Beneath Earth’s surface lies a complex and dynamic interior, a realm of immense pressure and heat that constantly influences our world. The journey from the surface to Earth’s core reveals a fascinating layered structure, each with distinct properties.
Measuring the Distance
The distance from Earth’s surface to its center is approximately 6,371 kilometers (3,959 miles). This is an average, as Earth is not a perfect sphere but an oblate spheroid, bulging at the equator and flattening at the poles due to its rotation.
The equatorial radius is about 6,378 kilometers (3,963 miles), while the polar radius measures approximately 6,357 kilometers (3,950 miles). This difference highlights the slight flattening at the poles, caused by the centrifugal force generated by Earth’s daily spin.
Inside Our Planet
The outermost layer is the crust, which varies significantly in thickness. Continental crust, forming the landmasses, typically ranges from 20 to 80 kilometers thick, averaging around 30 to 40 kilometers, and is primarily composed of felsic rocks like granite. Oceanic crust, found beneath the oceans, is much thinner, usually 5 to 10 kilometers thick (averaging about 7 kilometers), and consists of denser, mafic rocks such as basalt.
Beneath the crust lies the mantle, a thick layer extending roughly 2,880 to 2,900 kilometers deep. It constitutes about 80 percent of Earth’s volume and is composed of silicate rocks rich in iron and magnesium. While often described as solid, the mantle behaves like a viscous fluid over geological timescales, allowing for slow, convective flow.
The outer core sits beneath the mantle, a liquid layer approximately 2,200 to 2,300 kilometers thick. It is predominantly made of molten iron and nickel, with temperatures ranging from about 4,000 to 6,000 degrees Celsius.
At Earth’s very center is the inner core, a solid sphere with a radius of about 1,200 to 1,250 kilometers. Despite temperatures reaching approximately 5,000 to 6,000 degrees Celsius, the immense pressure keeps this iron-nickel alloy in a solid state.
Probing Earth’s Depths
Our understanding of Earth’s deep interior comes primarily from the study of seismology. When earthquakes occur, they generate seismic waves that travel through the planet. Scientists analyze two main types of these body waves: P-waves (primary waves) and S-waves (secondary waves).
P-waves are compressional waves that can travel through solids, liquids, and gases. S-waves are shear waves that can only propagate through solid materials. The speed and behavior of these waves change depending on the density, composition, and physical state of the materials they encounter. By observing how seismic waves reflect and refract at different depths, scientists can map the boundaries and properties of Earth’s internal layers. The absence of S-waves passing through the outer core, for example, provides strong evidence that this layer is liquid. Other methods, such as studying Earth’s gravity field and magnetic field, also provide data about the planet’s internal structure.
Why This Knowledge Matters
Understanding Earth’s internal structure is fundamental to comprehending many surface phenomena. The slow, convective movement within the mantle drives plate tectonics. This internal heat and motion also contribute to volcanic activity and the occurrence of earthquakes, particularly along plate boundaries.
The liquid outer core generates Earth’s magnetic field through the motion of electrically conductive iron. This magnetic field extends far into space, forming a protective shield that deflects harmful charged particles from the solar wind and cosmic radiation. Without this shield, life on Earth would be exposed to dangerous levels of radiation.