Understanding the 670 km Discontinuity
The Earth's mantle has distinct layers separated by discontinuities, which are depths where seismic wave velocities change abruptly. The 670 km discontinuity is a significant boundary within the mantle.
P-wave Velocity Increase Explained
At approximately 670 km depth, P-wave (compressional wave) velocity shows a noticeable increase. This indicates a change in the physical properties of the rock, such as density and elastic moduli.
Phase Transition: Garnet to Perovskite
The primary cause for this sharp increase in P-wave velocity at the 670 km discontinuity is a high-pressure phase transition. Specifically, the mineral garnet undergoes a transformation into denser mineral phases, primarily silicate perovskite and related phases (like magnesiowüstite).
- Garnet is a stable mineral in the upper mantle.
- As pressure increases with depth, garnet becomes unstable.
- Around 670 km, it transforms into perovskite and other dense phases, leading to increased seismic wave speeds.
This phase transition significantly alters the rock's properties, causing the observed jump in P-wave velocity.
Important Questions from Gross Composition
- The crust forming processes concentrate
Which one of the following represents the correct increasing order of abundance in the Earth’s crust?
- The relative abundance of elements, in decreasing order, in the Earth is
- Which of the following sets represents mechanical layering of the Earth from the surface to the centre?
- The masses of the Earth's crust, its mantle and core are in the ratio 7: 690: 303. Then their contributions to the Earth's gravity field will be in the ratio
