Geological or Genetic Classification

According to the geological classification, rocks are subdivided into three large groups, viz., igneous rocks, sedimentary rocks and metamorphic rocks.

1. Igneous rocks: Cooling and hardening of molten magma result in the formation of igneous rocks. Depending on the rate of cooling the sizes of crystals are formed. Granites, syenites, diorites and gabbros have large crystals, while basalts, rhyolites and andesites have small crystals. Igneous rocks are generally very hard.
2. Sedimentary rocks: Transportation agents like wind, water and ice may move the loose weathered rock materials and deposit them in the form of layers called sediments. Such sediments when subjected to heavy pressure undergo compaction and cementation, resulting in sedimentary rocks. Limestone, dolomite, shale, sandstone, etc., are sedimentary rocks.
3. Metamorphic rocks: Sedimentary rocks and to some extent igneous rocks when subjected to changes brought about by the combination of heat, pressure and plastic flow (called metamorphism) undergo changes in the structure, texture and mineral composition, and this results in the formation of metamorphic rocks. Gneiss, schist, slate, marble, etc., are metamorphic rocks.