TYPES OF ROCKS

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I. Igneous rocks:

These are the most abundant and makeup 95% of all the earth crust. Igneous rocks are “oldest rocks” and are also known as “crystalline (or) massive (or) fire rocks. These rocks mainly consist of primary minerals and more than half of the igneous rock is containing “feldspar” minerals. Igneous rocks based on their mode of formation are divided into two groups.

a. Extrusive (or) volcanic rocks: These are formed at the surface from the volcanic
magma. Eg: Basalt, diorite
b. Intrusive (or) plutonic rocks: These are formed by the cooling of the original magma
and occur below the earth crust. Eg: Granite.

Igneous rocks are also classified based on their chemical composition.

a. Acid rocks – contain more than 65% silica – Granite.
b. Neutral rocks – contain 50% – 65% silica – Diorite.
c. Basic rocks – contain less than 50% silica – Basalt.

II. Sedimentary rocks:

These rocks are formed through the transportation and deposition of weathered sedimens. The sedimetary rocks are mostly formed through the agency of water, which are called clastic, aqueous or stratified rocks. Based on origin, sedimentary. rocks are classfied into 4 groups.

1. Residual sediments: These are formed from the products of weathering in situe that is at
same place. Eg: Laterite, bauxite.

2. Mechanical sediments: These are formed due to the deposition of pebbles, sand and silt. Eg: Standstone, shale.

3. Chemical sediments: These are formed due to evaporation of water, precipitation and consequent accumulation of sediments. Eg: Limestone.

4. Organic sediments: These are fomred due to partial decompostion of organic remains under anaerobic conditions.
Eg: Peat.

III. Metamorphic rocks:

These are formed from the rocks by the action of heat and pressure on pre-existing igneous and / or sedimentary rocks. Eg: Sandstone – Quartzite, Shale – