Earth's core

Earth's core is the hot, innermost part of the planet. It cannot be studied directly because of its high heat and pressure and because of its unreachable location. Research into Earth's formation and structure requires seismology and other fields of study to make appropriate and logical inferences. As seismological technology improves, scientists' understanding of the core will increase. Earth's core is a critical element in the understanding of Earth's past, present, and future, especially because of the core's effect on Earth's magnetic field.

Characteristics of Earth's Core

Whether the structure of Earth is divided into rheological or chemical terms, the center is traditionally defined as a solid inner core surrounded by a liquid outer core. The existence of the inner core as a separate entity from the outer core was discovered in 1936 by Danish seismologist Inge Lehmann. Research in the early 2020s, which posited that the inner core consists of elements in an intermediate "superionic" state with both liquid and solid elements, challenged longstanding assumptions about the nature of the inner core.

Deeper than the crust and the mantle, the outer core begins at a depth of 2,890 kilometers (km), or 1,790 miles (mi), below Earth's surface and has a thickness of roughly 2,266 km (1,408 mi). Earth's inner core begins at a depth of 5,150 km (3,160 mi) below the surface, or about the length of 50,000 football fields. With a radius of about 1,226 km (760 mi), the inner core is roughly 70 percent the size of Earth's moon. Together, the inner and outer cores have a diameter about twice that of the moon.

Most substances can exist in different physical states of matter depending on the pressure and temperature to which they are subjected. Earth's core is a perfect example. Whereas both the inner and the outer core are composed mainly of iron and nickel, pressure and temperature differences contribute to their different states: the outer core is molten liquid, and the inner core, long held to be a normal solid, may, in fact, be in a highly unusual superionic state.

Iron is thought to account for about 80 percent of the inner core's composition, with nickel making up most of the remainder. Research suggests the presence of several siderophiles (“iron-loving” transition elements such as gold and platinum) in the innermost part of the core. These dense metals tend to bond readily with solid or molten iron. Trace amounts of lighter elements also are assumed to be present in both the inner and outer cores.

Temperature increases with distance from the surface; the core's temperature ranges from about 4,673 kelvins (K), or 4,400 degrees Celsius (C), at the outer edge of the outer core to about 6,373 K (6,100 degrees C) at the center of Earth, approximately the same as the surface of the sun. The extreme heat at the core results from three main sources: first, the residual heat from the processes that formed the Earth 4.5 billion years ago; second, frictional heat created as denser elements such as iron sank toward the center of the Earth during a major Earth-formation event known as the iron catastrophe; and third, heat released as a by-product of the decay of radioactive materials. The third source continues perpetually and likely accounts for the vast majority of the core's heat. The first two sources occurred billions of years ago but still contribute because Earth loses heat extremely slowly.

Since 1996, scientists have speculated that the inner core rotates faster than the rest of the Earth. Subsequent research confirmed this but added that the rotation was slower than initially thought. It is believed now that the inner core spins at a rate between 0.1 and 1 degree more than the rest of the Earth every one million years, much slower than earlier estimates of 1 degree each year.

Formation of Earth's Core

The solar system was born from the big bang 13.7 billion years ago. At first, the solar system was a solar nebula, a rotating cloud of helium, hydrogen, and other materials. An event 4.6 billion years ago, possibly a supernova, triggered the solar nebula to contract and flatten into a disc-like shape as it started rotating faster. Eventually, the sun formed in the center of this system, partly as a result of the nuclear fusion of hydrogen and helium. The remaining debris collided to form protoplanets, including one that would become Earth. These protoplanets continued to grow through the process of accretion. When the center of the Earth reached a high enough temperature, it was time for the interior of the planet to become more organized.

Earth's inner layers were created through a process called planetary differentiation, whereby the different substances that make up a planetary body separate into distinct layers because of different chemical and physical characteristics, particularly density. The core's formation took tens of thousands of years, which is considered fairly quick in planetary science. Modern isotopic studies suggest that this completion of the core layer occurred within 30 million years of Earth's beginning.

Before the core's formation, Earth comprised a mostly uniform blend of silicates and NiFe and other materials. Residual heat from Earth's formation and heat from the decay of radioactive materials in the Earth increased Earth's temperature to a point where the silicates began to melt. NiFe, which has a higher melting point and density than silicates, started sinking to Earth's center because of gravity. Even as the temperature became high enough to melt the NiFe, the material remained distinct from the silicates and “rained out” of the emulsion to continue sinking, ultimately forming Earth's core. This is referred to as the “rain-out model” when applied to Earth and other planetary bodies. In the Earth's planetary history, the formation of the core through the rain-out model is part of an event known as the “iron catastrophe,” which ultimately explains the core's effect on Earth's magnetic field.

