How the crust and mantle divide rocky worlds
The rigid crust of a rocky planet floats atop a much thicker layer called the mantle. Earth's crust ranges from 5 to 70 kilometers thick, but the mantle extends 2,900 kilometers down. Despite its solid appearance, the mantle is plastic: it flows very slowly, carrying heat from the deep interior upward over millions of years. This movement powers plate tectonics, volcanism, and the slow upwelling that replenishes seafloor spreading. The boundary between crust and mantle is defined not by rock type alone, but by a change in seismic wave velocity called the Mohorovicic discontinuity.
The crust itself splits into a thin oceanic layer (denser basalt, younger) and thicker continental crust (lighter granite, older). This duality means rocky planets with active geology tend toward a two-layer surface system: young, dark seafloor and older, lighter continents.
The iron-rich core and early planet evolution
Below the mantle lies a liquid outer core of molten iron and nickel, surrounding a solid inner core. The sheer weight of the entire planet compresses the inner core so intensely that iron remains solid despite temperatures above 6,000 Kelvin. The liquid outer core, churning due to heat differences and rotation, generates the planet's magnetic field through the movement of charged iron. Early in a rocky planet's life, the core was even hotter and more active; as planets cool over billions of years, core activity gradually weakens, and the magnetic field can fade entirely. This explains why Mars, smaller and cooled faster than Earth, has essentially no magnetic field today.