Lehmann discontinuity
Also called: Inner core boundary · Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"
Meaning
The Lehmann discontinuity is the boundary between the liquid outer core and the solid inner core. It lies about 5,150 km below the surface. A discontinuity is a boundary inside the earth where the speed of seismic waves (shaking waves sent out by earthquakes) suddenly changes. Danish seismologist Inge Lehmann found this one in 1936. She noticed faint P-waves arriving inside the P-wave shadow zone, the band 105°-145° from the epicentre where no direct P-waves should reach. She worked out that a solid inner core was reflecting them.
Example
A large earthquake in the Pacific is recorded at stations around the world. Some faint P-waves show up at stations inside the shadow zone. They have bounced off the Lehmann discontinuity, deep under the earth's centre region.
Don't confuse with
- Gutenberg discontinuity: the boundary between the mantle and the core at about 2,900 km (1913). Lehmann is the deeper boundary inside the core.
- Mohorovicic discontinuity (Moho): the boundary between the crust and the mantle, only 5-70 km deep.