Earth’s core is the very hot, very dense center of our planet. The ball-shaped core lies beneath the cool, brittle crust and the mostly-solid mantle. The core is found about 2,900 kilometers (1,802 miles) below Earth’s surface, and has a radius of about 3,485 kilometers (2,165 miles).
 
Planet Earth is older than the core. When Earth was formed about 4.5 billion years ago, it was a uniform ball of hot rock. Radioactive decay and leftover heat from planetary formation (the collision, accretion, and compression of space rocks) caused the ball to get even hotter. Eventually, after about 500 million years, our young planet’s temperature heated to the melting point of iron—about 1,538° Celsius (2,800° Fahrenheit). This pivotal moment in Earth’s history is called the iron catastrophe.
 
The iron catastrophe allowed greater, more rapid movement of Earth’s molten, rocky material. Relatively buoyant material, such as silicates, water, and even air, stayed close to the planet’s exterior. These materials became the early mantle and crust. Droplets of iron, nickel, and other heavy metals gravitated to the center of Earth, becoming the early core. This important process is called planetary differentiation.
 
Earth’s core is the furnace of the geothermal gradient. The geothermal gradient measures the increase of heat and pressure in Earth’s interior. The geothermal gradient is about 25° Celsius per kilometer of depth (1° Fahrenheit per 70 feet). The primary contributors to heat in the core are the decay of radioactive elements, leftover heat from planetary formation, and heat released as the liquid outer core solidifies near its boundary with the inner core. 
 
Unlike the mineral-rich crust and mantle, the core is made almost entirely of metal—specifically, iron and nickel. The shorthand used for the core’s iron-nickel alloys is simply the elements’ chemical symbols—NiFe. 
 
Elements that dissolve in iron, called siderophiles, are also found in the core. Because these elements are found much more rarely on Earth’s crust, many siderophiles are classified as “precious metals.” Siderophile elements include gold, platinum, and cobalt. 
 
Another key element in Earth’s core is sulfur—in fact 90% of the sulfur on Earth is found in the core. The confirmed discovery of such vast amounts of sulfur helped explain a geologic mystery: If the core was primarily NiFe, why wasn’t it heavier? Geoscientists speculated that lighter elements such as oxygen or silicon might have been present. The abundance of sulfur, another relatively light element, explained the conundrum.
 
Although we know that the core is the hottest part of our planet, its precise temperatures are difficult to determine. The fluctuating temperatures in the core depend on pressure, the rotation of the Earth, and the varying composition of core elements. In general, temperatures range from about 4,400° Celsius (7,952° Fahrenheit) to about 6,000° Celsius (10,800° Fahrenheit).
 
The core is made of two layers: the outer core, which borders the mantle, and the inner core. The boundary separating these regions is called the Bullen discontinuity. 
 
Outer Core
 
The outer core, about 2,200 kilometers (1,367 miles) thick, is mostly composed of liquid iron and nickel. The NiFe alloy of the outer core is very hot, between 4,500° and 5,500° Celsius (8,132° and 9,932° Fahrenheit). 
 
The liquid metal of the outer core has very low viscosity, meaning it is easily deformed and malleable. It is the site of violent convection. The churning metal of the outer core creates and sustains Earth’s magnetic field.
 
The hottest part of the core is actually the Bullen discontinuity, where temperatures reach 6,000° Celsius (10,800° Fahrenheit)—as hot as the surface of the sun.
 
Inner Core
 
The inner core is a hot, dense ball of (mostly) iron. It has a radius of about 1,220 kilometers (758 miles). Temperature in the inner core is about 5,200° Celsius (9,392° Fahrenheit). The pressure is nearly 3.6 million atmosphere (atm).
 
The temperature of the inner core is far above the melting point of iron. However, unlike the outer core, the inner core is not liquid or even molten. The inner core’s intense pressure—the entire rest of the planet and its atmosphere—prevents the iron from melting. The pressure and density are simply too great for the iron atoms to move into a liquid state. Because of this unusual set of circumstances, some geophysicists prefer to interpret the inner core not as a solid, but as a plasma behaving as a solid.
 
The liquid outer core separates the inner core from the rest of the Earth, and as a result, the inner core rotates a little differently than the rest of the planet. It rotates eastward, like the surface, but it’s a little faster, making an extra rotation about every 1,000 years. 
 
Geoscientists think that the iron crystals in the inner core are arranged in an “hcp” (hexagonal close-packed) pattern. The crystals align north-south, along with Earth’s axis of rotation and magnetic field.
 
