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In this article we are discussing the remaining types of sedimentary rocks, followed by explanations of metamorphic rocks and the rock cycle.
2) Organically formed sedimentary rocks:
a) These rocks are formed by the lithification (solidification of sediments/fragments under excessive pressure) of the remains of living organisms like corals, shellfish, algae and plankton sinks. Animals’ flesh gets decomposed, leaving behind the hard shell.
b) Rocks formed in this way are called calcareous rocks. For example: limestone, chalk, etc.
c) On the other hand, rocks that are formed of vegetative matter from swamps and forests are called carbonaceous rocks. In this type, the pressure mounted by other sediments compresses the plant sediments into compact masses of carbon. For example: peat, lignite and coal.
d) Being great sources of fuel, carbonaceous rocks have a high economic value.
3) Chemically formed sedimentary rocks:
a) Such rocks are formed due to precipitation of minerals from the water i.e. when dissolved minerals come out of water.
b) Chemically precipitated sediment is made up of solid and inorganic mineral compounds.
c) These types of rocks are also called evaporites.
d) Rock salts, a type of chemical sedimentary rocks, are derived from the beds of seas and lakes.
e) Rocks such as gypsum is formed by the evaporation of salt lakes like the Dead Sea.
f) Another example is dripstone, which is formed by calcium carbonate that has precipitated from dripping water.
Metamorphic rocks:
a) All other types of rocks that have changed their form under excessive heat and pressure are called metamorphic rocks.
b) Metamorphism results in crystallisation and reorganisation of materials that make up rocks.
c) Particularly during the Earth’s movements or extreme temperatures, the character and appearance of a rock may get metamorphosed (transformed). Rocks on the Earth’s crust are pushed down to its interior where molten magma comes in contact with them.
d) Grains or minerals in such rocks get arranged in layers. This arrangement is called foliation or lineation.
e) Sometimes the minerals get arranged in alternative thin and thick layers, appearing in light and dark shades. This structure is called banding and such rocks are called banded rocks.
f) Due to metamorphism, clay may change into slate, limestone into marble, sandstone into quartzite, granite into gneiss, shale into schist and coal into graphite.
Rock Cycle:
a) The rock cycle is a continuous process wherein old rocks get transformed into new rocks over time.
b) Igneous rocks are primary rocks whereas sedimentary and metamorphic rocks are formed out of primary rocks.
c) Igneous rocks can be changed into metamorphic rocks. Also, fragments of igneous and metamorphic rocks form sedimentary rocks.
d) Again, sedimentary rocks can break down into fragments, which may form another sedimentary rock.
e) Also, all the types of rocks may get carried down into the mantle through the process of subduction. These rocks in the Earth’s interior to form molten magma, which then becomes the source for another igneous rock.
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