How Hot Is the Earth's Core? Temperature, Pressure & Structure

The temperature at Earth's center is approximately 5,400°C (9,800°F), roughly as hot as the surface of the Sun. This extraordinary heat exists at the boundary between Earth's liquid outer core and solid inner core, at a depth of about 5,150 kilometers below the surface. The inner core itself may be even hotter, with estimates ranging up to 6,000°C.

Introduction

We have never directly measured the core's temperature — the deepest drill hole reaches only 12.26 kilometers, barely scratching the surface. Instead, scientists estimate core temperature by studying how seismic waves travel through the Earth, replicating core pressures in laboratory experiments, and modeling the physics of iron at extreme conditions.

The Short Answer

• Inner Core Temperature: ~5,400°C (comparable to the Sun's surface)
• Outer Core Temperature: ~4,400-5,400°C
• Depth to Core: ~2,891 km (outer core); ~5,150 km (inner core)
• Pressure at Center: ~360 GPa (3.6 million atmospheres)

Earth's core is divided into two parts: a liquid outer core (2,891-5,150 km depth) and a solid inner core (5,150-6,371 km depth). Both are composed primarily of iron and nickel. The outer core is liquid because the temperature exceeds the melting point of iron at that pressure. The inner core is solid because the even greater pressure at the center raises the melting point above the actual temperature.

The core temperature of ~5,400°C is comparable to the surface temperature of the Sun (~5,500°C). However, the comparison is somewhat misleading — the Sun's interior reaches 15 million degrees. Earth's core is hot enough to melt iron, but it is far cooler than the interiors of stars.

The Science Behind It

• How We Know: Seismic waves, high-pressure lab experiments, theoretical modeling
• Heat Sources: Primordial heat from formation + radioactive decay
• Cooling Rate: Inner core grows ~1 mm per year as it solidifies
• Discovery: Inge Lehmann discovered the solid inner core in 1936

Seismologist Inge Lehmann discovered the solid inner core in 1936 by analyzing seismic waves from a large earthquake in New Zealand. She noticed that certain waves, which should have been blocked by the liquid outer core, were arriving at unexpected locations, indicating they had been refracted by a solid inner body. Her discovery was one of the most important insights into Earth's interior structure.

The core's heat comes from two main sources: primordial heat left over from Earth's formation 4.6 billion years ago (when gravitational compression and meteorite impacts heated the planet intensely), and ongoing radioactive decay of elements like uranium, thorium, and potassium. The core is slowly cooling, and the inner core is growing as the outer core gradually solidifies — at a rate of roughly 1 millimeter per year.

Types & Variations

• Outer Core: Liquid iron-nickel alloy; 2,891-5,150 km depth; generates magnetic field
• Inner Core: Solid iron-nickel; 5,150-6,371 km depth; ~1,220 km radius
• Mantle: Solid but flows slowly; 35-2,891 km depth; ~2,000-3,700°C
• D" Layer: Mysterious boundary layer between mantle and core

The liquid outer core is critical for life on Earth because its convective motion generates the planet's magnetic field through a process called the geodynamo. Electrically conducting liquid iron, driven by heat from the inner core and Earth's rotation, creates self-sustaining electric currents that produce the magnetic field. This field shields the atmosphere from solar wind, without which Earth would lose its atmosphere like Mars did.

The inner core, despite its extreme temperature, remains solid because of the immense pressure — roughly 360 gigapascals (3.6 million times atmospheric pressure). At this pressure, the melting point of iron is raised above the actual temperature. The inner core is roughly the size of the Moon (radius ~1,220 km) and is growing as the outer core slowly crystallizes onto it.

Famous Examples

• Sun's Surface: ~5,500°C (similar to Earth's core)
• Kola Borehole: 12.26 km deep, reached 180°C (barely into the crust)
• Diamond Anvil Cell: Lab device that replicates core pressures to estimate temperature
• Mars: Smaller core, likely mostly solid, weak/no global magnetic field

Scientists estimate core temperature using diamond anvil cells — devices that squeeze tiny samples between two diamonds to replicate the pressures found at Earth's center. By measuring the melting point of iron at these extreme pressures, researchers can estimate the temperature at the inner-outer core boundary. A landmark 2013 study at the European Synchrotron placed this temperature at approximately 6,000°C, higher than previous estimates.

Mars provides a natural comparison. Mars's smaller core has cooled more rapidly, and its global magnetic field died roughly 4 billion years ago. Without magnetic field protection, the solar wind stripped away much of Mars's atmosphere, contributing to the loss of surface water and the planet's transition to a cold, dry world. Earth's still-active core is essential for maintaining the conditions for life.

Why It Matters

• Magnetic Field: Liquid core generates the field that protects life from solar radiation
• Plate Tectonics: Core heat drives mantle convection, which drives plate movement
• Volcanism: Ultimate energy source for volcanic activity
• Future: Core will eventually cool; Earth will lose its magnetic field in billions of years

Earth's core is the engine that drives virtually all geological processes. The heat flowing from the core into the mantle powers the convection cells that move tectonic plates, build mountains, open oceans, and generate earthquakes. Without this internal heat source, Earth's surface would be geologically dead, like the Moon.

In the very long term (billions of years), Earth's core will cool enough that the outer core solidifies completely and the geodynamo shuts down. When this happens, Earth will lose its magnetic field, and the solar wind will gradually strip away the atmosphere, as happened on Mars. However, this process is so slow that other factors (like the Sun's evolution) will affect Earth's habitability long before the core cools.

Key Facts

• Earth's inner core temperature is ~5,400°C — comparable to the surface of the Sun.
• The inner core is solid iron-nickel despite the extreme heat, because pressure raises the melting point.
• The liquid outer core generates Earth's magnetic field, which protects life from solar radiation.
• Inge Lehmann discovered the solid inner core in 1936 using seismic wave analysis.
• The inner core is growing at ~1 mm/year as the outer core slowly solidifies.

Fun Facts

• Earth's core is as hot as the Sun's surface (~5,400-6,000°C) but under 3.6 million times atmospheric pressure.
• The inner core is roughly the size of the Moon, with a radius of about 1,220 km.
• Mars lost its global magnetic field ~4 billion years ago when its smaller core cooled.
• If Earth's magnetic field disappeared, solar wind would gradually strip away our atmosphere.

Final Thoughts

Earth's core, at approximately 5,400°C, is a furnace that powers the planet's geology and protects its biosphere. The liquid outer core generates the magnetic field that shields life from solar radiation, while the heat flowing upward drives the tectonic plates that shape continents and oceans. Understanding the core's temperature and behavior is fundamental to understanding why Earth is a living, dynamic planet — and why other planets, with cooler or solid cores, are geologically dead.