Layers of the Earth

Think of a hard-boiled egg cut in half.

It has a thin shell, a thick white and a yellow yolk. Earth is a bit like that. It has a thin outside, a thick middle part and a center made of metal.

The U.S. Geological Survey (USGS) compares Earth to a boiled egg. The crust is like the thin, brittle shell. The mantle is like the thick egg white. The core in the middle is like the yolk.

Earth is about 12,750 kilometers across, so it is a long way to the middle. People knew Earth's size more than 2,000 years ago, in ancient Greece, but scientists only worked out its three main layers around the start of the 1900s. The core turned out to have two parts, which makes four layers in all: crust, mantle, outer core and inner core.

Earth is about 12,750 km in diameter, a figure the ancient Greeks had already estimated. Yet it was only at the turn of the 20th century, the USGS notes, that scientists established that the planet is made of three main layers: crust, mantle and core. The USGS compares this to a boiled egg, with the crust as a thin brittle shell, the mantle as the white and the core as the yolk, except that the core has two distinct parts, a liquid outer core and a solid inner core.

The layers are sorted by density. NASA describes how Earth formed about 4.5 billion years ago from gas and dust pulled together by gravity; National Geographic explains that as the young planet heated up, heavy iron and nickel sank to the center to form the core, while lighter rocky material stayed nearer the surface and became the mantle and crust. Geologists call this sorting planetary differentiation.

We live on the crust.

It is the hard, rocky outside of the Earth. Mountains, oceans and cities all sit on it. The crust is very thin compared with the whole Earth.

The crust is the rocky outer layer we live on. It is thinnest under the oceans, usually only about 5 to 10 kilometers thick. Under the continents it is thicker: the British Geological Survey (BGS) gives about 25 to 70 kilometers, and the USGS says that under big mountain ranges such as the Alps it can reach about 100 kilometers.

That sounds deep, but Earth is about 6,400 kilometers from the surface to the center. For the size of the planet, the crust is about as thin as the shell on an egg. And like an eggshell, it is brittle and can crack.

The crust is the brittle outermost layer. Oceanic crust is about 5 to 10 km thick and made mostly of basalt; continental crust is about 25 to 70 km thick (BGS), mostly granite-type rock, and can reach about 100 km beneath ranges such as the Alps or the Sierra Nevada (USGS). National Geographic notes that the crust holds only about 1 percent of Earth's mass, yet it contains all known life. Its base is the Mohorovicic discontinuity, or Moho, where the temperature is about 200 to 400 °C.

Oceanic crust is constantly recycled, so it is young: the oldest, under the Ionian Sea in the eastern Mediterranean, is about 270 million years old, while parts of the continental crust are more than four billion years old. The crust and the cooler, rigid top of the mantle together form the lithosphere, which the USGS says averages at least 80 km thick and is broken into the moving plates.

Under the crust is the mantle.

It is the thickest layer. It is made of very hot rock. The rock is solid, but over a very long time it moves very slowly.

Below the crust is the mantle, the thickest layer of all. It is about 2,900 kilometers thick. The deeper you go, the hotter it gets and the more the rock is squeezed.

The mantle is not a sea of melted rock. Earthquake waves that can only pass through solids travel right through it, so scientists know it is solid. But it is so hot that, over millions of years, parts of it can flow very slowly, like a very stiff putty. Earth's plates, made of the crust and the stiff top of the mantle, ride on top of this slowly moving layer.

The mantle reaches down to about 2,890 km (BGS) and is made of dense silicate rock, richer in iron, magnesium and calcium than the crust (USGS). Temperature and pressure rise with depth; National Geographic gives the geothermal gradient, the rise in temperature with depth, as about 25 °C per kilometer.

Both P-waves and S-waves cross the mantle, and S-waves cannot travel through liquids, so the mantle is solid. Yet over geological time it behaves as a very slow fluid. The cooler top of the mantle is welded to the crust as the lithosphere; beneath it, the USGS describes the asthenosphere, a weaker zone of hot rock that can soften and flow, on which the lithosphere moves. BGS notes that slow convection in the mantle is one of the forces proposed to move the plates, alongside ridge push and slab pull, and that research points to slab pull as the main driver for most plates.

In the middle of the Earth is the core.

