The layers of the Earth

Three compositional layers

The Earth is about 12,750 kilometres in diameter, and its interior is organised into three main compositional layers: a thin outer crust, a thick rocky mantle, and a metallic core.^[1,2]

The crust is strikingly thin compared with the planet it wraps. Beneath the oceans it generally extends only to about five kilometres; beneath the continents it averages about thirty kilometres, thickening to as much as a hundred kilometres under great mountain ranges.^[1,2]

Below the crust lies the mantle: a dense, hot layer of semi-solid rock roughly 2,900 kilometres thick, richer in iron, magnesium and calcium than the crust above it.^[1]

A second way to slice the planet

Composition is not the only way to divide the interior. Mechanically, the crust and the cool, rigid uppermost mantle behave as a single strong layer — the lithosphere — which averages at least eighty kilometres thick over much of the Earth and is broken into the moving plates that carry the continents and oceans.^[1,2]

What remains unknown

The mantle convects, carrying heat slowly toward the surface — but basic questions about that circulation remain open: how many convection cells exist, where they originate, and what structure they take.^[1]

Lithoen records open questions like these as first-class claims with an explicit UNKNOWN status, so that the limits of current knowledge are as citable as the knowledge itself.