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The inner core is a solid iron-and-nickel sphere at Earth’s center; the outer core is a liquid, electrically conducting iron-and-nickel shell around it. Their different physical states are the key to their different jobs: moving fluid in the outer core sustains most of Earth’s geomagnetic field through the geodynamo, while the solid inner core is not a permanent magnet.
Inner core and outer core at a glance
| Comparison | Inner core | Outer core |
|---|---|---|
| Position | Earth’s central sphere | Shell surrounding the inner core |
| Broad composition | Iron and nickel metals | Molten iron and nickel |
| Physical state | Solid | Liquid or fluid |
| Approximate NASA dimension | Radius of about 1,221 kilometers | Thickness of about 2,250 kilometers |
| Magnetic-field role | Not a permanent magnet; heat flow associated with the inner core is shown as helping drive motion in the outer core | Moving, electrically conducting fluid generates currents that sustain the geomagnetic field through the geodynamo |
| How scientists know about it | Inferred from earthquake waves, gravity, rotation, magnetic-field theory, and laboratory evidence rather than a direct sample | Inferred from the same converging geophysical and experimental evidence |
NASA Science gives the dimensions above as approximate values in an explainer published in 2021 and last updated October 23, 2024. They describe the idealized layer geometry, not exact boundaries measured by drilling or sampling.
Where each core layer is located
The inner core is Earth’s center
The inner core is the innermost region of the planet, enclosed on every side by the outer core. Its approximately 1,221-kilometer radius makes it a sphere about 2,442 kilometers across. The figure is small compared with Earth as a whole, but it is still larger in diameter than many continents. “Inner” refers to location, not to a separate material family: its broad metallic composition is similar to that of the layer around it.
The outer core is a surrounding shell
The outer core forms a roughly 2,250-kilometer-thick shell between the inner core and the rocky mantle. A shell thickness is not the same measurement as the inner core’s radius, so the two numbers should not be compared as though they were equivalent dimensions. Together, the layers make up the metallic core, but the outer core occupies more radial space because it wraps around the central sphere.
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Boundaries are inferred, not seen directly
Scientists cannot look at the core with a camera or recover a core sample from Earth’s center. The quoted dimensions are model-based approximations derived from how seismic waves travel, how Earth rotates, how gravity and inertia behave, and how materials respond in laboratory experiments. Different models can refine the boundary locations without changing the basic conclusion that a solid inner region is surrounded by a liquid outer region.
What the two layers are made of
Iron and nickel are the supported broad description
NASA’s overview describes both core layers broadly as iron-and-nickel metal. That wording is important: it identifies the dominant metallic materials without claiming that either layer is a chemically pure, two-element block. The available evidence in this comparison does not establish a complete inventory of minor elements or a precise percentage for each component.
“Molten” describes the outer core’s state, not a different substance
When an explanation calls the outer core molten iron and nickel, “molten” describes its physical state. The outer and inner cores can therefore share a broad composition while behaving differently. The outer core is fluid enough for large-scale motion; the inner core is solid. State and composition answer different questions and should not be conflated.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchDo not read the model as a directly sampled alloy
Earth’s deepest materials have not been directly sampled in the cited evidence. Researchers combine seismic observations with experiments on rocks, minerals, melting, and iron alloys, then test whether a proposed interior model can account for Earth’s gravity, rotation, heat flow, and magnetic field. This makes the iron-nickel description a well-established inference from multiple lines of evidence, rather than a laboratory analysis of a retrieved specimen.
Why the inner core is solid and the outer core is liquid
Physical state is the decisive difference
The inner core is modeled as solid, whereas the outer core is liquid or fluid. The distinction matters because a solid body cannot circulate in the way a liquid shell can. Fluid movement allows the outer core to transport heat and reorganize electrically conducting material throughout the layer. The inner core instead participates through its boundary and heat flow, not by flowing as a liquid.
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Temperature alone does not explain the whole structure
It is tempting to imagine that the deepest material must be liquid because it is extremely hot. The core’s state depends on the combined physical conditions at depth, not temperature in isolation. The evidence supports a solid center enclosed by a liquid shell, even though both regions are intensely hot. The USGS summarizes the temperature issue in a different context by stating that “The core of the Earth has a temperature of several thousand degrees Celsius, and is not permanently magnetized.”
The inner-core boundary enables a coupled system
The solid inner core and fluid outer core are not independent layers with unrelated behavior. NASA’s illustration presents heat flow associated with the inner core as helping drive convection in the outer core. That relationship is a contribution to the fluid system; it does not turn the inner core into a magnet or mean that all of the geodynamo’s energy comes from one specifically quantified source. The cited material does not establish a precise split between thermal and compositional contributions.
How the outer core sustains Earth’s magnetic field
The geodynamo starts with conducting fluid
Iron-rich fluid conducts electricity. When that fluid moves through the outer core, its motion generates electric currents. Those currents create magnetic fields, and the resulting field interacts with the fluid motion in a self-sustaining process called the geodynamo. NASA Science describes this directly: “This mechanism, which is responsible for maintaining Earth’s magnetic field, is known as the geodynamo.”
Convection supplies organized motion
Convection is circulation driven by differences in heat and buoyancy within the fluid. In the outer core, convection keeps electrically conducting material moving rather than allowing it to settle into a static layer. The mechanism is not simply random sloshing: sustained circulation is necessary for maintaining large-scale currents over geological time.
Earth’s rotation organizes the flow
Earth’s rotation influences the fluid motions through the Coriolis effect. NASA identifies convection and rotation as parts of the process that organize outer-core motion and generate currents. The field is therefore a dynamo produced by moving conductor, not a permanent magnetic object placed at the center of the planet.
