The layers deep within the Earth cannot be observed directly, but geophysicists use seismology—the study of energy released by earthquakes—to map the planet’s interior. This technique treats the Earth as a naturally occurring laboratory where seismic energy acts as a probe. By recording and analyzing how these waves travel, reflect, and bend, scientists reveal the physical state of the core, mantle, and crust. The distinct behavior of seismic waves provides indirect evidence regarding the composition and physical properties of the deepest layers, including whether the outer core is solid or liquid.
Defining Seismic Waves and Their Movement
The energy from an earthquake propagates through the Earth in two main types of body waves: Primary (P) waves and Secondary (S) waves. P-waves are the fastest seismic waves, traveling as longitudinal or compressional waves, much like sound. They push and pull the material in the direction the wave is moving, allowing them to transmit energy through any medium—solids, liquids, or gases.
S-waves, in contrast, are shear or transverse waves that move the material in a side-to-side motion, perpendicular to the direction of wave travel. This shearing motion requires the medium to have rigidity, or the ability to resist a change in shape. Because of this physical requirement, S-waves can only travel through solids, such as the Earth’s crust and mantle. The inability of S-waves to travel through liquids is a fundamental principle of wave physics.
The Outer Core: A Liquid Barrier
The direct answer is that S-waves do not travel through the outer core. This failure to transmit S-waves is the most convincing evidence for the outer core’s liquid state. The outer core is a vast, fluid layer composed primarily of molten iron and nickel, extending from approximately 2,889 kilometers to 5,150 kilometers below the surface.
When an S-wave encounters the core-mantle boundary, it is effectively stopped, absorbed, and reflected because the molten iron alloy cannot sustain the required side-to-side shear motion. This liquid state exists despite the immense pressure because the temperature, which ranges from 4,000 to 6,000 degrees Celsius, is high enough to keep the iron and nickel molten. P-waves, however, can pass through the liquid outer core, although their speed changes dramatically due to the difference in density and compressibility between the solid mantle and the liquid core.
Mapping Earth’s Interior Through the Shadow Zone
The liquid nature of the outer core is a conclusion drawn from a specific global observation known as the S-wave shadow zone. Following a major earthquake, seismographs around the world record the arrival of seismic waves. S-waves are detected across the globe, but they completely vanish beyond an angular distance of approximately 103 degrees from the earthquake’s epicenter. This large, ring-shaped area where no S-waves are recorded is the definitive S-wave shadow zone.
P-waves, which can travel through the liquid, behave differently; they are refracted, or bent, as they pass through the core-mantle boundary, creating a separate, smaller P-wave shadow zone between 104 and 140 degrees from the epicenter. This contrast between the complete blockage of S-waves and the mere refraction of P-waves is the observational evidence that allowed early 20th-century scientists to deduce the presence of a vast, liquid layer at the heart of the Earth.