Seismological Research of Earth's Core

Earth's core is too hot and too deep to be studied by humans directly, and even sending down equipment to make remote observations has proven to be impossible: Scientists have been able to drill 12 km (7.5 mi), only 0.2 percent toward the center. A Russian team of researchers started a drilling project in 1962, planning and building for eight years before breaking ground, and had to halt the project twenty-four years later as the drill reached the 12-km mark. The temperature was hotter than anticipated (180 degrees C; 356 degrees Fahrenheit [F]) instead of the originally estimated 100 degrees C (212 degrees F). The rocks in the crust began to behave like heated plastic, melting and blocking the progress of the drill.

Much of what is known about Earth's core comes from seismological research, although other approaches have included pressure and temperature research with crystalline solids, measurements of Earth's gravitational field, and the assumption that Earth's core is like a chondrite meteorite. Seismology, the study of the propagation of waves through a planetary body (particularly waves caused by earthquakes), allows scientists to peer into the virtual black box of the Earth to gather data and gain insight into its interior structure.

The fluidity of the outer core and the boundary between the mantle and core (2,890 km or 1,790 mi below the surface) were both discovered by observation of how seismic waves, particularly P (“pressure” and “primary”) waves and S (“shear” and “secondary”) waves, travel during earthquakes. P waves and S waves are types of body waves, which are a type of seismic wave that travels through substances. P waves involve direct longitudinal motion in the direction of propagation, whereas S waves behave transversely, moving perpendicular to the direction of propagation. P waves can travel through solids and fluids, but S waves can travel only through solids; the viscosity of fluid is too low to support the shear stresses of S waves. Scientists have observed that the waves behave differently in the core and mantle: P waves travel more slowly in the outer core than in the mantle, and S waves do not exist in the core at all. These observations helped clarify the location of the core-mantle boundary and helped to show that the outer core is liquid.

Seismology, the study of seismic waves, also accounts for the discovery of the two distinct parts of the core: the solid inner portion and the liquid outer portion. This discovery was made by Inge Lehmann in 1936. Lehmann had analyzed the data from a large earthquake, a magnitude 7.8 quake that hit New Zealand in 1929 and killed fifteen people. Lehmann noticed that the data showed the quake had strange P-wave behavior: Some waves were detected at unexpected observation points on Earth's surface as if they had been deflected within the Earth's interior. This would not have happened had the core been homogeneously liquid throughout, so Lehmann hypothesized about the existence of a solid inner core. (Lehmann later found another seismic discontinuity closer to Earth's crust within the mantle; this discontinuity is generally called the Lehmann discontinuity.)

Lehmann's discovery of the inner/outer core boundary is similar to earlier work with P waves and S waves done by Croatian seismologist Andrija Mohorovičić, who discovered the boundary between Earth's crust and Earth's mantle. The boundary is now called the Mohorovičić discontinuity.

Seismology research continues to be one of the most useful studies of Earth's interior. Seismological instruments are becoming more and more sensitive, leading to a greater body of data.

Other Research Methods Used to Study Earth's Core

Some properties of the Earth's core are determined mainly by inference. It is known that the core is highly dense, for example, because of the discrepancy between the Earth's average density (5,515 kg per cubic meter, or 344 pounds per cubic foot) and the average density of materials at the Earth's surface (3,000 kg per cubic meter or 187 pounds per cubic foot), which can be observed directly. (The Earth's average density is known by calculating the Earth's mass based on the force of its gravitational pull and then estimating its volume.)

Geophysicists also infer much about the core based on the assumption that Earth is like an ordinary chondrite meteorite, an assumption initially put forth by American geophysicist Francis Birch in 1940. Chondrites are the most common type of meteorite to land on Earth, and about 90 percent of those are classified as “ordinary.” Ordinary chondrites are composed mainly of NiFe alloys and iron sulfide (FeS), also called troilite. Chondrite meteorites are born from asteroids that were too small to melt or undergo planetary differentiation during the formation of the solar system.

Modern laboratory experimentation provides further insight into the core's structure and properties. In August 2011, for example, Kei Hirose, a scientist at the Tokyo Institute of Technology, became the first to examine what conditions in the center of the core must be like by recreating these conditions in his laboratory. After using a diamond-tipped vise to subject a NiFe alloy to three million times atmospheric pressure and 4,500 degrees C (8,132 degrees F), Hirose found that the alloy structure realigned to form a “forest” of large crystals. Computer models in the mid-1990s had suggested the presence of one giant iron crystal in the Earth's core, but subsequent research leaned toward an alignment of smaller crystals, even going so far as to pinpoint the alignment in a way that allows north-south seismic waves to travel faster through the Earth than do east-west waves. Hirose's research adds support to the hypothesis of many smaller crystals.

Effect of the Core on Earth's Magnetic Field

Earth's magnetic field protects the planet from the effects of solar wind, a stream of charged particles that escapes from the sun's surface. While most solar wind particles are deflected outward by the magnetic field, some are trapped by the Van Allen radiation belt, a region held in place by the magnetic field. Some particles even make it through to the Earth's upper atmosphere, causing geomagnetic storms and aurorae (such as the northern lights).