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The inner core is not the source of a bar-magnet field
A bar magnet retains aligned magnetization in its material. Earth’s core is far too hot for that kind of permanent magnetization. The inner core does not function as a giant bar magnet whose field simply extends through the planet. Its solid structure and associated heat flow can influence the outer-core system, but the field-maintaining currents are generated in the moving liquid outer core.
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For the main field measured around Earth, the accepted explanation is the outer-core geodynamo. This statement concerns the field generated inside the planet; it does not imply that every magnetic signal detected near Earth has the same origin. External space-weather effects and smaller crustal magnetic features are separate phenomena, while the deep, global field is sustained by the core dynamo.
How scientists know what cannot be visited
Earthquake waves reveal internal boundaries
Earthquakes send seismic waves through the planet. Waves change speed, direction, and whether they can pass through a region depending on the material’s properties and state. Patterns of arrivals at seismic stations allow researchers to infer a liquid outer region and a solid inner region, even though no instrument has traveled to the core.
Rotation, gravity, and inertia constrain the model
Earth’s rotation and moment of inertia contain information about how mass is distributed inside the planet. Gravity measurements add another independent constraint. A proposed core structure must fit these global observations as well as the seismic evidence; a model that matches one data type but fails the others is not sufficient.
Magnetic-field behavior tests the dynamo explanation
The existence and long-term behavior of Earth’s global magnetic field provide a physical test. A model with only a solid, permanently magnetized center cannot account for the field under core temperatures. A model with moving, electrically conducting liquid in the outer core does account for a self-sustaining dynamo, consistent with NASA’s explanation.
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Laboratory experiments connect observations to materials
Researchers study rocks, minerals, melting, and iron alloys in the laboratory. These experiments help determine which compositions and states could produce the seismic, gravitational, rotational, and magnetic observations measured at Earth’s surface. Comparisons with other planetary materials provide an additional check on plausible interior models.
Converging evidence is stronger than one measurement
The USGS account emphasizes that Earth’s interior is inferred from earthquake waves, heat flow, magnetic field and gravity observations, rotation and inertia, laboratory work, and comparisons with other planetary materials. The strength of the inner-outer-core model comes from these independent lines of evidence agreeing on the broad structure, not from a single photograph or sample.
Common misconceptions to avoid
“The inner core creates the magnetic field”
Correction: The main field is sustained by electric currents in moving outer-core fluid. The inner core can affect heat flow at the boundary, but it is not the current-generating liquid layer.
“The whole core is liquid”
Correction: The outer core is liquid; the inner core is solid. Treating the core as one uniform fluid erases the boundary that is central to the geodynamo model.
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“The core is a giant permanent magnet”
Correction: The USGS says the several-thousand-degree core is not permanently magnetized. Earth’s field is dynamo-generated, not the retained magnetization of a hot metal sphere.
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“Iron and nickel means a pure iron-nickel mixture with known percentages”
Correction: Iron and nickel are the broad compositional description supported here. Exact abundances and all minor elements are not established by the cited overview and should not be presented as settled numbers.
“The NASA dimensions are exact measurements”
Correction: The approximately 1,221-kilometer inner-core radius and approximately 2,250-kilometer outer-core thickness are values in an official NASA explainer. They are useful scale estimates for an inferred model, not exact universal boundary measurements.
How to use the comparison as a mental model
Start with the state, then connect it to the job
- Identify the inner core as solid and central.
- Identify the outer core as liquid and surrounding.
- Ask which layer can circulate: the outer core.
- Ask which circulating, electrically conducting layer can generate currents: the outer core.
- Use the inner core’s role as a boundary and heat-flow participant, not as a permanent magnet.
This sequence prevents the most common causal error: assigning the magnetic field to the inner core merely because it sits at the center.
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NASA’s inner-core figure is a radius, while its outer-core figure is a shell thickness. To picture scale, the inner core is about 2,442 kilometers across, whereas the outer core extends roughly 2,250 kilometers radially around it. Those derived and quoted values are approximate and describe different geometric features.
Separate established conclusions from unresolved detail
The solid-versus-liquid distinction, broad iron-nickel composition, approximate dimensions, and outer-core geodynamo are well-established conclusions of the cited evidence. More detailed claims about exact elemental abundances, temperature and pressure profiles, or the relative power supplied by different convection sources require additional, specifically sourced evidence and should be labeled accordingly.
What the evidence can—and cannot—tell us
Strongly supported conclusions
- Earth has a solid inner core and a liquid outer core.
- Both are broadly metallic, dominated in the cited description by iron and nickel.
- The inner core has an approximate radius of 1,221 kilometers, and the outer core has an approximate thickness of 2,250 kilometers in NASA’s overview.
- Motion in electrically conducting outer-core fluid, organized by convection and rotation, sustains the main geomagnetic field through the geodynamo.
- The core is too hot for permanent magnetization.
Claims that need additional qualification
- Exact concentrations of elements beyond the broad iron-and-nickel description.
- A precise temperature or pressure profile from the center outward.
- A definitive numerical division between thermal and compositional convection.
- Exact locations of every internal boundary to more precision than the cited NASA overview provides.
Keeping those limits visible makes the comparison more accurate without weakening its central answer. The central distinction remains clear: solid inner core, liquid outer core, and a magnetic field maintained by the moving conductor in the outer layer.
Bottom line
Earth’s inner core is a solid, iron-and-nickel central sphere; the outer core is a much thicker shell of liquid, electrically conducting iron-and-nickel fluid. Their different states explain their different roles. The outer core’s convection, organized by Earth’s rotation, generates the electric currents of the geodynamo that maintain the planet’s main magnetic field. The inner core influences that system through its boundary and heat flow, but it is not a permanent magnet. Scientists reach this picture by combining seismic, gravitational, rotational, magnetic, heat-flow, and laboratory evidence because neither core layer can be directly sampled.
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