Key Terms & Concepts — UPSC Mains
Mantle Transition Zone
"The region between 410 km and 660 km depth in Earth's interior, defined by mineral phase transitions and capable of storing vast water reserves"
The mantle transition zone is the region of Earth's interior between 410 km and 660 km depth, sandwiched between the upper mantle and the lower mantle. It is defined by two sharp seismic discontinuities where mineral phase transitions occur: at 410 km, olivine transforms to wadsleyite, and at 660 km, ringwoodite dissociates into bridgmanite and periclase. Both wadsleyite and ringwoodite can store 2-3 wt% water as hydroxyl groups — far more than minerals above or below — creating a natural water trap. A 2014 study estimated the transition zone holds water equivalent to three times all surface oceans. The 660 km discontinuity has a negative Clapeyron slope, which partially inhibits whole-mantle convection.
Critical for UPSC GS-1 (Physical Geography — Interior of the Earth). UPSC Prelims frequently tests seismic discontinuities (Moho, Gutenberg, Lehmann), mineral composition at different depths, and the behaviour of seismic waves. The mantle transition zone is key to understanding plate tectonics, deep water cycling, and mantle convection models.
- 1 Located between 410 km and 660 km depth; approximately 250 km thick
- 2 Bounded by two seismic discontinuities caused by mineral phase transitions
- 3 At 410 km: olivine transforms to wadsleyite; at 660 km: ringwoodite to bridgmanite + periclase
- 4 Wadsleyite and ringwoodite can store 2-3 wt% water — creating a natural water trap
- 5 Estimated to hold water equivalent to three times all surface oceans
Seismic waves from over 500 earthquakes recorded by the USArray network slowed significantly when passing through the mantle transition zone (410-660 km), indicating the presence of hydrous (water-bearing) minerals like ringwoodite and wadsleyite.
