By Lauren Corona • Updated Aug 30, 2022
Geophysics investigates what lies beneath Earth's surface, using seismic waves, gravity, magnetic fields, and heat flow to infer the planet’s internal structure. The Earth is divided into distinct layers—crust, mantle, outer core, inner core—each with unique composition and physical properties.
The crust is the outermost shell that we directly touch. It consists primarily of aluminosilicate minerals. Continental crust, which underlies continents, ranges from 35 to 70 km (22–44 mi) in thickness, while oceanic crust that forms the ocean floor is thinner, about 5–10 km (3.1–6.2 mi).
The mantle extends from the base of the crust to 2,900 km (1,800 mi) deep, making up about 80 % of Earth's volume. It is subdivided into an upper and a lower mantle. Both are solid but the lower mantle is denser and composed mainly of ferro‑magnesium silicates. While not molten, temperatures rise to 100–200 km below the surface, approaching the melting point. Convection within the mantle drives plate tectonics by transferring heat from the deep interior to the surface.
The outer core spans 2,300 km (1,400 mi) and consists of liquid iron and nickel with trace amounts of sulfur. Its fluid motion generates Earth's magnetic field, a fact inferred from the way seismic shear and compressional waves travel through this region. Though we cannot sample it directly, seismic data provide compelling evidence of its molten state.
At the center, the inner core is a solid sphere roughly 1,200 km (750 mi) in diameter, almost entirely iron with about 10 % of other elements such as sulfur, nickel, or oxygen. The combination of outer and inner core constitutes more than half of Earth's diameter, and its solid nature contrasts sharply with the fluid outer core.
