Internal structure of Earth

The solid inner core of Earth is a single giant crystal, a discovery that challenges the assumption that the planet's center is a chaotic jumble of molten metal. This massive iron-nickel sphere, with a radius of about 1,220 kilometers, was confirmed through experiments where samples of iron-nickel alloy were subjected to core-like pressures between diamond tips and heated to approximately 4,000 Kelvin. The resulting X-ray observations revealed that these giant crystals run north to south, creating a unique structure that has remained stable for billions of years. This finding emerged from a long history of scientific inquiry, beginning with the realization that the Earth's mass is approximately 5.9722 times 10 to the 24th kilograms, a figure derived from gravitational experiments and satellite observations that have evolved from early pendulum studies to modern orbital mechanics. The density of the planet, averaging about 5.5 grams per cubic centimeter, hints at a composition far denser than the rocks we walk on, suggesting that the bulk of the planet's mass is concentrated in its deepest layers.

The Crust's Ancient Secrets

The oldest known mineral grains on Earth are approximately 4.4 billion years old, indicating that the planet has possessed a solid crust for nearly its entire existence. These ancient fragments, found within younger rocks, serve as time capsules from the Hadean Eon, a period when the surface was likely molten and hostile. The crust itself is a thin veneer, ranging from 5 to 10 kilometers in depth beneath the ocean basins to much thicker formations under continents. This outer shell is divided into two major categories: sial, which is rich in aluminum and silicon, and sima, which contains magnesium and iron. The boundary between the crust and the mantle, known as the Mohorovičić discontinuity, marks a distinct change in seismic wave velocity, where primary seismic waves travel at speeds consistent with basalt above and peridotite below. This discontinuity is not always distinct, and in some continental regions, the transition is absent, complicating the geological map of the planet's outermost layer.

The Flowing Mantle Engine

The mantle extends to a depth of approximately 2,900 kilometers, making it the thickest layer of the planet and accounting for 83.7 percent of Earth's volume. Although solid, the mantle's extremely hot silicate material can flow over very long timescales, a property that drives the motion of tectonic plates in the crust. This convection is powered by the decay of radioactive isotopes in the crust and mantle combined with the initial heat from the planet's formation, released when a large amount of matter collapsed into a gravity well. The viscosity of the mantle ranges between 10 to the 21st and 10 to the 24th pascal-seconds, increasing with depth, which means the lower part flows less easily than the upper regions. The lowest part of the mantle, known as the D-double-prime layer, sits next to the core-mantle boundary where pressure reaches approximately 140 gigapascals, creating conditions that are unlike anything found on the surface.

Common questions

What is the radius of the solid inner core of Earth?

The solid inner core of Earth has a radius of about 1,220 kilometers. This massive iron-nickel sphere was confirmed through experiments where samples were subjected to core-like pressures between diamond tips and heated to approximately 4,000 Kelvin. X-ray observations revealed that these giant crystals run north to south, creating a unique structure that has remained stable for billions of years.

When did Inge Lehmann discover the inner core of Earth?

Inge Lehmann discovered the inner core of Earth in 1936 by analyzing seismic waves. She noted that some waves were reflected back to the surface in a way that suggested a solid inner sphere within the liquid outer layer. This discovery proved that the Earth is not a uniform ball of rock but a complex system of distinct layers.

How thick is the mantle of Earth and what percentage of Earth's volume does it account for?

The mantle extends to a depth of approximately 2,900 kilometers, making it the thickest layer of the planet. It accounts for 83.7 percent of Earth's volume. Although solid, the mantle's extremely hot silicate material can flow over very long timescales, a property that drives the motion of tectonic plates in the crust.

What is the composition of the outer core of Earth and how does it generate the magnetic field?

Earth's outer core is a fluid layer about 2,200 kilometers in height, composed mostly of iron and nickel. This layer generates the Earth's magnetic field through a process known as the geodynamo, where convection combined with the Coriolis effect creates a magnetic field that is 50 times stronger than the field at the surface. The average magnetic field in the outer core is estimated to measure 25 microteslas.

How old are the oldest known mineral grains found on Earth?

The oldest known mineral grains on Earth are approximately 4.4 billion years old. These ancient fragments, found within younger rocks, serve as time capsules from the Hadean Eon, a period when the surface was likely molten and hostile. They indicate that the planet has possessed a solid crust for nearly its entire existence.

What is the estimated time until the core of Earth freezes up?

One estimate suggests that the core of Earth would not be expected to freeze up for approximately 91 billion years. This timeline is well after the Sun is expected to expand, sterilize the surface of the planet, and then burn out. The solid inner core is too hot to hold a permanent magnetic field, as the temperature exceeds the Curie temperature.