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In this article we are discussing about the Earth’s interior. 

The Mohorovičić discontinuity: 

a) Usually known as the Moho discontinuity, it is the boundary between the Earth’s crust and the mantle. It was discovered by Croatian seismologist Andrija Mohorovičić (hence the name).

b) It separates both the oceanic crust and continental crust from the mantle underneath.

c) It lies entirely within the lithosphere (upper layer of the mantle). 

d) The Moho discontinuity runs 5 to 10 kilometres deep beneath the ocean floor, and 20 to 90 kilometres deep (average depth being 32-35 kilometres) beneath the continents.

e) According to this concept, the velocity of p and s waves gets increased with depth.

Crust: 

a) It is the topmost layer of the Earth’s interior.

b) The average thickness of the crust is 40km. 

c) This comprises 1% of the Earth’s total volume and 0.5% of the Earth’s mass.

d) The composition of Earth’s crust is as follows,

Element               Mass percent

Oxygen                      46.6%

Silicon                       27.7%

Aluminium                  8.1%

Iron                              5%

Calcium                      3.6%

Sodium                       2.8%

The crust is further divided into 2 distinct parts, namely:

1) Continental crust:

a) This is the upper layer of the crust formed of granite rocks. This part forms the continents.

b) This layer is thicker though less dense- with a density of about 2.7gm/cm3 – as compared to the oceanic crust. 

c) This part is composed of silica & aluminium. Thus, it is also known as ‘sial’.

2) Oceanic crust:

a) This is the lower part of the crust, which is formed of basaltic rocks. This part forms the ocean floors.

b) While it is thinner than the continental crust, it is denser with an average density 3 gm/cm3

c) This part is composed of silica, aluminium & magnesium and hence is called ‘sima’.

Hence,

Crust + Moho discontinuity + Mantle above Asthenosphere = Lithosphere

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