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Difficulty: HardInternal and External Structure of the Earth

Match each seismic discontinuity of the Earth's interior in Column I with its defining structural interface or wave behavior in Column II. Which alignment correctly matches each discontinuity to its boundary feature?

  • Conrad DiscontinuityIntra-crustal boundary separating the granitic upper layer (SIALSIAL) from the denser basaltic lower layer (SIMASIMA)
  • Mohorovičić DiscontinuityBoundary between the Earth's rigid crust and the underlying ultramafic peridotite mantle
  • Gutenberg DiscontinuityCore-mantle boundary at approximately 2,900 km2,900\text{ km} depth where transverse (SS) waves terminate completely
  • Lehmann DiscontinuityBoundary separating the liquid metallic outer core from the solid crystalline inner core

Answer

Conrad Discontinuity corresponds to the intra-crustal boundary between SIAL and SIMA; Mohorovičić Discontinuity corresponds to the boundary between the crust and mantle; Gutenberg Discontinuity corresponds to the core-mantle boundary where S-waves drop to zero; Lehmann Discontinuity corresponds to the boundary between the liquid outer core and solid inner core.
Each discontinuity represents a specific geophysical transition within the Earth's interior. The Conrad discontinuity is intra-crustal (SIAL/SIMA), the Mohorovičić discontinuity is crust-mantle, the Gutenberg discontinuity is mantle-outer core (where liquid stops S-waves), and the Lehmann discontinuity is outer core-inner core.

Step-by-Step Solution

1
Identify intra-crustal zoning features
The Conrad discontinuity lies within the crust, separating the granitic upper crust (SIALSIAL) from the basaltic lower crust (SIMASIMA).
It represents a density contrast within continental crustal material.
2
Identify the crust-mantle boundary
The Mohorovičić discontinuity separates continental and oceanic crust from the denser peridotite mantle below.
Seismic wave velocities increase markedly across this compositional boundary.
3
Identify the mantle-core transition
The Gutenberg discontinuity occurs at 2,900 km2,900\text{ km} depth, marking the change from the solid mantle to the liquid outer core where S-waves cannot propagate.
Shear waves cannot pass through liquid media.
4
Identify the inner-outer core boundary
The Lehmann discontinuity separates the liquid iron-nickel outer core from the solid inner core at a depth of roughly 5,150 km5,150\text{ km}.
Higher pressure at the center forces iron and nickel into a solid metallic crystal structure despite high temperatures.

Key Concept

Internal Structure of the Earth and Seismic Discontinuities
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Topics

Internal and External Structure of the Earth

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