The orientation of the crystal structure means that seismic waves—the most reliable way to study the core—travel faster when going north-south than when going east-west. Seismic waves travel four seconds faster pole-to-pole than through the Equator.
 
Growth in the Inner Core
As the entire Earth slowly cools, the inner core grows by about a millimeter every year. The inner core grows as bits of the liquid outer core solidify or crystallize. Another word for this is “freezing,” although it’s important to remember that iron’s freezing point more than 1,000° Celsius (1,832° Fahrenheit).
 
The growth of the inner core is not uniform. It occurs in lumps and bunches, and is influenced by activity in the mantle. 
 
Growth is more concentrated around subduction zones—regions where tectonic plates are slipping from the lithosphere into the mantle, thousands of kilometers above the core. Subducted plates draw heat from the core and cool the surrounding area, causing increased instances of solidification. 
 
Growth is less concentrated around “superplumes” or LLSVPs. These ballooning masses of superheated mantle rock likely influence “hot spot” volcanism in the lithosphere, and contribute to a more liquid outer core.
 
The core will never “freeze over.” The crystallization process is very slow, and the constant radioactive decay of Earth’s interior slows it even further. Scientists estimate it would take about 91 billion years for the core to completely solidify—but the sun will burn out in a fraction of that time (about 5 billion years).
 
Core Hemispheres
Just like the lithosphere, the inner core is divided into eastern and western hemispheres. These hemispheres don’t melt evenly, and have distinct crystalline structures.
 
The western hemisphere seems to be crystallizing more quickly than the eastern hemisphere. In fact, the eastern hemisphere of the inner core may actually be melting. 
 
Inner Inner Core
Geoscientists recently discovered that the inner core itself has a core—the inner inner core. This strange feature differs from the inner core in much the same way the inner core differs from the outer core. Scientists think that a radical geologic change about 500 million years ago caused this inner inner core to develop.
 
The crystals of the inner inner core are oriented east-west instead of north-south. This orientation is not aligned with either Earth’s rotational axis or magnetic field. Scientists think the iron crystals may even have a completely different structure (not hcp), or exist at a different phase.
 
Magnetism
 
Earth’s magnetic field is created in the swirling outer core. Magnetism in the outer core is about 50 times stronger than it is on the surface.
 
It might be easy to think that Earth’s magnetism is caused by the big ball of solid iron in the middle. But in the inner core, the temperature is so high the magnetism of iron is altered. Once this temperature, called the Curie point, is reached, the atoms of a substance can no longer align to a magnetic point. 
 
Dynamo Theory
Some geoscientists describe the outer core as Earth’s “geodynamo.” For a planet to have a geodynamo, it must rotate, it must have a fluid medium in its interior, the fluid must be able to conduct electricity, and it must have an internal energy supply that drives convection in the liquid. 
 
Variations in rotation, conductivity, and heat impact the magnetic field of a geodynamo. Mars, for instance, has a totally solid core and a weak magnetic field. Venus has a liquid core, but rotates too slowly to churn significant convection currents. It, too, has a weak magnetic field. Jupiter, on the other hand, has a liquid core that is constantly swirling due to the planet’s rapid rotation.
 
Earth is the “Goldilocks” geodynamo. It rotates steadily, at a brisk 1,675 kilometers per hour (1,040 miles per hour) at the Equator. Coriolis forces, an artifact of Earth’s rotation, cause convection currents to be spiral. The liquid iron in the outer core is an excellent electrical conductor, and creates the electrical currents that drive the magnetic field.
 
The energy supply that drives convection in the outer core is provided as droplets of liquid iron freeze onto the solid inner core. Solidification releases heat energy. This heat, in turn, makes the remaining liquid iron more buoyant. Warmer liquids spiral upward, while cooler solids spiral downward under intense pressure: convection.
 
Earth’s Magnetic Field
Earth’s magnetic field is crucial to life on our planet. It protects the planet from the charged particles of the solar wind. Without the shield of the magnetic field, the solar wind would strip Earth’s atmosphere of the ozone layer that protects life from harmful ultraviolet radiation.
 
Although Earth’s magnetic field is generally stable, it fluctuates constantly. As the liquid outer core moves, for instance, it can change the location of the magnetic North and South Poles. The magnetic North Pole moves up to 64 kilometers (40 miles) every year.
 
