Earth’s internal structure is layered, with each shell presenting a distinct physical state and composition. The “center” of the planet is defined as its gravitational and geometric midpoint, located at the heart of the innermost layer. To reach this point, one must traverse a distance equal to the planet’s average radius, which is approximately 6,371 kilometers (3,959 miles) from the surface.
The Earth’s Primary Layers
The crust is the planet’s thinnest and outermost layer, varying significantly in thickness. Underneath the oceans, the oceanic crust is only about 5 kilometers (3 miles) thick. The continental crust averages about 30 kilometers (19 miles) and can extend up to 70 kilometers (44 miles) beneath mountain ranges. This brittle, rocky shell is broken into tectonic plates that move slowly across the surface.
Beneath the crust lies the mantle, Earth’s thickest layer, extending to a depth of roughly 2,890 kilometers (1,800 miles). This layer is composed of hot, dense, silicate rock rich in iron and magnesium. While solid, it flows like an extremely viscous fluid over millions of years due to intense heat and pressure. The slow convection currents within the mantle are the driving force behind the movement of the surface tectonic plates.
The mantle ends abruptly at the core-mantle boundary, marking the transition to the outer core, a vast layer approximately 2,300 kilometers (1,400 miles) thick. This region is composed primarily of liquid iron and nickel, along with lighter elements like sulfur and oxygen. The flowing motion of this molten metal generates Earth’s magnetic field, which shields the planet from solar radiation.
The Innermost Core: Location and Composition
The true center of the Earth is reached at the boundary between the liquid outer core and the solid inner core, approximately 5,150 kilometers (3,200 miles) beneath the surface. This solid sphere, the inner core, has a radius of about 1,220 kilometers (758 miles) and represents the planet’s geometric midpoint.
The inner core is a dense alloy of iron and nickel, with iron making up an estimated 80 to 90 percent of its mass. Despite temperatures that may reach 5,400 degrees Celsius (9,800 degrees Fahrenheit), comparable to the surface of the Sun, the inner core remains solid. This solid state is a result of the immense pressure, roughly 3.6 million times greater than the pressure at the Earth’s surface, effectively preventing the iron-nickel alloy from melting.
Determining the Internal Structure
Since drilling efforts have only penetrated a little more than 12 kilometers into the crust, scientists rely on indirect methods to map the deep interior. The most informative method for deducing the internal structure is seismology, the study of seismic waves generated by earthquakes. These waves travel through the Earth and their behavior changes depending on the material they encounter.
Primary waves (P-waves) are compressional waves that travel through both solids and liquids, while secondary waves (S-waves) are shear waves that can only pass through solid material. By measuring the travel times and paths of these waves at seismograph stations, scientists identify abrupt changes in density, temperature, and physical state. For instance, the complete absence of S-waves in the shadow zone on the opposite side of the planet confirmed that the outer core is liquid. This analysis of seismic wave refraction and reflection allows researchers to precisely map the boundaries and physical properties of each layer.