Earth's interior becomes hotter and experiences greater pressure with increasing depth. Understanding the distributions of density, temperature, and pressure makes it possible to explain the physical states (solid or liquid) of each layer.
Basic Knowledge
Density: Approximately 2.7〜3.0 g/cm³ in the crust → 3.3〜5.6 g/cm³ in the mantle → 10〜12 g/cm³ in the outer core → approximately 13 g/cm³ in the inner core. Density increases sharply at discontinuities (the Moho and Gutenberg discontinuity).
Temperature: The rate at which temperature increases from the surface is called the geothermal gradient, and in the crust it averages approximately 25〜30℃/km. The gradient becomes smaller in the deep mantle, but the absolute temperature is high, reaching approximately 4000℃ at the base of the mantle and an estimated 5000〜6000℃ at the outer-core–inner-core boundary (close to the Sun's surface temperature).
Pressure: Approximately 360 GPa (3.6 million atmospheres) at Earth's center. It increases steadily with depth.
The inner core is solid because its extremely high pressure raises the melting point. The liquid iron in the outer core has a lower melting point because the pressure is lower, so it remains liquid.
Geothermal gradient（temperature increase at a rate of approximately 25〜30℃/km in the crust）
Geothermal heat flow（amount of heat flowing from Earth's interior to the surface）
Melting point（temperature at which a substance begins to melt. The melting points of many substances increase under high pressure）
Gutenberg discontinuity（at a depth of approximately 2900 km. Boundary between the mantle and outer core）
Further Exploration
Earth's interior has two main heat sources. ① Heat from the formation of the early Earth (heat from collisions of planetesimals; Earth has not yet completely cooled) and ② heat from the decay of radioactive isotopes (such as ²³⁸U, ²³⁵U, ²³²Th, and ⁴⁰K). More than half of the current geothermal heat flow is thought to result from radioactive decay.
Although the mantle is solid, mantle convection occurs over timescales of several million to several hundred million years and drives plate movement. Mantle convection is inferred from seismic tomography, which visualizes cold, dense slabs (subducted plates) sinking into the deep mantle.
Geothermal energy is attracting attention as a renewable energy source, and geothermal power generation is carried out in Iceland and Japan.
- Geothermal gradient: approximately 25〜30℃/km in the crust
- Temperature at Earth's center: approximately 5000〜6000℃
- Pressure at Earth's center: approximately 360 GPa
- Inner core is solid = high pressure raises the melting point
- Heat sources inside Earth = heat from formation + radioactive decay heat
- Mantle convection drives plate movement
Points to Note
① “The inner core is solid because its temperature is low” is incorrect. It is solid because high pressure raises its melting point. ② The geothermal gradient value applies to the crust, and the gradient becomes smaller with increasing depth. ③ Earth's central temperature is close to the Sun's surface temperature (approximately 5500℃), but it is completely different from the Sun's core temperature (approximately 16 million℃).
Practice
- What is the approximate geothermal gradient in the crust, in ℃/km?
- Explain why the inner core is solid using the terms “melting point” and “pressure.”
- State the two internal heat sources of Earth.