It is made of metal, mostly iron. The outside part is runny, like hot soup. The very middle is a solid ball of metal.

The core starts about 2,900 kilometers down. It is made mostly of the metals iron and nickel, so it is nearly twice as dense as the mantle.

The outer core is about 2,200 kilometers thick and is liquid metal. National Geographic gives its temperature as about 4,500 to 5,500 °C. As Earth spins, this liquid metal swirls around, and that makes Earth's magnetic field.

The inner core is a solid ball of metal. From its center to its edge is about 1,220 kilometers, and it is about 5,200 °C. It is hotter than the temperature at which iron melts, but the weight of the whole planet pressing down keeps it solid.

The core begins at about 2,900 km and is an iron-nickel alloy, nearly twice as dense as the mantle (USGS). The liquid outer core is about 2,200 km thick at roughly 4,500 to 5,500 °C (National Geographic). Convection in this electrically conducting liquid, organized by Earth's rotation, acts as a geodynamo that generates the magnetic field.

The inner core has a radius of about 1,220 km (National Geographic; NASA gives 1,221 km and the USGS about 1,250 km) and a temperature of about 5,200 °C, with NASA giving up to 5,400 °C. It is solid despite being far above the melting point of iron, because the pressure there, nearly 3.6 million atmospheres, keeps the iron from melting. National Geographic adds that the inner core grows by about a millimeter a year as the outer core slowly crystallizes onto it, and that it rotates slightly faster than the rest of the planet. Precise core temperatures are difficult to determine, which is why sources give ranges.

Nobody can dig to the middle of the Earth.

So scientists listen to earthquakes. Earthquakes send waves through the whole planet. The waves change when they reach a new layer.

No one has ever seen the mantle or the core. So how do we know they are there? Earthquakes send waves of shaking through the whole planet. Machines called seismometers around the world record when the waves arrive.

The waves bend and change speed as they pass from one layer to another, a little like light bending in a glass of water. One kind of wave, the S-wave, cannot travel through liquid. It never makes it across the outer core, and that is how scientists know the outer core is liquid. In 1936, the Danish scientist Inge Lehmann studied faint waves that should not have arrived and worked out that a solid inner core sits inside the liquid one.

Seismic waves from large earthquakes travel through the whole Earth. Their speed depends on the material they cross, so they refract at boundaries, as light does in a prism, and their travel times let seismologists map how density changes with depth (BGS). Two kinds matter most: fast compressional P-waves, which cross solids and liquids, and slower shear S-waves, which cannot cross liquids.

The core refracts P-waves and casts a shadow zone between about 103° and 143° from an earthquake, and S-waves do not pass through the outer core at all, which shows it is liquid. Harold Jeffreys concluded in 1926 that the core must be fluid. Inge Lehmann, who ran Denmark's seismograph network, noticed weak P-waves arriving inside the shadow zone; in her 1936 paper, titled simply "P′", she proposed that they had been deflected by a solid inner core inside the liquid outer core (Earth magazine).

People once dug a very deep, thin hole.

It went down about 12 kilometers. That is still only a tiny part of the crust. The center of the Earth is much, much deeper.

The deepest hole people have ever dug is the Kola Superdeep Borehole in northern Russia. Scientists drilled for about 20 years and reached 12,262 meters, about 12 kilometers. Drilling stopped in 1992.

It got much hotter down there than they expected: about 180 °C, almost twice what they had predicted. The heat bent the drill parts. Even this record hole went only part of the way through the crust on land, and the center of the Earth is about 6,400 kilometers down, more than 500 times deeper.

The Kola Superdeep Borehole, near Murmansk in Russia, is still the deepest artificial point on Earth at 12,262 m, reached after about 20 years of drilling, mostly for science; drilling stopped in 1992 (Scientific American). The team had expected to reach about 14,500 m, but rock at the bottom, 2.7 billion years old, was around 180 °C, nearly twice the prediction, and it deformed the drill bits and pipes. Some later oil wells are longer, but none is deeper.

An American project in 1958, Project Mohole, tried to drill into the Pacific Ocean floor to reach the boundary between the crust and the mantle; funding ended in 1966 after the drillers had reached only 183 m. Compared with the 6,400 km to Earth's center, even Kola reached less than 0.2 percent of the way, which is why seismic waves remain the main window into the deep Earth.