Key Terms & Concepts — UPSC Mains
Seismic Discontinuity
"A boundary within Earth's interior where seismic wave velocities change abruptly due to changes in composition or mineral phase"
A seismic discontinuity is a boundary or interface within Earth's interior where the velocity, direction, or behaviour of seismic waves (P-waves and S-waves) changes abruptly. These changes result from differences in rock composition, density, or mineral phase transitions. Major seismic discontinuities include: the Conrad discontinuity (15-20 km, separating upper and lower crust), the Mohorovicic or Moho discontinuity (5-70 km, separating crust and mantle, discovered by Andrija Mohorovicic in 1909), the Gutenberg discontinuity (2,900 km, separating mantle and outer core, discovered by Beno Gutenberg in 1914), and the Lehmann discontinuity (5,100 km, separating outer and inner core, discovered by Inge Lehmann in 1936). S-waves are completely blocked at the Gutenberg discontinuity, proving the outer core is liquid.
One of the most frequently tested topics in UPSC Prelims (Physical Geography). UPSC regularly asks about which discontinuity separates which layers, who discovered them, and how P-wave and S-wave shadow zones prove the structure of Earth's interior. This is a must-know topic for both Prelims and GS-1 Mains.
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- 5 S-waves blocked at Gutenberg discontinuity — proves outer core is liquid
The 2014 ringwoodite study relied on seismic discontinuities at 410 km and 660 km — boundaries where olivine transforms to wadsleyite and ringwoodite respectively — to detect water-bearing minerals in the mantle transition zone.
