Model Answer
0 min readIntroduction
The Earth’s interior remains largely inaccessible for direct observation. However, the study of seismic waves, generated by earthquakes and controlled explosions, provides the most comprehensive information about the structure and composition of our planet. These waves, as they travel through the Earth, undergo changes in velocity and direction due to variations in density and physical state of the different layers. Understanding these wave behaviors is crucial for constructing models of Earth’s internal structure, a field known as seismology.
Seismic Waves: Types and Properties
Seismic waves are broadly classified into two types: Body waves and Surface waves. Body waves travel through the Earth’s interior, while surface waves travel along the Earth’s surface. This answer will focus on Body waves.
- P-waves (Primary waves): These are compressional waves, meaning they cause particles to move in the same direction as the wave is traveling. They can travel through solids, liquids, and gases.
- S-waves (Secondary waves): These are shear waves, causing particles to move perpendicular to the wave’s direction. They can only travel through solids.
Revealing Earth’s Interior
The differing behavior of P and S waves as they traverse the Earth provides key insights into its internal structure.
1. Crust and Mantle Boundary (Mohorovičić Discontinuity – Moho)
A sudden increase in P-wave and S-wave velocities at a depth of approximately 30-50 km beneath continents and 5-10 km beneath oceans indicates the boundary between the crust and the mantle. This boundary, known as the Mohorovičić discontinuity (Moho), marks a change in rock composition from primarily granitic/basaltic in the crust to denser peridotite in the mantle.
2. Mantle Structure
Variations in P-wave and S-wave velocities within the mantle suggest compositional and phase changes. For example, the 410-km discontinuity is attributed to a phase transition in olivine to wadsleyite, while the 660-km discontinuity is linked to a transition to perovskite and magnesiowüstite. These transitions are driven by increasing pressure and temperature with depth.
3. Core-Mantle Boundary (CMB)
A significant decrease in S-wave velocity and a decrease in P-wave velocity at a depth of approximately 2900 km indicates the core-mantle boundary. The complete absence of S-waves beyond this boundary confirms that the outer core is liquid, as S-waves cannot propagate through fluids. P-wave velocities decrease due to the change in density and composition.
4. Inner Core
P-wave velocities increase again at a depth of approximately 5150 km, indicating the boundary between the liquid outer core and the solid inner core. The inner core is primarily composed of iron and nickel. Anomalous P-wave behavior within the inner core suggests variations in crystal alignment and composition.
5. Shadow Zones
The existence of S-wave shadow zones (areas where S-waves are not detected after an earthquake) and P-wave shadow zones (areas with reduced P-wave intensity) are crucial evidence for Earth’s layered structure. The S-wave shadow zone is a direct result of the liquid outer core blocking S-wave propagation. The P-wave shadow zone is caused by refraction of P-waves at the core-mantle boundary.
| Layer | Depth (km) | State | Key Seismic Characteristics |
|---|---|---|---|
| Crust | 0-70 | Solid | Relatively low velocities; Moho discontinuity |
| Mantle | 70-2900 | Solid | Increasing velocities with depth; 410 & 660 km discontinuities |
| Outer Core | 2900-5150 | Liquid | S-waves absent; P-wave velocity decrease; large shadow zone |
| Inner Core | 5150-6371 | Solid | P-wave velocity increase |
Conclusion
Seismic waves serve as a vital tool for probing the Earth’s interior, providing a detailed understanding of its layered structure, composition, and physical state. The analysis of P and S wave velocities, refraction, reflection, and shadow zones has revolutionized our knowledge of the planet’s internal dynamics. Continued advancements in seismology, coupled with computational modeling, will further refine our understanding of Earth’s complex interior and its role in geological processes.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.