Earth's inner core facts for kids
- continental crust
- oceanic crust
- upper mantle
- lower mantle
- outer core
- inner core
- Mohorovičić discontinuity
- core–mantle boundary
- outer core–inner core boundary
The Earth's inner core is the deepest part of our planet. Imagine a solid ball right at the very center of Earth. It's mostly made of iron–nickel alloy and is super hot, about the same temperature as the surface of the Sun.
Even though it's incredibly hot, the inner core stays solid. This is because of the immense pressure from all the layers of Earth above it. Scientists can't directly visit or take samples from the core. Instead, they learn about it by studying seismic waves from earthquakes and observing Earth's magnetic field.
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Who Found the Inner Core?
Scientists learned about the solid inner core in 1936. A Danish seismologist named Inge Lehmann made this amazing discovery. She studied seismographs, which are tools that record ground movements from earthquakes.
Lehmann noticed that seismic waves from earthquakes in New Zealand bounced off something deep inside Earth. She figured out there must be a solid inner core distinct from the liquid outer core. She even estimated its size, which was very close to what we know today.
After Lehmann's discovery, other scientists like Beno Gutenberg and Charles Richter used more data. They helped confirm the inner core's existence and refine its estimated size. In 1940, people started to believe the inner core was made of solid iron.
Later, in 1952, Francis Birch further supported the idea that the inner core was crystalline iron. The boundary between the inner and outer cores is sometimes called the "Lehmann discontinuity" in her honor. In 1971, scientists finally confirmed that the inner core is indeed rigid and solid.
How Scientists Study the Core
Using Seismic Waves to See Inside
Most of what we know about the inner core comes from seismic waves. These are vibrations that travel through Earth after an earthquake. Think of them like sound waves moving through different materials.
There are two main types of seismic waves:
- P-waves (Primary or Pressure waves): These are like sound waves. They can travel through both solid and liquid materials.
- S-waves (Secondary or Shear waves): These waves move differently. They can only travel through solid, rigid materials.
Scientists pay close attention to how these waves travel. For example, "PKIKP" waves go all the way through the inner core. By studying their speed and how they change, scientists can figure out what the inner core is like. The fact that S-waves can pass through the inner core confirms it's solid.
Other Clues About the Core
Besides seismic waves, other things help us understand the inner core:
- Earth's Magnetic Field: The liquid outer core creates Earth's magnetic field. But the solid inner core affects these currents and the heat flow, which influences the magnetic field.
- Earth's Mass and Gravity: The weight and pull of Earth are affected by how dense its inner layers are.
- Earth's Vibrations: After big earthquakes, Earth can "ring" like a bell. How it vibrates tells us a lot about the density, size, and shape of its inner layers.
What is the Inner Core Like?
How Fast Do Waves Travel Through It?
Seismic S-waves travel through the inner core at about 3.5 to 3.7 kilometers per second. This is much slower than in the deep mantle, which is just above the outer core. P-waves travel faster, around 11.1 to 11.4 kilometers per second through the inner core.
Size and Shape of the Inner Core
The inner core is about 1,221 kilometers in radius. That's about 19% of Earth's total radius. To give you an idea, it's about 70% the size of our Moon.
Its total volume is about 7.6 billion cubic kilometers. This means it makes up less than 1% of the entire Earth's volume. The inner core is almost a perfect sphere. It's slightly flattened at the poles, but much less so than the Earth's surface.
Pressure and Gravity Inside
The pressure in the inner core is incredibly high. It ranges from about 330 to 360 gigapascals. That's like having the weight of many thousands of elephants pressing down on every square inch!
The pull of gravity at the surface of the inner core is about 4.3 meters per second squared. This is less than half the gravity we feel on Earth's surface.
How Dense and Heavy is It?
The inner core is very dense. Its density is about 12.8 to 13.0 kilograms per liter. For comparison, the average density of the Earth's upper crust is only about 3.4 kilograms per liter. This high density means the inner core has a mass of about 1023 kilograms. That's about 1.7% of the entire Earth's mass.
What is the Temperature?
Scientists estimate the temperature of the inner core to be around 5,400 to 5,700 Kelvin. That's incredibly hot, similar to the temperature on the surface of the Sun! Iron can be solid at such high temperatures because the extreme pressure raises its melting point.
Does It Have a Magnetic Field?
While the main magnetic field comes from the liquid outer core, the inner core also has a magnetic field. In 2010, scientists estimated the magnetic field in the outer core to be about 2.5 milliteslas. This is about 40 times stronger than the magnetic field at Earth's surface. The field inside the inner core is likely similar.
How Viscous is It?
Even though seismic waves show the inner core as solid, it might be extremely viscous. This means it's like a super-thick, slow-moving liquid, much thicker than tar. Some scientists think there might be very slow movement or "convection" happening inside the inner core. This could help explain why seismic waves travel differently in different directions through it.
What is the Inner Core Made Of?
Scientists believe the inner core is mostly an iron–nickel alloy. This is based on how planets form and what we know about the elements in our Solar System.
Pure iron at the core's conditions would be too dense. This suggests there are lighter elements mixed in. These could include silicon, oxygen, or sulfur. Recent studies suggest up to 10% nickel and 2-3% of other lighter elements.
When the iron in the outer core freezes to form the inner core, most of the oxygen is left behind in the liquid outer core. Experiments suggest the inner core's iron has a specific crystal structure called hexagonal close-packed (hcp) ε-iron.