Earth's magnetic field moves over time, even completely reversing direction every few hundred thousand years. The geographic North Pole and geographic South Pole are, therefore, not the same as the magnetic North Pole and magnetic South Pole, respectively.

The outer core is largely responsible for maintaining Earth's magnetic field through a phenomenon described by the dynamo theory, which explains the existence of long-lived magnetic fields that do not collapse from ohmic decay over time. According to the dynamo theory, these fields can be created and maintained by a fluid that meets three requirements: It rotates (providing kinetic energy), convection is present within it (caused by an internal source of heat), and it can conduct electricity. The liquid outer core of the Earth fulfills all of these requirements.

The Coriolis effect (an inertial force powered by Earth's rotation) produces the necessary rotation within the core fluid, the liquid composition conducts electricity, and a variety of sources provide the energy for convection, including gravitational energy still released in the aftermath of core formation and the radioactive decay of trace elements found in the core. The particulars of convection within the outer core (imagine a turbulently churning molten sea of metal) is a widely researched topic. The strength of the magnetic field within the outer core is estimated to be about fifty times that of the Earth's magnetic field at the surface.

The inner core is too hot to maintain a magnetic field on its own. Under high heat, the orientation of iron's molecules becomes randomized, which prevents it from exerting a magnetic force. Additionally, the inner core cannot meet the fluidity requirements set forth by the dynamo theory (because it is at least somewhat solid), but the inner core is thought to somehow help the outer core stabilize Earth's magnetic field.

Continuing Research About Earth's Core

Modern research continues to examine Earth's core to try to confirm past observations and to get a more detailed picture of its formation, composition, and effects on Earth's magnetic field. In 2010, for example, a team at Université Joseph Fourier in Grenoble, France, used models of fluid flow in the core to solve the discrepancies among various past inferences of the strength of the magnetic field within the outer core. Meanwhile, collaboration between Université Paris Diderot (in France) and the Lawrence Livermore National Laboratory (in Livermore, California) confirmed the percentages of nickel and silicon assumed to be present in the inner core (about 5 percent nickel and 2 percent silicon). Earlier estimates were inferred by seismological research; the contemporary team took high-pressure sound velocity measurements from a crystalline iron-nickel-silicon alloy to arrive at the same results.

Outer core convection is a popular topic of core-related research. In February 2010, for example, a team of researchers from Japan examined the process of convection in the outer core, part of what maintains the Earth's magnetic field, and found evidence of zonal flows: The convection system in the core actually contains two types of motion (sheet-like radial plumes and the newly discovered zonal flows). Also in 2010, a Chinese team of computer scientists developed an improved “solver” for modeling the core's convection. In September 2011, a research team in California examined the thermal and chemical buoyancy forces driving convection in the outer core; the team created a buoyancy model to be used with geodynamo models.

In February 2022, physicists at the Chinese Academy of Scientists published a study in which they revealed the discovery of elements in a highly unusual superionic state in Earth's inner core. They found that these elements, primarily hydrogen, oxygen, and carbon, exist in an intermediate state between liquid and solid due to the intense heat and pressure of the inner core, upending long-held assumptions about the solid nature of the inner core. In 2025, researchers theorized that the Earth's inner core may be less solid than previously thought. Researchers used specialized seismograms to show deformities in the inner core, showing that it changes its shape over time.

An actual “journey to the center of the Earth” is still impossible by all modern standards. However, scientists can continue to improve upon understandings of Earth's core with the development of better computer models and other research strategies.

Principal Terms

accretion: the growth of a planet through gravitational attraction of matter in space; the process by which Earth and other large planetary bodies formed

chondrite meteorite: the most common type of meteorite to fall to Earth; much core research relies on the assumption that Earth is like a chondrite meteorite

convection: transfer of heat via the movement of molecules in liquids and gases

Coriolis effect: an inertial force that makes objects in motion deflect to a side; caused by Earth's rotation

dynamo theory: explains the creation of long-lived magnetic fields by a fluid that convects, conducts electricity, and rotates

iron catastrophe: an early Earth event in which iron and other dense elements sank to the center of the planet, leading to the formation of Earth's core

magnetic field: an invisible area produced by an electric field that can exert a magnetic force on certain things in or around it

pain-out model: the stage of planetary differentiation when the core is formed during the “raining out” of iron from the molten NiFe (nickel and iron) from silicate emulsion toward the center of the planet

planetary differentiation: a process in which a planetary body separates into distinct layers because of the different chemical and physical characteristics of the substances that make up its composition

radioactive decay: the spontaneous emission of ionizing particles from an unstable atom; the rate of radioactive decay is described as the “half-life” of a given substance

seismic waves: energy waves that travel through a planetary body because of processes, such as earthquakes, which produce low-frequency acoustic energy

seismology: the study of the propagation of waves through a planet; mostly focused on earthquake research

superionic state: an intermediate state of matter with both liquid and solid features

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