Depending upon the mode of formation, sedimentary rocks are classified into three major groups:
- mechanically formed — sandstone, conglomerate, limestone, shale, loess etc.
- organically formed — geyserite, chalk, limestone, coal etc.
- chemically formed — chert, limestone, halite, potash etc.
Enrich Your Learning:
There are many different kinds of rocks which are grouped under three families on the basis of their mode of formation. They are:
(i) Igneous Rocks– solidified from magma and lava
(ii) Sedimentary Rocks– the result of deposition of fragments of rocks by exogenous processes
(iii) Metamorphic Rocks– formed out of existing rocks undergoing recrystallization
- As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks.
- The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma in its upward movement cools and turns into solid form it is called igneous rock.
- The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.
- Igneous rocks are classified based on texture. Texture depends upon size and arrangement of grains or other physical conditions of the materials.
- If molten material is cooled slowly at great depths, mineral grains may be very large.
- Sudden cooling (at the surface) results in small and smooth grains. Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks.
- Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.
- The word ‘sedimentary’ is derived from the Latin word sedimentum, which means settling.
- Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments. Such fragments are transported by different exogenous agencies and deposited.
- These deposits through compaction turn into rocks. This process is called lithification.
- In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification.
- Hence, a number of layers of varying thickness can be seen in sedimentary rocks like sandstone, shale