How is the Inner Core Structured?
Scientists first thought the inner core would be perfectly uniform. Some even suggested it might be a single giant crystal of iron. However, studies have shown it's more complex.
Why Do Waves Travel Differently?
In 1983, scientists noticed something interesting. Seismic waves traveling through the inner core from north to south moved about 1% faster than waves traveling along the equator. This difference in speed depending on direction is called "anisotropy."
Later studies confirmed this. It means the inner core isn't perfectly uniform in all directions. The difference in speed is usually between 0.5% and 1.5%.
The crystals of ε-iron, which make up the inner core, naturally allow waves to travel faster in certain directions. So, if these crystals are mostly lined up in a north-south direction, it would explain the observed anisotropy.
One idea is that there's a very slow flow or "creep" inside the inner core. This flow could cause the crystals to align themselves. This flow might be due to the inner core freezing faster at the equator than at the poles. Another idea is that slow thermal convection (heat movement) inside the inner core could cause this alignment.
Are There Multiple Layers?
In 2002, scientists found evidence that the inner core might have an "innermost inner core" (IMIC). This central part seems to have slightly different properties than the outer part of the inner core. Its exact size is still being debated, but it could be between 300 and 750 kilometers in radius.
More recently, in 2018, a three-layer model was suggested. It includes an "inner inner core" (IIC) about 500 kilometers in radius, an "outer inner core" (OIC) about 600 kilometers thick, and an outer shell. In this model, the fastest wave direction changes between the IIC and OIC.
Does It Vary From East to West?
Some scientists have suggested that the inner core's properties might be different in its "eastern" hemisphere (under Borneo) compared to its "western" hemisphere (under Colombia). This could be due to melting in one area and freezing in another. However, more recent studies suggest these east-west differences are very small or not clearly present.
How Does the Inner Core Grow?
The Earth's inner core is slowly getting bigger. This happens as the liquid outer core cools down and freezes onto the surface of the inner core. This process is very slow, with Earth's interior cooling by about 100 degrees Celsius every billion years.
As iron freezes to form the inner core, the liquid around it becomes richer in oxygen. This makes the liquid less dense and creates currents in the outer core. These currents are thought to be the main reason Earth has a magnetic field. The inner core also helps stabilize these movements in the outer core, which helps keep the magnetic field steady.
How Does the Inner Core Move?
Does the Inner Core Spin?
Since the inner core isn't rigidly attached to the rest of Earth, scientists have wondered if it spins at a different speed. In the 1990s, seismologists claimed to have found evidence that the inner core "super-rotates." This means it spins slightly faster than the rest of Earth.
Early estimates suggested it spun about one degree faster per year. Later, this was refined to 0.3 to 0.5 degrees per year. However, in 2023, new research suggested that the inner core might have stopped spinning faster around 2009. It might even be rotating slower than the surface now. This movement is thought to be part of a cycle that takes about 70 years.
Does the Inner Core Wobble?
The gravity from the Sun and Moon causes tides in Earth's oceans. These same forces also affect the Earth's rotation. Scientists have found that the inner core might wobble slightly, or "nutate," with a period of about one day. This complex movement is influenced by the currents and magnetic fields in the core.
How Old is the Inner Core?
Scientists believe the solid inner core formed from a completely liquid core as Earth slowly cooled down. But exactly when this happened is still a mystery.
Scientists use two main ways to estimate the inner core's age:
- Thermodynamic Modeling: This involves creating computer models of how Earth has cooled over time.
- Paleomagnetic Evidence: This looks at the history of Earth's magnetic field, which is recorded in ancient rocks.
Estimates for the inner core's age range from about 0.5 billion to 2 billion years old. This is much younger than Earth itself, which is about 4.5 billion years old.
Clues from Earth's Cooling
One way to guess the inner core's age is by modeling how Earth's heat flows. The Earth's magnetic field needs a certain amount of heat flow from the core to keep working. By estimating this heat flow, scientists can work backward to figure out when the inner core might have started to form.
Early estimates, assuming no radioactive elements in the core, suggested the inner core was about 1 billion years old. If there are radioactive elements, it could be a few hundred million years older. More recent studies have changed the estimated electrical and thermal conductivity of iron, leading to different age estimates. Some suggest it could be less than 700 million years old, while others say it could be up to 4.2 billion years old.
Clues from Ancient Magnetism
Another way to estimate the age is by studying the magnetic field trapped in old rocks. This is called the "paleomagnetic record." The presence or absence of a solid inner core could change how the magnetic field behaves.
In 2011, scientists found that Earth's magnetic field was more like a simple bar magnet between 2.8 and 2.5 billion years ago. They thought this change might be due to the growth of the solid inner core between 3.5 and 2.0 billion years ago.
In 2015, a study of very old rocks showed a big increase in Earth's magnetic field strength about 1 to 1.5 billion years ago. This change had not been seen before. Scientists thought this could be when the solid inner core was born.
More recently, in 2019, an analysis of rocks from about 565 million years ago showed unusual magnetic field patterns. These patterns, along with frequent magnetic field reversals, could mean that the inner core started forming around 0.5 billion years ago. This idea is still being explored by scientists.
See also
In Spanish: Núcleo interno de la Tierra para niños
- Geodynamics
- Internal structure of Earth
- Iron meteorite
- Thermal history of Earth
- Travel to the Earth's center
