Earth’s crust consists of rocks and the loose material derived from these rocks, most of which is modified into soil. There are about a dozen elements which constitute the bulk of earth’s crust-nearly 99% by weight. These elements go into the formation of minerals which in turn form rocks from which all soils are derived.

Rocks in Earth’s Crust:

Parent rocks in earth’s crust are a complex aggregate of minerals whose properties determine the physical and chemical nature of the rock which they form. Though there are more than two thousand minerals known, almost 99% of earth’s surface is made up of only 10-12 minerals (Table 1.4). About 58% of the mass of terrestrial rocks is constituted by Feldspars, 16% by Pyroxenes, 13% by Quartz and about 4% by Micas. These four mineral groups make up well over 90% of the rocks. The remaining 10% of the bulk is made up of other minerals.

Rocks which occur in earth’s crust are basically of three types. These are : 1. Igneous rocks 2 Sedimentary rocks 3. Metamorphic rocks


1. Igneous rocks:

Igneous rocks are formed by cooling and solidification of molten rock material called Magma. Some of the common igneous rocks are: Diorite and Basalts.

2. Sedimentary rocks:

Sedimentary rocks are those rocks which develop as a result of gradual accumulation, consolidation and hardening of products of weathering of mineral particles brought about by wind or water. These rocks are characterized by the presence of distinct sedimentary layers. Some of the important sedimentary rocks are: Shale, Sandstone, Limestone etc.


3. Metamorphic rocks:

Metamorphic rocks are rocks formed as a result of metamorphosis of igneous and sedimentary rocks. This transformation is brought about by intense heat and high pressures. Important metamorphic rocks are: Schists, Slate, Marble, Quartzite etc.

Disintegration of rocks results into the formation of gravel, sand, silt and clay. Burial, compaction and hardening of the products of disintegration of rocks result in the formation of sedimentary rocks. When sedimentary rocks are subjected to high temperatures and pressures, metamorphic rocks are produced.

Igneous rocks also transform themselves into metamorphic rocks when subjected to intense heat and high pressures. Melting, slight upward movement and subsequent cooling form igneous rocks again. The disintegration and formation of different types of rocks is a cyclic process which operates on the geological time scale. Clay, silt, sand and gravel are just intermediates in the cycle-the material from which soil formation starts.