The Discovery of the Earth's Core

Our understanding of Earth's deep interior—composed of the crust, mantle, and core—is almost entirely derived not from direct observation but from the analysis of **seismic waves**. In the early 20th century, scientists recognized that the speed and path of waves generated by **earthquakes** change abruptly at certain depths, indicating transitions between layers of different composition and density.

The existence of the core was established by **Richard Dixon Oldham** in 1906. By studying seismograph records from distant earthquakes, Oldham noted that P-waves (compressional waves) and S-waves (shear waves) behaved unexpectedly as they travelled through the Earth. He observed a distinct **shadow zone** where P-waves disappeared or arrived late, leading him to postulate the presence of a central boundary separating the **mantle** from a dense, liquid outer region. This became known as the **outer core (OC)**.

Further complexity was added by Danish seismologist **Inge Lehmann** in 1936. By meticulously examining faint P-waves that unexpectedly appeared within Oldham’s shadow zone, Lehmann deduced that these waves had refracted (bent) off a second, even deeper boundary. She correctly proposed that the Earth’s core must consist of two distinct parts: a **liquid outer core** and a solid, **innermost sphere**—the **inner core (IC)**. This crucial discovery was made without ever directly sampling the material.

The current model states that the liquid Outer Core, primarily composed of **iron and nickel**, is responsible for generating Earth's **magnetic field** through the motion of conductive molten material (the geodynamo). The solid Inner Core, despite being hotter than the sun's surface, remains solid due to the immense pressure at the planet's centre. The boundary between the OC and IC is often referred to as the **Lehmann Discontinuity**, a testament to her groundbreaking work. The continuous study of seismic wave propagation remains the only effective means of refining our picture of Earth’s inaccessible interior.