Mohorovicic discontinuity

Geography glossary

Also called: Moho · Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"

Meaning

The Mohorovicic discontinuity (Moho) is the boundary between the earth's crust and the mantle below it. At this boundary, seismic waves (earthquake waves) suddenly speed up [2].

It matters because it was the first clear proof that the earth is built in layers. It also marks where the light crustal rock ends and the heavier mantle rock begins.

Explanation

How it was found (cause → process → result)

  • Who and when: the Croatian seismologist Andrija Mohorovicic found it in 1909. A seismologist is a scientist who studies earthquakes.
  • What he did:
  • He studied earthquake waves recorded at stations at different distances from the quake.
  • Some waves reached far-away stations sooner than expected.

  • They could only do this by going down into a deeper layer where waves travel faster, and then coming back up.

  • Result: he worked out that a sharp boundary lies under the crust. Below it, the rock lets waves travel much faster.

What happens at the Moho

  • Seismic wave speeds jump sharply (rise quickly) when the waves cross from the crust into the mantle [2].
  • Why they jump: the rock changes type.
  • Above the Moho is the crust. It is lighter rock: granite (sial) on the continents and basalt (sima) under the oceans.
  • Below the Moho is the mantle. It is silicate rock rich in iron and magnesium, and it is denser (heavier for its size).

  • Waves move faster through denser, more rigid rock.

  • A sharp boundary like this, where wave speed suddenly changes, is called a discontinuity.

How deep it lies

  • Under the oceans: 5-10 km below the ocean floor. Britannica gives about 7 km below oceanic crust [2].
  • Under the continents: 30-70 km. Britannica gives about 35 km as the usual depth [2].
  • Deepest under high mountains such as the Himalayas.
  • Big mountains have a deep "root" of light crust pushed down into the mantle.
  • This root holds the high land up, the way the large part of an iceberg under the water holds it up. This balance is called isostasy.

  • Result: the crust, and so the Moho, reaches about 70 km deep under the Himalayas.

  • Simple rule: the depth of the Moho = the thickness of the crust above it.

Where the Moho sits in the two ways of dividing the earth

  • By rock type (chemistry): crust → Moho → mantle. The Moho is a chemical boundary.
  • By strength (rigid or soft rock): lithosphere (rigid shell, crust plus uppermost mantle) → asthenosphere (weak, plastic zone that can flow slowly).
  • These two systems do not line up. The Moho lies almost entirely inside the lithosphere. It forms the lithosphere-asthenosphere boundary only beneath mid-ocean ridges, where the rigid layer is very thin [2].

Where it occurs

  • Everywhere under the earth's surface. It lies under every continent and every ocean, but at different depths.
  • Ocean floors (world): shallow, at 5-10 km below the sea floor (about 7 km) [2]. This is because oceanic crust is thin, about 5 km on average.
  • Continents (world): at 30-70 km, usually about 35 km [2].
  • Himalayas (India): among the deepest Moho anywhere, at about 70 km. The Indian plate keeps pushing into the Eurasian plate, which has thickened the crust here.
  • Mid-ocean ridges: underwater mountain chains where plates move apart. Here the Moho forms the base of the lithosphere, because the rigid layer is very thin [2].
  • Croatia: the home country of Andrija Mohorovicic, who found the boundary in 1909.

Don't confuse with

  • Conrad discontinuity: this is a boundary inside the continental crust, between the granitic upper crust and the basaltic lower crust. It is not found everywhere and is absent in oceanic crust. The Moho is the crust-mantle boundary and is found everywhere.
  • Lithosphere-asthenosphere boundary: this separates rigid rock from weak rock, so it is a strength boundary. The Moho separates crustal rock from mantle rock, so it is a chemical boundary. The Moho lies inside the lithosphere and meets its base only beneath mid-ocean ridges [2].
  • Mantle-core boundary (at 2,900 km): this marks the bottom of the mantle. The Moho marks the top of the mantle.

Prelims Hooks

  • Moho = crust-mantle boundary. It was found by Andrija Mohorovicic (Croatia) in 1909.
  • Depth: 5-10 km below the ocean floor (about 7 km) and 30-70 km below the continents (about 35 km) [2]. It is deepest under the Himalayas (about 70 km).
  • Order from the surface downward: crust → Conrad (continental crust only) → Moho → mantle → mantle-core boundary at 2,900 km.
  • At the Moho, seismic wave speeds jump sharply [2]. They rise; they do not fall.
  • Trap: the Moho is not the base of the lithosphere. It lies almost entirely inside the lithosphere and meets its base only beneath mid-ocean ridges [2].
  • Trap: the Conrad discontinuity is absent in oceanic crust, but the Moho is present under both oceans and continents.

Mains Points

  • Seismic waves are our only "window" into the earth's interior.
  • No one can drill or dig down to the mantle.
  • The sudden jump in wave speed at the Moho showed where the crust ends.

  • This is why the layered model of the earth (crust, mantle, core) rests on earthquake records.

  • The depth of the Moho shows how thick the crust is, and this links to mountain building and isostasy.

  • Oceanic crust is thin and young, so the Moho is shallow (5-10 km).
  • Under the Himalayas, the Indian plate pushing into the Eurasian plate has thickened the crust to about 70 km, so the Moho lies deep.

  • This thick, stressed crust helps explain why the Himalayan belt and North-East India are among India's most earthquake-prone areas (GS-III, disaster management).

  • The Moho shows that chemical layers and strength layers are not the same.

  • Plates are made of the lithosphere (crust plus uppermost mantle), not just the crust.
  • So the plates that move and cause earthquakes, such as the Indian, Pacific and Arabian plates, carry both crust and some mantle rock. The Moho lies inside the moving plate.

Related concepts

Read more

Sources

  1. 1Class 11, Ch 3 "Interior of the Earth" (primary)
  2. 2Britannica, "Mohorovičić discontinuity (Moho)"britannica.com · tier 3