Short Essay on the Crust of the Earth

The Crust It is the uppermost shell of the earth that covers the rocks of the interior thinly. Its thickness over the oceanic areas is generally 5 to 10 kms; whereas on the continental areas it is about 35 kms and the thickness rangesfrom55to70kmsinorogenic belts. The Mohorovicic discontinuity marks its lower boundary.

From the study of the shallow focus earthquakes and artificial seismic explosions, it has been inferred that there are two zones of crustal rocks beneath the continents, although only one occurs beneath the oceans.

In the continental regions, underneath a zone of superficial sediments, the crust can be divided into two layers, the upper layer called ‘Sial’ and the lower one ‘Sima’. The boundary between the sial and the sima is called the Conrad Discontinuity.

(i) Sial

It is also known as the upper continental crust. It consists of all types of rocks, igneous, sedimentary and metamorphic, which are exposed at the land surface. This layer is rich in silica and aluminium. The rocks in this layer are of granitic to grano dioritic composition.

The Conrad discontinuity which is located at a depth of 11 kms separates the sial layer from the underlying sima layer.

(ii) Sima

It is also known as Lower Continental Cnsst. Its thickness is about 22 kms. It extends from the Conrad discontinuity up to the Mohorovicic discontinuity. This layer is rich in silica and magnesium and is basaltic in composition. It includes two parts:

(a) Outer Sima and (b) Inner Sima

Outer sima extends upto a depth of 19 kms and consists of rocks of intermediate composition.

Inner sima is located at a depth of about 19 kms and extends upto 33 kms. It is basic to ultrabasic in composition.

It has been observed that the L-waves while passing through ocean floors acquire more velocity in comparison to its propagation through the land masses. This indicates that the sialic layer (granitic material) with which the landmasses are usually composed of, is practically absent on the ocean floors.

The ocean floor is of basaltic composition, which are poorer in potassium and iJaer in aluminium than the basalts of the land surface and are called ‘oceanic tholeiites’.

A change from continental to oceanic crust takes place at the peripheries of the major continents where there are marginal seas and island arcs.