NDA (II) 2022 General Ability Test · Question 92 of 150

Correct — B, Mohorovicic. The Mohorovicic discontinuity, usually shortened to the Moho, is the boundary between the crust and the mantle. The Croatian seismologist Andrija Mohorovicic identified it in 1909 after noticing that earthquake waves arriving from a certain distance came in two sets, because part of the energy had sped up on entering denser rock below. It lies only about 5 to 10 km down under the oceans but roughly 30 to 40 km down under the continents, and deeper still beneath high mountains.

• (a)Gutenberg — The Gutenberg discontinuity is far deeper — it separates the mantle from the outer core, at about 2,900 km, where S-waves stop passing through.
• (c)Conrad — The Conrad discontinuity lies inside the continental crust itself, between the lighter upper crust and the denser lower crust, so it never reaches the mantle.
• (d)Repetti — In the layering used by standard geography texts the Repetti discontinuity is placed within the mantle, between its upper and lower parts, not at the crust-mantle boundary.

We cannot sample the Earth's interior directly, so its layers are mapped from the behaviour of earthquake waves. Wherever the speed or path of seismic waves changes abruptly, geologists mark a discontinuity and infer a change in the material. The named discontinuities are therefore the internal boundaries of the Earth, and the Moho is the shallowest major one.

The four names sit at four different depths, so the item is a depth-ordering test in disguise. Working outward from the surface the order is Conrad inside the crust, Moho at the base of the crust, Repetti within the mantle and Gutenberg at the mantle-core boundary. Note also that the lithosphere is not the same as the crust — it is the crust plus the rigid uppermost mantle, so the lithosphere straddles the Moho.

• The Mohorovicic discontinuity separates the crust from the mantle and was identified by Andrija Mohorovicic in 1909.
• The Moho lies at roughly 5 to 10 km beneath the oceans and about 30 to 40 km beneath the continents.
• The Gutenberg discontinuity marks the mantle-outer core boundary at about 2,900 km depth.
• The Conrad discontinuity separates the upper and lower parts of the continental crust.

• Treating 'crust' and 'lithosphere' as the same thing — the lithosphere includes the uppermost solid mantle.
• Swapping Moho and Gutenberg, which are the two most frequently confused names.

A one-line recall item matching a named discontinuity to the internal boundary it marks.

• practice — not a real PYQ

  The Gutenberg discontinuity separates

  • (a)the crust from the mantle
  • (b)the upper crust from the lower crust
  • (c)the mantle from the outer core
  • (d)the outer core from the inner core

  Answer(c) the mantle from the outer core — at a depth of about 2,900 km.

• practice — not a real PYQ

  The Mohorovicic discontinuity is shallowest beneath

  • (a)young fold mountains
  • (b)ocean basins
  • (c)continental shields
  • (d)plateaus

  Answer(b) ocean basins — oceanic crust is only about 5 to 10 km thick, against 30 to 40 km for continental crust.