Fluctuations in the core can cause Earth’s magnetic field to change even more dramatically. Geomagnetic pole reversals, for instance, happen about every 200,000 to 300,000 years. Geomagnetic pole reversals are just what they sound like: a change in the planet’s magnetic poles, so that the magnetic North and South Poles are reversed. These “pole flips” are not catastrophic—scientists have noted no real changes in plant or animal life, glacial activity, or volcanic eruptions during previous geomagnetic pole reversals.
 
Studying the Core
 
Geoscientists cannot study the core directly. All information about the core has come from sophisticated reading of seismic data, analysis of meteorites, lab experiments with temperature and pressure, and computer modeling.
 
Most core research has been conducted by measuring seismic waves, the shock waves released by earthquakes at or near the surface. The velocity and frequency of seismic body waves changes with pressure, temperature, and rock composition.
 
In fact, seismic waves helped geoscientists identify the structure of the core itself. In the late 19th century, scientists noted a “shadow zone” deep in the Earth, where a type of body wave called an s-wave either stopped entirely or was altered. S-waves are unable to transmit through fluids or gases. The sudden “shadow” where s-waves disappeared indicated that Earth had a liquid layer.
 
In the 20th century, geoscientists discovered an increase in the velocity of p-waves, another type of body wave, at about 5,150 kilometers (3,200 miles) below the surface. The increase in velocity corresponded to a change from a liquid or molten medium to a solid. This proved the existence of a solid inner core.
 
Meteorites, space rocks that crash to Earth, also provide clues about Earth’s core. Most meteorites are fragments of asteroids, rocky bodies that orbit the sun between Mars and Jupiter. Asteroids formed about the same time, and from about the same material, as Earth. By studying iron-rich chondrite meteorites, geoscientists can get a peek into the early formation of our solar system and Earth’s early core.
 
In the lab, the most valuable tool for studying forces and reactions at the core is the diamond anvil cell. Diamond anvil cells use the hardest substance on Earth (diamonds) to simulate the incredibly high pressure at the core. The device uses an x-ray laser to simulate the core’s temperature. The laser is beamed through two diamonds squeezing a sample between them.
 
Complex computer modeling has also allowed scientists to study the core. In the 1990s, for instance, modeling beautifully illustrated the geodynamo—complete with pole flips.
core
The core is the hottest, densest part of the Earth.
Buried Treasure
Although the inner core is mostly NiFe, the iron catastrophe also drove heavy siderophile elements to the center of the Earth. In fact, one geoscientist calculated that there are 1.6 quadrillion tons of gold in the core—that’s enough to gild the entire surface of the planet half-a-meter (1.5 feet) thick.
Geoneutrinos
One of the most bizarre ways geoscientists study the core is through “geoneutrinos.” Geoneutrinos are neutrinos, the lightest subatomic particle, released by the natural radioactive decay of potassium, thorium, and uranium in Earth’s interior. By studying geoneutrinos, scientists can better understand the composition and spatial distribution of materials in the mantle and core.
Subterranean Fiction
“Subterranean fiction” describes adventure stories taking place deep below the surface of the Earth. Jules Verne’s Journey to the Center of the Earth is probably the most well-known piece of subterranean fiction. Other examples include Dante Alighieri’s Divine Comedy, in which the deepest center of Earth is Hell itself; the movie Ice Age: Dawn of the Dinosaurs, in which an underground world allows dinosaurs to survive into the present day; and the rabbit hole of Alice’s Adventures in Wonderland—which was originally titled Alice’s Adventures Under Ground.
Inge Lehman
Inge Lehman, who called herself “the only Danish seismologist” working in the 1930s, was a pioneering figure in the study of Earth’s interior. Lehman was the first to identify Earth’s solid inner core, and became a leading expert in the structure of the upper mantle as well. She was the first woman to receive the prestigious William Bowie Medal, the highest honor awarded by the American Geophysical Union. In 1997, the AGU created the Inge Lehman Medal, recognizing a scientist’s “outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth's mantle and core.”

