The Crust
The crust is the uppermost shell of the earth. It is 5-10 km thick over the oceanic area, 35 km in the continental area, and thickness ranges from 55 to 70 km in orogenic belts. The Mohorovicic discontinuity marks its lower boundary of the crust. From the study of shallow-focus earthquakes and artificial seismic explosions, it has been inferred that there are two zones of crustal rocks beneath the continents. However, 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 layer ‘Sima’. The boundary between the Sial and the Sima is called Conrad Discontinuity.
Sial is also known as the upper continental crust, and contains igneous, metamorphic, and sedimentary rocks that are exposed on the land surface. This layer is rich in silica and aluminum. The rocks in this layer are of granitic to granodioritic composition. The Conrad Discontinuity, located at a depth of 11kms separates the Sial from Sima.
Sima is also known as the lower continental crust and is about 22 km thick. It extends from Conrad discontinuity up to the Mohorovicic discontinuity. This layer is rich in silica and magnesium and is basaltic in composition. Sima includes two parts-
Outer Sima: It extends up to a depth of 19 km and consists of rocks of intermediate composition.
Inner Sima: It is located at a depth of about 19 km and extends up to 33 km. It is basic to ultrabasic in composition.
It has been observed that the L-waves while passing through ocean floors acquire more velocity than their propagation through the land masses. This indicates that the sialic layer (granitic material) with which the landmasses are usually composed is practically absent on the ocean floors. The ocean floor is of basaltic composition, which is poorer in potassium and richer in aluminum than the basalt of the land surface and is called ‘oceanic tholeiites’. A change from continental to oceanic crust occurs at the peripheries of the major continents where there are marginal seas and island arcs.Â