• In general, a rock is any mass of mineral matter, whether consolidated or not, which forms part of the earth's crust or lithosphere. 
  • About 90 per cent of rock-forming minerals are silicates (compounds containing silicon, oxygen or more metals). 
  • The important rock-forming silicate mineral groups are feldspar, quartz, and ferro-magnesium. 
  • Feldspar is the most abundant rock-forming silicate mineral (making up 54 per cent of the minerals in the earth's crust). 
  • Limestones are marble rocks made up of calcite, an important mineral of the carbonate group. 
  • Although most rocks are made of minerals, some substances of organic origin, such as peat and guano, are accepted as rocks. 
  • Rocks are classified on the basis of their mode of formation into three broad categories.

Igneous Rocks (Primary Rock or Parent Rock) 

  • Igneous rocks are formed by the cooling, solidification, and crystallization of molten earth materials, known as magma and lava. 
  • These are granular and crystalline rocks. The sizes of the crystals vary from one rock to another. 
  • Since water does not percolate through them easily, these rocks are less affected by chemical weathering. 
  • These rocks are more prone to mechanical weathering due to their granular structure. 
  • These rocks are non-fossiliferous. 
  • Most of the igneous rocks consist of silicate minerals. 
  • On the basis of chemical composition, igneous 'rocks can be divided into the following types: 
    (i) Acidic Igneous Rocks having more silica. They are relatively light rocks, e.g.Granites 
    (ii) Basic Igneous Rocks have lower amount of silica. They are dark-coloured, due to the pre-dominance of ferro-magnesium, e.g.— Gabbro. Basalt, etc. 
  • On the basis of the mode of occurrence, igneous rocks are classified into two major groups, as follows:
    Intrusive Igneous Rocks: When the rising magma is cooled and solidifies below the surface of the earth, they are known as Intrusive Igneous Rocks. These are further sub-divided into: 
    (a) Plutonic Igneous Rocks: They result from the cooling of magma very deep inside the earth. Due to very slow cooling at that great depth, large grains are developed, e.g. Granite. 
    (b) Hypabyssal Igneous Rocks: They are formed when magma cools and solidifies just beneath the earth's surface. They take different shapes and forms depending upon the hollow places in which they solidify.

Extrusive Igneous Rocks 

  • These rocks are formed by the cooling and solidification of molten lava on the earth's surface. 
  • Basalt is the most important example of extrusive igneous rocks, others being Gabbro and Obridian. 
  • They are generally fine-grained or glassy because of the quick rate of cooling of lava. 
  • The extrusive igneous rocks are further divided into two sub-groups: 
    (i) Explosive Type: Volcanic materials of violent volcanic eruptions including bombs (big fragments of rocks), lapilli (pea-sized fragments), and volcanic dusts and ashes. 
    (ii) Quiet Type: In this, lava appears on the surface through cracks and fissures, and its continuous flow forms extensive lava plateaus, e.g., the Deccan Plateau, Columbia Plateau (USA).

Sedimentary Rocks 

Rocks formed from material derived from pre-existing rocks and from organic sources by the process of denudation are known as sedimentary rocks. 

  • Sedimentary rocks contain different layers of sediments. 
  • Fossils are found in these rocks. 
  • About 75 per cent of the surface area of the globe is covered by sedimentary rocks, while the balance 25 per cent area is occupied by igneous and metamorphic rocks. 
  • Though sedimentary rocks cover the largest area of the earth's surface, they constitute only 5 per cent of the composition of the crust, while 95 per cent of the crust is composed of igneous and metamorphic rocks. 
  • Layers of sedimentary rocks are seldom found in original and horizontal manner. They are prone to folding and faulting due to compressional and tensional forces. 
  • Most of the sedimentary rocks are permeable and porous, but a few of them are also non-porous, such as clay. 
  • Shale is the most abundant sedimentary rock.

Classification of Sedimentary Rocks 

  • Mechanically-Formed or Clastic Rocks 
    (a) Rocks formed by water action : E.g. (i) Sandstone, (ii) Conglomerate, (iii) Clay (iv) Shale, etc. 
    (b) Rocks formed by wind action: E.g. Loess. 
    (c) Rocks formed by glacial action: E.g. Boulder Clay. 
  • Organically-Formed Sedimentary Rocks 
    (i) Limestone (ii) Dolomite (iii) Coal (iv) Peat 
  • Chemically-Formed Sedimentary Rocks 
    (i) Gypsum (ii) Salt rock 

Metamorphic Rocks 

  • Metamorphic rocks are formed by the change in the texture, mineral com-position and structure of the pre-existing rocks due to temperature & pressure. 
  • The pre-existing rocks may be igneous, sedimentary or even metamorphic rocks. 
  • The fossils of the original sedimentary rocks are destroyed by the heat and pressure. 
  • When already formed metamorphic rocks are again metamorphosed, they are known as Remetamorphosed Rocks.