Planetary Cores
All known planets have metal cores. Even the gas giants of our solar system, such as Jupiter and Saturn, have iron and nickel at their cores.

abundance
Noun

large amount.
accretion
Noun

process by which a substance grows by the collection and clustering of different parts.
alloy
Noun

mixture of two or more metals.
alter
Verb

to change.
analysis
Noun

process of studying a problem or situation, identifying its characteristics and how they are related.
artifact
Noun

material remains of a culture, such as tools, clothing, or food.
asteroid
Noun

irregularly shaped planetary body, ranging from 6 meters (20 feet) to 933 kilometers (580 miles) in diameter, orbiting the sun between Mars and Jupiter.
atmosphere (atm)
Noun

(atm) unit of measurement equal to air pressure at sea level, about 14.7 pounds per square inch. Also called standard atmospheric pressure.
atom
Noun

the basic unit of an element, composed of three major parts: electrons, protons, and neutrons.
axis of rotation
Noun

single axis or line around which a body rotates or spins.
body wave
Noun

seismic wave that travels through the interior of the Earth.
border
Verb

to exist on the edge of a boundary.
boundary
Noun

line separating geographical areas.
brittle
Adjective

fragile or easily broken.
Bullen discontinuity
Noun

seismic boundary between Earth's liquid outer core and solid inner core.
buoyant
Adjective

capable of floating.
catastrophic
Adjective

very bad.
chondrite
Noun

type of stony meteorite containing hardened droplets, called chondrules, of silicate minerals.
churn
Verb

to mix vigorously or violently.
circumstance
Noun

condition or situation.
collision
Noun

crash.
complex
Adjective

complicated.
composition
Noun

arrangement of the parts of a work or structure in relation to each other and to the whole.
compression
Noun

instance of being pressed together or forced into less space.
concentrated
Adjective

items gathered closely together in one place.
conduct
Verb

to transmit, transport, or carry.
conundrum
Noun

puzzling question or problem.
convection
Noun

transfer of heat by the movement of the heated parts of a liquid or gas.
convection current
Noun

movement of a fluid from a cool area to a warm area.
core
Noun

the extremely hot center of Earth, another planet, or a star.
Coriolis force
Noun

force that explains the paths of objects on rotating bodies.
crucial
Adjective

very important.
crust
Noun

rocky outermost layer of Earth or other planet.
crystal
Noun

type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules.
Curie point
Noun

temperature at which a ferromagnetic material loses its ferromagnetism—its ability to possess magnetism in the absence of a magnetic field.
current
Noun

steady, predictable flow of fluid within a larger body of that fluid.
data
Plural Noun

(singular: datum) information collected during a scientific study.
deform
Verb

to put out of shape or distort.
dense
Adjective

having parts or molecules that are packed closely together.
diamond anvil cell
Noun

device that compresses a test substance to up to 6 million atmospheres of pressure.
dissolve
Verb

to break up or disintegrate.
distinct
Adjective

unique or identifiable.
earthquake
Noun

the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.
electricity
Noun

set of physical phenomena associated with the presence and flow of electric charge.
element
Noun

chemical that cannot be separated into simpler substances.
Equator
Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.
exterior
adjective, noun

on the outside or outdoors.
fluctuate
Verb

to constantly change back and forth.
fluid
Noun

material that is able to flow and change shape.
freezing point
Noun

temperature at which liquid becomes solid; the freezing point of water is 0 degrees Celsius (32 degrees Fahrenheit).
frequency
Noun

rate of occurrence, or the number of things happening in a specific area over specific time period.
furnace
Noun

device used for heating by burning a fuel, such as wood or coal.
geodynamo
Noun

process by which a celestial body generates a magnetic field.
geologic
Adjective

having to do with the physical formations of the Earth.
geomagnetic pole reversal
Noun

change in a celestial body's magnetic field so that the magnetic North and South Poles are switched.
geothermal gradient
Noun

gradual change in temperature from the Earth's core (hot) to its crust (cool), about 25° Celsus per kilometer of depth (1° Fahrenheit per 70 feet of depth).
glacial activity
Noun

process of a glacier moving and changing the landscape.
gravitate
Verb

to move toward or be attracted to something.
heavy metal
Noun

chemical substance with a specific gravity of at least 5.0.
hemisphere
Noun

half of a sphere, or ball-shaped object.
hot spot
Noun

intensely hot region deep within the Earth that rises to just underneath the surface. Some hot spots produce volcanoes.
inner core
Noun

deepest layer of the Earth, beneath the outer core.
inner inner core
Noun

oddly crystallized structure at the heart of our planet, with iron crystals oriented east-west instead of north-south (as with the inner core).
interpret
Verb

to explain or understand the meaning of something.
iron
Noun

chemical element with the symbol Fe.
iron catastrophe
Noun

(~4 billion years ago) point in Earth's planetary formation when the temperature reached the melting point of iron and heavy elements (mostly iron and nickel) gravitated toward the center of the planet.
laser
Noun

