The Core

The story of the Earth's interior is told by earthquake waves. It was the behaviour of these waves — how their velocities changed as they travelled through the planet — that first revealed the existence of a distinct innermost layer, the core. The boundary between the core and the mantle above it lies at a depth of 2,900 kilometres from the surface. This is a sharp, well-defined boundary, and it marks one of the most dramatic changes in the Earth's interior.

The core itself is not uniform. It is divided into two parts with very different physical states. The outer core is in a liquid state, while the inner core is in a solid state. This difference is crucial — it explains why certain earthquake waves behave the way they do when they pass through the deep interior. The liquid outer core blocks one type of wave entirely, while the solid inner core allows waves to pass through it at changed velocities. …