2 groups of Rocks classified according to their mode of origin and chemical composition

2 groups of Rocks classified according to their mode of origin and chemi­cal composition

Rocks can be classified according to their mode of origin and chemi­cal composition. The three such groups are: (1) Igneous rock (2) Sedi­mentary rock

Igneous rock (Latin, ignis fire):

Igneous rocks are the primary rocks, the ancestors of all the rocks and make up about 80 per cent of the earth’s crust. Due to sudden release in the pressure, the heavier and denser materials of the earth’s interior turn to a molten form or magma. - Geology Stage 1.5: Sedimentary Rocks

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When magma gushes out onto the earth’s surface as lava, it cools rapidly and forms fine grained dark coloured igneous rocks also known as volcanic rocks such as basalt rocks.

Magma may also cool slowly, deep under the ground and form large crystals. Examples of such rocks are granite which is also known as Plutonic rocks (Pluto is God of the underworld).

Igneous rocks are unlayered or unstratified, hard and mainly crystal­line in structure. They do not contain fossils due to their fiery origin.


Igneous rocks are divided into four classes depending on chemical composition.

(1) Acidic rock (65 per cent Si02)

(2) Intermediate rock (55 per cent-65 per cent SiO)

(3) Basic rock (45-55 per cent Si02)


(4) Ultrabasic (45 per cent Si02)

Sedimentary rock (Latin, sedimentium settling down):

The pri­mary igneous rocks undergo weathering since their formation. The weathered rocks are eroded and this eroded material is carried away to different places by fast winds and rain and ‘sediments’ are accumu­lated in layers over long periods usually under water.

As time progresses these sediments undergo physical and chemical changes and gradually become compressed and harden to form rocks called sedimentary rocks. On the basis of formation they are classified as (1) Mechanically formed (2) Organically formed (3) Chemically formed.

(i) Institutional wastes:

Schools, colleges, universities, research and development organizations, religious places, and community halls generate wastes like paper, rejected office stationary, polythene, rejected pen, pencils, etc.

(ii) Industrial solid wastes:

The MSW includes the wastes of domestic and commercial nature generated from the office, staff quarters, canteen of the small and cottage industries operating in the municipalities and industrial estates.

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