia, reached 12,262 m in 1989. Work stopped when the rock reached about 180 °C, too hot for the drill to go on. That is the whole depth humans have reached directly. Here is how far that is:

1. The distance to the center is 6,371 km, or 6,371,000 m.
2. 12,262 ÷ 6,371,000 = 0.0019, so the hole went 0.19% of the way down.
3. On a globe 30 cm across, the radius is 15 cm, or 150 mm. 0.19% of 150 mm is about 0.3 mm.
4. A credit card is about 0.8 mm thick. The deepest hole is less than half of it, and even the continental crust, at about 35 km, would be under a millimeter.

Everything deeper is known by indirect evidence that fits together. Three lines matter most.

Earthquake waves

An earthquake sends waves through the whole planet, and seismometers around the world record when they arrive. Two kinds matter here. P waves are push-and-pull waves that travel through solids and liquids. S waves are side-to-side shaking waves that cannot travel through a liquid at all, because a liquid can't spring back from being sheared. How fast waves travel depends on what they pass through, and they bend where the material changes, as light bends entering water.

The discoveries came step by step:

1. 1906. Richard Dixon Oldham, a British geologist who had worked for the Geological Survey of India, showed that P waves arriving on the far side of the world from an earthquake were late, which he explained with a core of different material in the middle.
2. 1909. Andrija Mohorovičić noticed waves speeding up at a few tens of kilometers depth under Croatia: the base of the crust, now called the Moho.
3. 1914. Beno Gutenberg placed the edge of the core at about 2,900 km.
4. The shadow zone. Seen from an earthquake, S waves disappear beyond about 104° of the way around the planet, and direct P waves vanish between about 104° and 140°, a band about 4,000 km wide. A liquid outer core explains both: it stops S waves and bends P waves away, like a lens.
5. 1936. Inge Lehmann, a Danish seismologist, studied the 1929 Murchison earthquake in New Zealand and found faint P waves arriving inside the shadow zone where none should be. She explained them as waves bent by a separate inner core, and published the idea in a paper titled simply P′. That the inner core is solid was argued in the following years and later confirmed.

Later instruments confirmed both cores and now map the mantle in three dimensions, a technique called seismic tomography.

Meteorites

Meteorites are leftovers of the material the planets formed from. Stony ones called chondrites have, element for element, about the mix the whole Earth should have. Iron meteorites are pieces of the metal cores of small worlds that broke apart, which shows what planetary cores are made of. The Moon, most likely made from the outer layers of a giant collision with the young Earth, is the opposite case: it has very little iron, as the note on what the Moon is made of explains.

Weight and spin

Earth's average density is 5.51 grams per cubic centimeter, according to NASA's Earth fact sheet, but surface rocks are only about 2.7 to 3. The interior must be much denser to make up the difference, which points to a metal core. The way Earth wobbles as it spins, and the way its liquid outer core churns to make the planet's magnetic field, agree.

What do people get wrong about the inside of the Earth?

The most common picture is a thin crust floating on molten lava. The mantle is solid. It flows only in the sense that a glacier flows: slowly, over long times, while staying solid. Molten rock is the exception, found in pockets where pressure drops or water lowers the melting point.

A second mistake is that the deep Earth is known only by guesswork. The layers' depths are pinned down to within a few kilometers by millions of seismic records. What is uncertain is detail: the exact temperatures, which lighter elements are mixed into the core, and the structure of the inner core. Ideas of a hollow Earth are ruled out by the same seismic waves, which pass through a solid, dense center.

Still argued

The temperature of the inner core isn't settled. It is estimated from how hot iron must be to melt at the pressure of the inner core's edge, and methods disagree. Calculations from first principles by Dario Alfè and colleagues put that edge at about 5,400 to 5,700 kelvin in a 2007 review, roughly 5,100 to 5,400 °C. A 2013 experiment by Anzellini and colleagues found iron melts there at about 6,230 kelvin, give or take 500, near 6,000 °C. Either way the center is about as hot as the surface of the Sun.

Where does this lead next?

The layers explain most of what the surface does. The churning outer core makes the magnetic field that steers the northern lights. Heat rising through the mantle moves the plates, which build mountains, open oceans and dig the deepest trench on Earth, the Mariana Trench. And the same sorting by density, iron in the middle and rock outside, shaped the other rocky planets of the solar system.

Check yourself

• Without looking back: why is the inner core solid when it is hotter than the liquid outer core?
• How does the S-wave shadow zone show that the outer core is liquid?
• Why is the list of elements in the whole Earth so different from the list in the crust?

Sources

• Inside the Earth, in This Dynamic Earth — U.S. Geological Survey
• Earth fact sheet — NASA Goddard Space Flight Center
• Core — National Geographic Society
• The deepest hole we have ever dug — BBC Future, 2019
• Internal structure of Earth — Wikipedia
• Chemical composition of Earth, Venus, and Mercury — John W. Morgan and Edward Anders, PNAS, 1980