Model Answer
0 min readIntroduction
The Earth’s interior, inaccessible for direct observation, is primarily investigated through the study of seismic waves generated by earthquakes and controlled explosions. These waves, as they propagate through the Earth, undergo refraction and reflection at boundaries between layers with differing physical properties, creating what are known as seismic discontinuities. These discontinuities represent significant changes in composition, density, or phase of Earth materials. Understanding these discontinuities is fundamental to deciphering the Earth’s internal structure and its dynamic processes. This answer will detail the major seismic discontinuities and their corresponding petrological explanations.
Seismic Discontinuities and Petrological Explanations
Seismic discontinuities are surfaces within the Earth where seismic wave velocities change abruptly. These changes are caused by variations in density, composition, and phase of the Earth’s materials. The primary discontinuities are:
1. Mohorovičić Discontinuity (Moho)
Depth: Approximately 5-70 km (thinner under oceans, thicker under continents). Seismic Characteristic: A sharp increase in P-wave velocity from ~6.8 km/s to ~8.0 km/s and a significant increase in S-wave velocity. Petrological Explanation: The Moho marks the boundary between the crust and the mantle. Above the Moho lies the relatively less dense, silica-rich crust (granitic composition for continents, basaltic for oceans). Below the Moho lies the denser, iron and magnesium-rich mantle, primarily composed of peridotite. The change in composition and density causes the velocity increase. The transition from hydrated crustal rocks to anhydrous mantle rocks also contributes.
2. Gutenberg Discontinuity
Depth: Approximately 2900 km. Seismic Characteristic: A dramatic decrease in P-wave velocity and the complete disappearance of S-waves. Petrological Explanation: This discontinuity marks the boundary between the mantle and the outer core. The mantle is primarily solid, while the outer core is liquid, composed mainly of iron and nickel. S-waves cannot propagate through liquids, explaining their disappearance. The decrease in P-wave velocity is due to the change in density and the transition from solid silicate materials to liquid metallic iron.
3. Lehmann Discontinuity
Depth: Approximately 2200 km. Seismic Characteristic: A slight increase in P-wave velocity and a subtle change in S-wave velocity. Petrological Explanation: This discontinuity represents the boundary between the upper and lower mantle. It is associated with a phase transition in silicate minerals, specifically the transition from a spinel structure to a perovskite + magnesiowüstite structure. This phase transition is driven by increasing pressure and temperature with depth. The change in mineral structure leads to a change in density and seismic velocity.
4. D'' (D double prime) Discontinuity
Depth: Approximately 2700-2900 km (at the base of the lower mantle). Seismic Characteristic: Complex and variable, with both velocity increases and decreases, and evidence of highly scattered seismic waves. Petrological Explanation: This region is characterized by post-perovskite phase transitions of silicate minerals, along with the accumulation of subducted slabs and potentially iron-rich post-perovskite. The complexity of the D'' layer is attributed to the dynamic interaction between the mantle and the core, including thermal plumes and compositional heterogeneity. The presence of partially molten material is also suggested.
5. Inner Core Boundary
Depth: Approximately 5150 km. Seismic Characteristic: A sharp increase in P-wave velocity. Petrological Explanation: This boundary marks the transition from the liquid outer core to the solid inner core. Despite the high temperature, the immense pressure at this depth forces iron into a solid state. The alignment of iron atoms under extreme pressure contributes to the increased velocity. The inner core is believed to be primarily composed of iron with some nickel and trace elements.
| Discontinuity | Depth (km) | Seismic Characteristic | Petrological Explanation |
|---|---|---|---|
| Mohorovičić | 5-70 | Increase in P & S wave velocity | Crust-Mantle boundary; Silica-rich to Iron-Magnesium rich |
| Gutenberg | 2900 | Decrease in P wave velocity, S waves disappear | Mantle-Outer Core boundary; Solid Silicate to Liquid Iron-Nickel |
| Lehmann | 2200 | Slight increase in P & S wave velocity | Upper-Lower Mantle boundary; Spinel to Perovskite + Magnesiowüstite |
| D'' | 2700-2900 | Complex velocity changes | Post-perovskite phase transitions, subducted slabs |
| Inner Core Boundary | 5150 | Increase in P wave velocity | Liquid Outer Core to Solid Inner Core; Iron phase transition |
Conclusion
Seismic discontinuities provide invaluable insights into the Earth’s layered structure and the physical and chemical processes occurring within. These boundaries are not simply sharp transitions but rather complex zones of change, influenced by mineral phase transitions, compositional variations, and dynamic interactions between different layers. Continued research using advanced seismological techniques and high-pressure mineral physics experiments is crucial for refining our understanding of the Earth’s deep interior and its evolution.
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.