Classify rocks according to the origin (or mode of formation)
Rocks can be classified into three different families based on the way they originated. These include:- Igneous rocks : These were formed by the cooling of heated liquid minerals. The word Igneous comes from theLatin word Ignus which means fire. The rocks thatare formed deep within the earth from the molten material(magma) are called intrusive igneous rocks whereas the ones that are formed on the surface are called extrusiveigneous rocks. The surface magma is called Lava.- Sedimentary rocks : the name sedimentary refers to the fact that the rocks were formed from layers of sediment.Sediment is created when rocks are broken into smaller and smaller pieces by outside forces involvingtemperature, wind, water and gravity.Sedimentary rocks can be further classified into three groups based upon their origin or source:(i) Mechanically formed sedimentary rock: These are formed from rock fragments which become cementedtogether with the aid of pressure and other chemical reactions. Examples are: Sandstone, Mudstone etc.(ii) Chemically formed sedimentary rocks : Are formed from dissolved minerals that are left when the liquids evaporate. Chemical reaction and evaporation causes these minerals to fall out of solution and settle to thebottom. Examples are: Rocksalt, Potash etc.(iii)Organically formed sedimentary rocks: Are formed either directly or indirectly from substances that were once living. When plants and animals die their soft parts decay. The hard parts such as the shells of marineorganisms or the bones of animals remain. Examples are: Limestone, Coal etc.