Earth’s Interior Structure: Unveiling the Depths with Seismic Waves and Meteorites
Theme 3: Structure and Composition of the Earth
1. Methods of Study
According to evidence from various research methods, the Earth’s interior is divided into concentric layers. But how do scientists study depths of hundreds and thousands of kilometers when the deepest well drilled has only reached about 12 km? Several research methods allow us to study the composition and structure of Earth’s interior, including:
A. Meteorites
Meteorites are solid, metallic, or stony bodies that move at high speed through space. Upon entering the atmosphere, they heat up due to friction, causing them to glow and creating “shooting stars”. While most are consumed in the atmosphere, larger ones can reach the surface, appearing polished due to atmospheric friction.
Meteorites are considered an indirect source of information about Earth’s interior composition. Initially thought to be fragments of a planet formed between Mars and Jupiter, they are now believed to be remnants of the material that formed the solar system. Consequently, scientists look for similarities with Earth. Chemical analysis shows that meteorites contain no elements not found on Earth, and some of their minerals are unknown here. The most common components include iron, diamonds, graphite, magnetite, and quartz.
B. Seismic Waves
Seismic waves are caused by various events, such as passing trains or construction vibrators. However, only those generated by large earthquakes, volcanic eruptions, asteroid impacts, and nuclear explosions can traverse the entire Earth. These waves are of two types:
- Surface Waves: These cause most of the damage during earthquakes.
- Interior Waves: These are the most useful for studying Earth’s interior and are the focus of our discussion.
An earthquake generates two types of interior seismic waves: P waves and S waves.
P Waves (Primary Waves)
P waves are the fastest and can pass through solids, liquids, and gases. They are compressional waves, similar to sound waves, where the material moves back and forth along the direction of wave propagation. As a P wave passes through a material, it is compressed and expanded, returning to its original size and shape once the wave has passed.
S Waves (Secondary Waves)
S waves are generally slower than P waves and can only pass through solids. They are transverse waves, meaning they move the material perpendicular to the direction of wave propagation, deforming it transversely. Since liquids and gases are not rigid, they cannot be deformed transversely, and S waves cannot travel through them.
The velocities of P and S waves depend on the density and elasticity of the materials they pass through. They travel slower through high-density rocks but faster through rocks with high elasticity. Elasticity is a property of solids, like rocks, that allows them to regain their original shape after being deformed by an applied force.
Since P waves are faster than S waves in all materials, they always arrive first at seismic stations. Seismic waves move outward as wavefronts from their origin, visualized as seismic rays, which are lines showing the direction of wavefront movement.
The behavior and travel times of these waves through Earth’s interior provide scientists with valuable information about its internal structure. The speed of P and S waves depends on the density and elasticity of the materials they traverse. Both properties generally increase with depth, although elasticity increases more rapidly than density, resulting in an overall increase in seismic wave speed with depth.
If Earth’s interior were uniform, seismic rays of P and S waves would travel in straight lines. However, when a seismic ray passes from one material to another with different density or elasticity, its speed and direction change, a phenomenon called refraction.
As waves pass through materials of varying density and elasticity, they are continually refracted, resulting in curved paths. In addition to refraction, seismic rays can also undergo reflection, similar to how light reflects off a mirror.
When seismic rays encounter a discontinuity that separates materials of different elasticity or density within the Earth, they are refracted, and some of their energy is reflected back to the Earth’s surface. By knowing the wave velocity and the time it takes to travel from its source to the interface and back to the surface, scientists can calculate the depth of the interface. This information is used to determine the depth of the layers within the Earth.