THE CRUST

                            THE CRUST 

                    It is the top most layer of the earth. Its thickness over the oceanic area is generally 5 to 10 kms, whereas on the continental area, it is 35 kms and the thickness ranges from 55 to 70 kms in Orogenic belts.

Sub-divisions of the crust : 

                    The crust of the earth is sub-divided into sub layers as follows :

                    1.Sial,

                    2.Sima.

1.Sial : 

           👉 It is also known as the Upper - Continental - Crust. It's consists of all types of rocks like igneous, Sedimentary and metamorphic, which are exposed at the land - surface. It is rich in silica and aluminum. 

           👉 It's composition is usually granitic to grano - dioritic. In the ocean-basins, they are floored by a basaltic - horizon which are poorer in potassium and richer in aluminum then the basalts of land surface and are called 

“ Oceanic - Tholeiites ”.

          👉 Conrad Discontinuity separates the sial- layer from the under lying sima - layer. This discontinuity is a second - order discontinuity and is located at a depth of 11 kms.

2.Sima : 

              It is also known as Lower - Continental crust. Its thickness is about 22kms. It includes two parts,

               a) Outer Sima,

               b) Inner Sima. 

👉 Outer sima extends up to a depth of 19kms and comprises rocks of intermediate composition.

👉 Inner sima is located at a depth of 19 kms and extends up to 33 kms. It is of basic to ultra - basic in composition.