(acronym for light amplification by stimulated emission of radiation) an instrument that emits a thin beam of light that does not fade over long distances.
lithosphere
Noun

outer, solid portion of the Earth. Also called the geosphere.
LLSVP
Noun

(large low shear velocity province) seismically anomalous region at the deepest part of Earth's mantle. Also called a superplume or thermo-chemical pile.
magnetic
Adjective

able to produce a force field that can attract or repel certain substances, usually metals (magnets).
magnetic field
Noun

area around and affected by a magnet or charged particle.
magnetic north
Adjective

direction that all compass needles point.
malleable
Adjective

flexible and capable of reforming itself without breaking when under stress.
mantle
Noun

middle layer of the Earth, made of mostly solid rock.
melting point
Noun

temperature at which a solid turns to liquid.
meteorite
Noun

type of rock that has crashed into Earth from outside the atmosphere.
mineral
Noun

inorganic material that has a characteristic chemical composition and specific crystal structure.
modeling
Noun

representation of a process, concept, or system, often created with a computer program.
molten
Adjective

solid material turned to liquid by heat.
NiFe
Noun

nickel-iron alloys that form Earth's core.
orbit
Verb

to move in a circular pattern around a more massive object.
orientation
Noun

relative positions of specific atoms or molecules in a chemical compound.
outer core
Noun

liquid, iron-nickel layer of the Earth between the solid inner core and lower mantle.
ozone layer
Noun

layer in the atmosphere containing the gas ozone, which absorbs most of the sun's ultraviolet radiation.
peek
Verb

to look quickly or from a secret location.
pivotal
Adjective

very important or crucial point.
planet
Noun

large, spherical celestial body that regularly rotates around a star.
planetary differentiation
Noun

process of separating different layers of a planetary body by chemical and physical mechanisms.
plasma
Noun

state of matter with no fixed shape and molecules separated into ions and electrons.
precious metal
Noun

valuable metal, such as gold, silver, or platinum.
precise
Adjective

exact.
prefer
Verb

to choose or prioritize.
primary
Adjective

first or most important.
P-wave
Noun

seismic shock wave that represents longitudinal motion. Also called a primary wave or pressure wave.
radical
Adjective

extreme or drastic.
radioactive decay
Noun

transformation of an unstable atomic nucleus into a lighter one, in which radiation is released in the form of alpha particles, beta particles, gamma rays, and other particles. Also called radioactivity.
radius
Noun

ray extending from the center of a circle or sphere to its surface or circumference.
rapid
Adjective

very fast.
rock
Noun

natural substance composed of solid mineral matter.
rotation
Noun

object's complete turn around its own axis.
seismic wave
Noun

shock wave of force or pressure that travels through the Earth.
shock wave
Noun

moving, measurable change in pressure and density of a material.
siderophile
Noun

material that has a chemical affinity for iron.
silicate
Noun

most common group of minerals, all of which include the elements silicon (Si) and oxygen (O).
simulate
Verb

to create an image, representation, or model of something.
solar system
Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.
solar wind
Noun

flow of charged particles, mainly protons and electrons, from the sun to the edge of the solar system.
solidify
Verb

to make solid.
sophisticated
Adjective

knowledgeable or complex.
spatial processes
Noun

series of changes affecting natural and human activity on Earth's surface.
spatial processes
Noun

series of changes affecting natural and human activity on Earth's surface.
speculate
Verb

to consider or guess.
subduction zone
Noun

area where one tectonic plate slides under another.
S-wave
Noun

seismic shock wave that represents perpendicular motion. Also called a secondary wave or shear wave.
tectonic plate
Noun

massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.
temperature
Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.
transmit
Verb

to pass along information or communicate.
ultraviolet radiation
Noun

powerful light waves that are too short for humans to see, but can penetrate Earth's atmosphere. Ultraviolet is often shortened to UV.
uniform
Adjective

exactly the same in some way.
vast
Adjective

huge and spread out.
velocity
Noun

measurement of the rate and direction of change in the position of an object.
viscosity
Noun

measure of the resistance of a fluid to a force or disturbance.
volcanic eruption
Noun

activity that includes a discharge of gas, ash, or lava from a volcano.
volcanism
Noun

upward movement of molten material from within the Earth to the surface, where it cools and hardens.
X-ray
Noun

radiation in the electromagnetic spectrum with a very short wavelength and very high energy.

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    Editor

    Jeannie Evers, Emdash Editing

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    Caryl-Sue, National Geographic Society

    Last Updated

    Aug. 17, 2015

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