E1 Internal Structure of the Earth

Topic

Earth's internal structure is understood primarily through the speed at which seismic waves propagate through its interior. Compressional waves can travel through both solids and fluids, whereas shear waves can only travel through solids; it was precisely this difference that allowed scientists to identify Earth's outer core as liquid, as shear waves simply do not pass through it.

A distinct seismic discontinuity separates the crust from the mantle, marked by a sudden jump in seismic wave velocity; this discontinuity is typically found at a depth of seven kilometers beneath the oceans and thirty-five kilometers beneath the continents, extending to seventy kilometers beneath major mountain ranges, where the crust is thickest.

Lithostatic pressure increases with depth because each additional layer of rock adds its own weight to that of the overlying layers; this pressure reaches approximately 135 gigapascals at the mantle-core boundary and about 363 gigapascals at Earth's very center.

A well-established mathematical relationship allows for the estimation of the Earth's interior adiabatic density profile by integrating seismic velocities measured at various depths; this same relationship forms the basis of the standard seismological reference model used today to describe Earth's entire structure.

Primordial planetary differentiation gave rise to this entire structure: denser iron and nickel sank gravitationally toward the planet's center to form the core, while lighter silicates floated upward to form the crust. The direct legacy of this primordial differentiation is the concentric structure—crust, mantle, and core—that seismic waves continue to reveal today, and with which this description began.

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