What is the Structural Composition of Lithosphere?

It is the general term for the entire solid earth realm i.e. crust. According to the recent concepts, the term lithosphere is used for the crust and the upper part of mantle, which is considered to be elastically very strong.

This is the outer, cold pan of the earth which is about 50-100 kms thick. Compared to the whole earth, the lithosphere is quite thin. Beneath the lithosphere, the rocks are still solid but are capable of creeping a few millimeters per year if the load on them is changed.

The lithosphere is underlain by the asthenosphere which is considered to be a compara­tively weaker zone.

According to the estimation made by Clarke and Washington, the lithosphere consists of 95% igneous rocks, 4% shale 0.75% sandstone, and 0.25% limestone (the metamorphic rocks being the altered equiva­lents of one or other of these rocks).

The average chemical composition of the lithosphere has been computed by a number of geo-scientists in terms of elements by weight percentage as indicated below:

The above tables indicate that 99 percent of the upper crust is made up mainly of 10 elements, with oxygen accounting for slightly less than 50%. Besides, the above mentioned ten oxides constitute more than 98 percent of the lithosphere, with silica being the most abundant one.

The mineralogical composition of the lithosphere has been com­puted as follows by Clarke and Washington in terms of volume percent­age:

ATMOSPHERE, HYDROSPHERE & LITHOSPHERE 35

Quartz - 11%

Alkali feldspar - 16%

Plagioclase feldspar (Andesine) - 47%

Amphiboles and Biotites - 20%

Magnetite - 5%

Apatite - 1%

100%

The above analysis represents only the average mineralogical composition of the lithosphere but it does not represent in any way the composition of the earth as a whole or even the crust as a whole.