Inside Earth

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• The mantle

  The mantle is the largest part of the Earth, and almost everything commonly said about it is wrong in the same way: it is not molten. It is solid rock that flows.

• Lithosphere and asthenosphere

  There are two ways to slice the Earth’s interior, and they do not line up. One divides by what the layers are made of; the other by how they behave. Plate tectonics happens on the second division.

• Earth’s internal heat

  The engine that moves plates, builds mountains and melts rock runs on two fuels, not one: heat inherited from the planet’s formation, and heat still being made by radioactive decay. Separating the two took a particle detector.

• The layers of the Earth

  Beneath its thin, brittle shell, the Earth is organised into layers — by what they are made of, and by how they behave. The distinction between the two ways of slicing the planet is where plate tectonics begins.

• How Earth formed

  Earth has no birth certificate. Its own oldest rocks were destroyed by the planet it belongs to, so the age of the Earth had to be read from objects that never changed — and that indirection is the first thing worth understanding about deep time.

Compositional layers

Divisions defined by what the material is: the classification used when the question is chemistry rather than behaviour.

• Earth layer

  Crust

  The outermost compositional layer of the Earth: thin, rigid and brittle compared with the layers beneath it, and markedly thinner under the oceans than under the continents.

• Earth layer

  Oceanic crust

  The thin, dense crust underlying the ocean basins, generated continuously at mid-ocean ridges and returned to the mantle at subduction zones. It varies little in thickness compared with continental crust.

• Earth layer

  Continental crust

  The thicker, less dense crust that makes up the continents. Its buoyancy is why continental lithosphere resists being drawn down at convergent boundaries, and why continental collision builds mountains instead of trenches.

• Earth layer

  Mantle

  The dense, hot layer of semi-solid rock between the crust and the core, containing more iron, magnesium and calcium than the crust. Its slow internal motion is central to plate tectonics.

• Earth layer

  Core

  The innermost compositional layer of the Earth: a metallic iron–nickel region, denser than the mantle above it, divided into a liquid outer core and a solid inner core.

• Earth layer

  Outer core

  The liquid part of Earth’s metallic core. Its inability to transmit shear waves is the observation from which its liquid state is inferred, and motion within it generates Earth’s main magnetic field.

• Earth layer

  Inner core

  The solid innermost region of Earth’s core. Its solidity is an inference from the behaviour of seismic waves rather than a direct observation, and the reasoning behind that inference has itself been revised.

Mechanical layers

Divisions defined by how the material deforms. Plate tectonics operates on this classification, not the compositional one.

• Earth layer

  Lithosphere

  The rigid mechanical outer layer of the Earth, formed by the crust together with the cool, rigid uppermost mantle, and broken into the tectonic plates.

• Earth layer

  Asthenosphere

  A relatively narrow, mobile zone in the upper mantle beneath the lithosphere, composed of hot material that can soften and flow over geological time. The rigid plates are thought to move over it.

Processes of the interior

What the interior does: produce heat, move it outward, and deform while remaining solid.

• Geological process

  Radiogenic heating

  Heat produced inside the Earth by the decay of long-lived radioactive isotopes, principally uranium, thorium and potassium. It is one of two main sources of Earth’s internal heat, alongside heat left over from formation.

• Geological process

  Secular cooling

  The gradual loss of heat retained from Earth’s formation and differentiation. Together with radiogenic heating it supplies the energy that drives the planet’s interior dynamics.

• Geological process

  Mantle convection

  Slow circulation of the solid mantle driven by internal heat, in which hot material rises and cooler, denser material sinks. The geometry of that circulation is far less well constrained than the fact of it.

• Geological process

  Solid-state flow

  Deformation of solid rock by creep over geological timescales. It is the reason the mantle can convect while remaining, almost everywhere, solid — and the reason it is wrong to describe the mantle as a body of liquid rock.

• Geological process

  Planetary differentiation

  The separation of a planetary body into layers of contrasting composition, dense metal sinking inward and lighter silicate rising, producing Earth’s core, mantle and crust.