Basic Concept of Sedimentary Rocks

Definition and Formation :-

 Sedimentary rocks are formed by the worn-out sediments of previous rocks. The primary rocks are worn down by the different natural agents such as sun,rain,running water, wind , moving ice etc. The fragment of rocks and sediments are brought down by the glacier, river, stream, wind and wave etc.and they are subsequently deposited in layers on the sea-bed.In time,owing to pressure of their own layers, heat of the interior and the chemical reaction of lime , these layers of sedimentary undergo solidification and cementation to form hard rocks.Thus the sedimentary rocks are formed in layers of sediments and they are also know as stratified rock.

A layer of sedimentary rock rest upon another layer . The plane or the surface which separates one layer of sedimentary rock from another is know as bedding plane.

Fossil :-

Fig :- Fossil

Fig :- Fossil

  With the sediments,remains of animals and plants are also carried down and these undergo solidification to from rock.Thus organic rocks are formed ; they are also know as fossils.Thus fossils are the recognizable parts trace or imprint of a particular pre-historic animal or planet once buried in sediment and subsequently hardened like of rocks.

Example :-

sandstone, mud stone, limestone, conglomerate and shale are the examples of sedimentary rocks. These rocks and some fossils are found in the layers of the sedimentary rocks of the Himalayas.

Fig :- Sandstone

Fig :- Conglomerate

Fig- Lime stone

Fig :- Shale

Classification of sedimentary rock according to its mode of formation :  

 According to its mode of formation,sedimentary rocks can be divided into the following three types: (1) mechanically formed rocks, (2) organically formed rocks and (3) chemically formed rocks.

Mechanically formed rocks :-

These rocks are consolidated in a mechanical manner. They are coarse grained.

Example :- 

 Conglomerate, Mudstone and Breccia are of this type of sedimentary rock. Large rounded boulders, sand and fragments of rock together from conglomerate. Breccia is formed by triangular from of rock-particles. Minute particles of sedimentary constitute the Mud stone.

Fig-Breccia

Fig :- Mudstone

Organically formed rock   :-

Hard particles of once living organism may accumulate to from the organically formed rocks.

Example :- 

Limestone and Coal are the example of organically formed rocks.Calcareous Limestone are composed of shells and skeletons of animals largely consisting of calcium carbonate.Coal is carbonaceous rock.It is formed from plant accumulation beneath the surface of the earth .

Fig-Limestone and Coal

Chemically formed rocks :-

These rocks are mostly precipitated from solution of certain salts and other chemical compounds.

Example-

Rocks salt , Dolomite, Calcite, Gypsum borax etc. are the examples of chemically formed rocks.

Fig :- Dolomite

Fig :- Calcite

Characteristics and uses of sedimentary rocks :-

(1) The sedimentary rocks are formed by sediments and remains of the primary rocks. (2) They are formed mainly by the action of water and are, therefore, called aquatic rocks. (3) They are formed in states of horizontal layers and are,therefore, know as stratified rocks. (4) They contain particles of various sizes and shape. (5) They are less hard than igneous and metamorphic rocks. (6) They cover nearly 70% of the total surface of the earth.(8) They contain several kinds of fossil. (9) They are not resistant to erosion.Sedimentary rocks are very useful.Sandstone and limestone are of great economic value. Sandstone are useful as building and road materials. They are used to raw materials in the glass factory. Limestone are used in cement industry. The fossils are very useful to trace out the past history of the earth.

Basic Concept of Igneous Rock

Definition of Rocks :-

A rock may be difined as an aggregate of minerals of different kinds in varying proporation, occurring in natural condition. All meterials  that make the earth's crust are called rocks. They may be soft like clay or hard like granite . clay , sand , slate and basalt are all rocks in the widest term of the world.

Classification of rocks :-

 Rocks can be classified into three main types according to their mode of origin : (1) Igneous rocks , (2) Sedimentary rocks, (3) Metamorphic rocks.

IGNEOUS ROCK :-

The igneous rocks are formed by cooling and solidification of hot molten lava and magma poured out on the earth's surface or poured in below the solid rock of the earth's crust by volcanic activity  at the beginning when the earth born ,it was just a glowing ball of gas. Gradually, it cooled down and its molten volatile surface solidified. In this way the igneous rock formed at first on the earth's crust . Hence it is called primary rock. It has no layer and that is why it is called unstratified rock . 

Classification of igneous rock according to its mode of formation :

The igneous rock originates from the hot molten lava and magma under various circumstance. According to the mode of formation , it can be classified into the following types : (1) Extrusive rock and (2) Intrusive rock.

                                                                                                                                                         

Extrusive rocks :-

Extrusive rock is formed , when hot moltem lava pours out on the earth's surface and cools there. As its cools very rapidly , its forms very fine crystal. Thus extrusive rock is a fine-grained rock.

Example :-

Basalt contains extremely fine crystal . Basalt is a very common rock in Chotanagpur plateau. It is also found in some parts of the lava plateau of Maharastra.

Fig :- Bassalt

Fig :- Bassalt 

Intrusive rocks :-

Intrusive rock is formed when magma intrudes into the earth's crust. As it cools there slowly,it forms large crystal. Thus intrusive rock is a coarse - grained rock.

Example :- 

Granite, Gabbro, Dolerite are example of intrusive rock may be further into two more types are (1) Plutonic rock and  (2) Hypabyssal Rock

Fig :- Granite

Fig :- Gabbro

Fig :- Gabbro

Fig :- Dolerite

Plutonic rock :-

The magma which cools down very slowly in the deep underground of the earth's crust is called plutonic rock after pluto, the god of the under grounded. As it cools down very slowly , large crystals are formed in it . Thus plutonic rock is characterized by large crysalled, compactness and coarse texture.

Example :-

 Granite and Gabbro are plutonic rocks. these rocks are found in the Deccan plateau of India .

Fig :- Granite and Gabbro

Fig :- Deccan Plateau of india

Fig :- Granite and Gabbro

Hypabyssal rock :-

It is formed when intrusion of magma occurs in the cracks or along the lines of weakness in the earth's crust. As it cools down more rapidly  than the plutonic rock but slower than the extrusive rock, smaller crystals than the plutonic rock , but larger than the extrusive rock , are formed.

Examples :-

Dolerite and Prophyrie are typical examples of  hypabyssal rock. Dolerite and prophyrie are typical examples of Hypabyssal rock. Dolerite is commonly found in Jharia and Raniganj coalfields.

Characteristics and uses of igneous rocks :-

(1) Igneous rocks are formed by cooling and solidification of hot molten lava and magma. (2) They are compact , massive and crystalline. (3) They contain several kinds of minerals ,though they are not rich in mineral resources. (4) They do not contain any fossil. (5) They are very usefull as building and road materials.