Model Answer
0 min readIntroduction
Seismic discontinuities represent abrupt changes in seismic wave velocities within the Earth’s interior. These changes provide crucial insights into the layering and composition of our planet. The upper mantle, extending from the base of the crust (Mohorovičić discontinuity) to approximately 660 km depth, exhibits several such discontinuities. These are not sharp boundaries but rather transition zones where physical and chemical properties change significantly. Understanding the causes of these discontinuities is fundamental to deciphering the dynamic processes occurring within the Earth, including mantle convection and plate tectonics.
Causes of Seismic Discontinuities in the Upper Mantle
The upper mantle exhibits several key seismic discontinuities, each arising from distinct causes. These can be broadly categorized into compositional changes, phase transitions, and thermal variations.
1. Mohorovičić Discontinuity (Moho) – ~30-70 km depth
This is the boundary between the Earth’s crust and the mantle. The primary cause is a change in composition. The crust, being largely composed of lighter silicate minerals (granitic and basaltic rocks), overlies the denser peridotite mantle. This compositional contrast leads to a significant increase in seismic wave velocity as waves enter the mantle.
2. Lehmann Discontinuity – ~220 km depth
This discontinuity is less pronounced and its existence is debated. It’s thought to be caused by a phase transition in olivine, the dominant mineral in the upper mantle. At this depth, increasing pressure and temperature cause olivine to transform from α-olivine to β-olivine. This phase change results in a slight increase in density and seismic velocity.
3. Gutenberg Discontinuity – ~660 km depth
This is a major discontinuity marking the boundary between the upper and lower mantle. The primary cause is a phase transition. At this depth, the mineral ringwoodite (a high-pressure polymorph of olivine) undergoes a further phase transition to perovskite and magnesiowüstite. This transition is accompanied by a substantial increase in density and seismic velocity. The endothermic nature of this transition (requiring energy input) also plays a role in mantle dynamics.
4. Discontinuities at ~410 km depth
This discontinuity is caused by the phase transition of olivine to wadsleyite. Similar to the Lehmann discontinuity, this transition is driven by increasing pressure and temperature. Wadsleyite is denser than olivine, leading to an increase in seismic wave velocity. This discontinuity is more consistently observed than the Lehmann discontinuity.
5. Other Minor Discontinuities
- Thermal Variations: Localized temperature anomalies, such as those associated with mantle plumes or subducting slabs, can cause smaller-scale discontinuities.
- Partial Melting: The presence of even small amounts of partial melt in the asthenosphere (a partially molten layer within the upper mantle) can significantly reduce seismic wave velocities, creating a discontinuity.
- Water Content: The presence of water in mantle minerals can also affect their physical properties and contribute to discontinuities. Water lowers the temperature of phase transitions.
Geophysical Evidence: These discontinuities are identified through the analysis of seismic wave travel times and amplitudes. Sudden changes in these parameters indicate a change in the properties of the Earth’s interior. Seismic tomography, which uses seismic waves to create 3D images of the Earth’s interior, provides further evidence for the existence and characteristics of these discontinuities.
| Discontinuity | Depth (km) | Primary Cause | Mineralogical Change |
|---|---|---|---|
| Mohorovičić | 30-70 | Compositional Change | Crust (Granite/Basalt) to Mantle (Peridotite) |
| Lehmann | 220 | Phase Transition | α-Olivine to β-Olivine |
| ~410 | 410 | Phase Transition | Olivine to Wadsleyite |
| Gutenberg | 660 | Phase Transition | Ringwoodite to Perovskite + Magnesiowüstite |
Conclusion
Seismic discontinuities in the upper mantle are vital indicators of the Earth’s internal structure and dynamics. They are primarily caused by compositional changes and, more significantly, by phase transitions in mantle minerals due to increasing pressure and temperature. These discontinuities influence mantle convection, plate tectonics, and the overall evolution of our planet. Further research, utilizing advanced seismic techniques and laboratory experiments, is crucial for refining our understanding of these complex phenomena and their role in Earth’s processes.
Answer Length
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