Inner Core of Earth
The Earth's inner core is the Earth's innermost part and according to seismological studies, it has been believed to be primarily a solid ball with a radius of about 1220 kilometers, or 768 miles (about 70% of the Moon's radius). It is composed of an iron–nickel alloy and some light elements. The temperature at the inner core boundary is approximately 5700 K (5400 °C).
The Inner Core
The Inner Core is the final layer of the Earth. It is a solid ball made of metal. To learn about what metal the Inner Core is made of, read this section about the Inner Core. You can also learn how hot the Inner Core is, how thick it is and some interesting facts about the Inner Core.
The Inner Core is about 1250 km thick and is the second smallest layer of the Earth. Although it is one of the smallest, the Inner Core is also the hottest layer.
The Inner Core is a solid ball composed of an element named NiFe. Ni for Nickel and Fe for Ferrum also known as Iron.
The Inner Core is about 5000-6000 degrees Celsius. It melts all metal ores in the Outer Core causing it to turn into liquid magma.
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 ageologic 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.
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.
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).
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.
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.
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.
Inner Core Facts
Earth's innermost section is called its inner core, and is believed to be just as hot as the sun's surface. It was once believed that the earth's inner core was liquid, but Inge Lehmann - a seismologist - proved in theory in 1936 that the inner core was solid, and the outer core was liquid. The inner core is believed to be made up of an iron-nickel (metal) alloy. The earth, from the center moving outward, is made up of the inner core, the outer core, the lower mantle, the upper mantle, and the crust. Scientists continue to study the inner core, mostly through the use of seismic activity, as they try to learn more about it.
Interesting Inner Core Facts
- The temperature of the inner core is believed to be approximately 5400 degrees Celsius, or 5700 Kelvin. This heat is caused by three elements: residual heat from the formation of the earth, gravitational forces from the moon and the sun, and and radioactive decay of earth's inner elements.
- The nickel alloy that makes up the earth's inner core is referred to as NiFe. Ni represents nickel, and Fe represents iron.
- Scientists believe that the earth's inner core is growing slowly. It is also not believed to be uniform, as the seismic waves passing through do so at different speeds.
- Some believe that the earth's inner core formed between 2 and 4 billion years ago. This theory also states that it was entirely molten originally, and was not present when the earth originally formed 4.5 billion years ago.
- The earth's inner core is approximately 1500 miles wide. It is estimated to be approximately 1,802 miles below the earth's surface (crust).
- Some scientists believe that the earth's inner core spins at a faster speed than the rest of it - at a rate of 2/3rds of a second faster.
- Earth's magnetic field is not created by its solid iron and nickel inner core. Instead it is created by the earth's outer core, made of molten iron and nickel. The magnetic field is created when the outer core flows around the inner core.
- Some find it hard to believe that the inner core can be solid, considering how hot it is. This inner core is able to remain solid because the center of the earth exists in extremely high pressures.
- Although the inner core is made up primarily of nickel and iron, it also includes elements that are capable of dissolving in iron such as cobalt, gold, and platinum. These elements are called siderophiles.
- Some scientists believe that the earth's inner core actually has another inner-inner core. They believe it formed approximately 500 million years ago due to a geological change.
- In addition to seismic activity, scientists use a variety of other information to learn about the earth's inner core. These additional methods involve lab experiments, meteorite analysis, computer modeling, and reading of data.
- The inner core makes up approximately 1.7% of earth's total mass, while the outer core makes up approximately 30.8% of the earth's total mass.
Scientists say they have gained new insight into what lies at the very centre of the Earth.
Research from China and the US suggests that the innermost core of our planet has another, distinct region at its centre.
The team believes that the structure of the iron crystals there is different from those found in the outer part of the inner core.
The findings are reported in the journal Nature Geoscience.
Without being able to drill into the heart of the Earth, its make-up is something of a mystery.
So instead, scientists use echoes generated by earthquakes to study the core, by analysing how they change as they travel through the different layers of our planet.
Prof Xiaodong Song, from the University of Illinois at Urbana-Champaign said: "The waves are bouncing back and forth from one side of the Earth to the other side of the Earth."
Prof Song and his colleagues in China say this data suggests that the Earth's inner core - a solid region that is about the size of the Moon - is made up of two parts.
The scientists believe the Earth's inner core is composed of two parts
The seismic wave data suggests that crystals in the "inner inner core" are aligned in an east-to-west direction - flipped on their side, if you are looking down at our planet from high above the North Pole.
Those in the "outer inner core" are lined up north to south, so vertical if peering down from the same lofty vantage point.
Prof Song said: "The fact we are discovering different structures at different regions of the inner core can tell us something about the very long history of the Earth."
The core, which lies more than 5,000km down, started to solidify about a billion years ago - and it continues to grow about 0.5mm each year.
The finding that it has crystals with a different alignment, suggests that they formed under different conditions and that our planet may have undergone a dramatic change during this period.
Commenting on the research, Prof Simon Redfern from the University of Cambridge said: "Probing deeper into the solid inner core is like tracing it back in time, to the beginnings of its formation.
"People have noticed differences in the way seismic waves travel through the outer parts of the inner core and its innermost reaches before, but never before have they suggested that the alignment of crystalline iron that makes up this region is completely askew compared to the outermost parts.
"If this is true, it would imply that something very substantial happened to flip the orientation of the core to turn the alignment of crystals in the inner core north-south as is seen today in its outer parts."
He added that other studies suggest that the Earth's magnetic field may have undergone a change about half a billion years ago, switching between the equatorial axes and the polar axis.
"It could be that the strange alignment Prof Song sees in the innermost core explains the strange palaeomagnetic signatures from ancient rocks that may have been present near the equator half a billion years ago," he added.
"For the moment, however, the model proposed in this paper needs testing against other ways of analysing the seismic properties of Earth's innermost core, since no other researchers have previously considered evidence for the same conclusions in their studies."