On the basis of mode of formation, rocks are of three main classes.
1)Igneous rock: When the original molten material solidified Igneous rocks formed. These are of two types – Plutonic rocks and Volcanic rocks. Examples are Granite, Basalt, diorite, gabbro, syenite, Peridotite, Granodiorite.
2)Sedimentary rock: Such rocks are derived from igneous rocks. These rocks are formed by the consolidation of fragmentary rock materials.4/5th of visible rocks on the surface of the earth are sedimentary rocks. Examples are Limestone, Dolomite, Sandstone, Shale, Conglomerate.
3)Metamorphic rocks: Such rocks are formed from the Igneous or Sedimentary rocks by the action of intense heat and high pressure. Examples are Gneiss, marble, schists, slate, calcite, and quartzite.