The layered structure of Earth is a direct consequence of density, which is defined as mass per unit volume. Density dictates how matter arranges itself under the influence of gravity. The planet is stratified into concentric shells where materials with greater density reside beneath materials with lesser density, creating a fundamental gradient that increases steadily from the surface to the center. This arrangement is the single most important factor shaping the internal architecture of the planet.
Planetary Differentiation
The formation of Earth’s distinct layers began shortly after its accretion, in a process called planetary differentiation. Early Earth was extremely hot and largely molten due to heat generated by radioactive decay, meteorite impacts, and gravitational compression. This molten state allowed materials to move and separate based on density. Gravity caused the densest elements, primarily iron and nickel, to sink toward the center, establishing the metallic core. Simultaneously, lighter materials, mainly silicate compounds rich in elements like silicon, oxygen, and magnesium, floated upward, establishing a permanent compositional gradient, increasing uniformly with depth.
Density Profile of the Outer Layers
The outermost shell, the crust, is the least dense layer, with an average density around 2.7 grams per cubic centimeter (g/cm³). The crust is not uniform, consisting of two types with different densities. Continental crust is primarily granitic, which is less dense and thicker, allowing it to float higher on the underlying mantle. Oceanic crust is basaltic, containing more iron and magnesium, making it denser (typically around 3.0 g/cm³), and causing it to sit lower.
The boundary between the crust and the mantle, known as the Mohorovičić discontinuity or Moho, marks an abrupt density jump. The mantle, composed of silicate rocks rich in iron and magnesium, is substantially denser than the crust, with the upper mantle averaging approximately 3.4 g/cm³. Density continues to increase throughout the mantle, reaching up to 5.7 g/cm³ in the lower mantle, driven by the immense pressure that compacts the material. The combined effects of increasing pressure and temperature create distinct density zones within the mantle.
Density and Composition of the Core
The transition from the mantle to the core at a depth of 2,900 kilometers is marked by a massive increase in density. The core is predominantly an alloy of iron and nickel, the heaviest common elements in the planet. The liquid outer core begins with a density around 9.9 g/cm³ and increases to approximately 12.2 g/cm³ near the boundary with the inner core. The inner core, despite having a similar iron-nickel composition, is a solid ball due to the extreme pressure it experiences, reaching densities up to 13.1 g/cm³ at the center. The core contains between 85 and 90 percent of the Earth’s iron.
How Scientists Measure Earth’s Internal Density
Since direct sampling of the deep interior is impossible, scientists use the behavior of seismic waves to infer the density of the layers. Earthquakes generate P-waves (primary, compressional) and S-waves (secondary, shear) whose speed and path are directly affected by the density and rigidity of the material they pass through. Seismic wave velocity changes abruptly at boundaries between layers with contrasting densities, allowing scientists to map discontinuities like the Mohorovičić discontinuity. For example, S-waves drop to zero upon entering the liquid outer core because they cannot propagate through a fluid medium. By timing the arrival of these waves at seismograph stations across the globe, researchers calculate travel times and determine the material properties, including density, of each layer.