The term rock in the general usage implies something which is hard and resistant, but to a geologist the term comprises all the solid materials of the earth’s crust, whether it is hard like granite, soft like clay, or loose like sand.
A rock is not a chemical compound, but is usually a mixture of variety of minerals. In other words, rock may be defined as an aggregate of minerals.
Minerals are homogeneous, naturally occurring inorganic substances having definite chemical composition and physical properties. Minerals are usually crystalline in appearance. Whereas minerals are homogeneous in their composition, rocks, on the contrary, are heterogeneous.
Minerals are identified in a general way by various characteristics which are the following: (i) colour, (ii) lustre, (iii) hardness, (iv) number and arrangement of crystal faces, (v) cleavage, (vi) fracture, (vii) solubility in water and various acids, and (viii) their associations in rocks.
Remember that the chemical elements, singly or in combination, form an inorganic natural compound which is known as mineral.
There is certain elements, e.g. gold, copper, sulphur etc. which make up minerals by themselves. However, most of the minerals are compounds of two or more elements. According to this definition, ice may be considered as a mineral.
But coal, natural oil and gas are not minerals because they are not solid. Despite the fact that a very large number of minerals is known, their number being about 2000, but only a few are important as rock constituents. These minerals are referred to as rock-forming minerals.
The above table clearly shows that the principal rock-forming minerals are olivine, garnet, pyroxene, amphibole, micas, clays, feldspar and quartz. It may be pointed out that there are many discrete families of minerals.
There are certain elements which readily combine with various other elements. Such elements are silicon, oxygen, and carbon; therefore, the most common mineral groupings are the silicates, oxides, and carbonates.
Remember that the silicates form the largest group, constituting 92 per cent of the earth’s crust. Besides the above mineral groupings, there are sulphide minerals which include pyrites and iron sulphides. Metallic elements such as iron, aluminum form metal oxides when they come into contact with atmospheric oxygen.
From the above discussion it is clear that the rocks forming the earth’s crust consist of two or more minerals which have different characteristics. Thus, the erosional landforms are mainly governed by the rocks.
The structural characteristics and the composition of different varieties of rocks determine the nature and magnitude of erosion.
It is due to difference in the characteristics of rocks that the tectonic and gradational processes working together create the variety of landforms over the earth’s surface.
The earth’s crust is composed of numerous kinds of rocks and minerals that respond in different ways and at different rates to the earth-shaping processes.
Because of the above facts, it is imperative for the physical geographer to have some knowledge of the different types of rocks and of their chief characteristics, especially how they respond to the tectonic and gradational processes.
It is worthwhile to remember that the bedrock is the solid layer which is often covered by weathered material, soil or unconsolidated sediment.
Regolith is a layer of decomposed rock lying above the bedrock. Above the regolith, there may be soil. On a mountain slope running water removes the weathered material as soon as it forms leaving behind the exposed bedrock.