The Earth’s interior is largely inaccessible; the deepest boreholes penetrate only a tiny fraction of the planet’s radius. Seismology, the study of vibrations that travel through the Earth, offers the only realistic window into this hidden structure. These vibrations, known as seismic waves, are generated primarily by earthquakes and carry information about the materials they pass through. By recording the arrival times and behavior of these waves at stations across the globe, scientists construct a detailed, three-dimensional map of our planet’s internal layers.
Primary and Secondary Waves: The Tools of Seismology
Scientists use two types of body waves—waves that travel through the Earth’s interior: Primary waves (P-waves) and Secondary waves (S-waves). P-waves are the fastest seismic waves, arriving first at any recording station.
P-waves are compressional waves that move material back and forth in the same direction the wave travels, similar to sound. This motion allows P-waves to propagate through any state of matter: solids, liquids, and gases.
S-waves are slower and arrive second. They are shear waves, moving particles perpendicular to the direction of wave travel. Because fluids lack the necessary rigidity, S-waves cannot travel through liquids or gases. This difference in behavior is foundational evidence for identifying the planet’s deep, fluid layers.
Interpreting Wave Behavior: Reflection, Refraction, and Speed
Seismic wave information is decoded by observing three behaviors: changes in speed, reflection, and refraction. Wave velocity is directly proportional to the density and rigidity of the material they travel through. Waves generally speed up as they pass into deeper, more rigid, and compressed rock layers.
When a seismic wave encounters a boundary between two different materials, part of its energy bounces back, which is called reflection. Seismologists analyze these reflected waves to pinpoint the depth of an interface. The abrupt change in material properties also causes the wave to bend as it crosses the boundary, a process known as refraction.
Waves traveling through an increasingly dense medium continuously refract, causing their paths to curve upward toward the surface. Seismographs record the arrival times of these waves at various distances from the earthquake source. By comparing measured travel times to models, scientists calculate the depth and composition of the materials the waves traversed.
Mapping Earth’s Major Layers
The principles of wave behavior allowed scientists to delineate the three main layers: the crust, the mantle, and the core. The first major boundary is marked by a sudden increase in wave velocity, separating the relatively thin crust from the denser mantle beneath it. Wave speeds generally increase throughout the mantle due to increasing pressure, though zones of partially molten rock cause localized slowdowns.
The boundary between the mantle and the core is revealed by the behavior of S-waves. At approximately 2,900 kilometers deep, S-waves abruptly disappear, creating a large S-wave “shadow zone” on the opposite side of the planet from the earthquake source. Since S-waves cannot travel through liquid, their disappearance proves that the Earth’s outer core is a fluid layer.
P-waves, which pass through liquid, are sharply slowed and severely refracted as they enter the liquid outer core, confirming the massive change in density at this depth. Some P-waves refract again, indicating a final boundary deeper inside the Earth. This final refraction point separates the liquid outer core from the solid inner core, where P-waves speed up again as they pass through the high-pressure solid metal.
Identifying Internal Discontinuities and Boundaries
Refined seismic analysis has further identified smaller boundaries, or discontinuities, where wave velocities change abruptly. The Mohorovičić Discontinuity, commonly known as the Moho, marks the distinct compositional change between the crust and the underlying mantle. This boundary is shallower under the oceans, averaging about 8 kilometers deep, and deeper under continents, averaging around 35 kilometers.
The Gutenberg Discontinuity is situated at the core-mantle boundary, approximately 2,900 kilometers deep. It is defined by the sharp decrease in P-wave velocity and the complete cessation of S-waves as they encounter the liquid outer core. The Lehmann Discontinuity marks the transition from the liquid outer core to the solid inner core at a depth of about 5,150 kilometers, where P-waves suddenly increase in speed once more.