Earth's interior increases in density and temperature with depth due to increasing pressure and changing mineral composition. The crust (~10-70 km) is compositionally distinct from the denser mantle below, creating a fundamental density boundary. Iron-rich material in the core distinguishes it from silicate layers above.
Use density profiles and velocity-depth curves from seismic data to infer composition. Compare crustal samples from drilling with computed properties.
Earth is not uniform inside — it is layered, and the layering is defined by two distinct properties: composition (what material is present) and mechanical behavior (how that material responds to stress). The compositional layers are the most fundamental. The outermost layer, the crust, is thin (5–10 km under oceans, 30–70 km under continents) and composed of relatively light silicate rocks. Oceanic crust is basaltic (density ~3.0 g/cm³), while continental crust is more granitic (density ~2.7 g/cm³). Below the crust lies the mantle, a thick shell of denser silicate rock (density ~3.3–5.5 g/cm³) dominated by minerals like olivine and pyroxene. At the center sits the core, composed primarily of iron and nickel, with density reaching ~13 g/cm³ at Earth's center.
How do we know what is inside a planet we cannot directly access below the first few kilometers? The answer is seismic waves. When earthquakes generate waves that travel through Earth's interior, their speed changes at boundaries between materials of different density and rigidity. P-waves (compressional) travel through both solids and liquids; S-waves (shear) travel only through solids. The sharp velocity change at the Mohorovičić discontinuity (Moho) at the base of the crust marks the compositional transition from crustal rock to denser mantle rock. Deeper, S-waves disappear entirely at the outer core boundary — revealing that the outer core is liquid iron. These velocity-depth profiles, combined with laboratory experiments on minerals at high pressures, allow geophysicists to reconstruct the density and composition of each layer.
The density increase with depth is not smooth — it occurs in steps that correspond to compositional boundaries and phase transitions. Within the mantle, increasing pressure forces minerals into denser crystal structures even though composition changes relatively little. At about 410 km depth, olivine transforms to a higher-density structure (wadsleyite), and at 660 km, another transition produces even denser minerals (bridgmanite). These phase transitions cause abrupt jumps in seismic velocity without requiring a change in chemical composition. Understanding this distinction — between compositional layering and phase-change layering — is essential for interpreting Earth's interior structure correctly and forms the foundation for understanding why tectonic plates move, why some regions of the mantle convect differently from others, and how Earth has differentiated over geologic time.
This is a foundational topic with no prerequisites.
No prerequisites — this is a starting point.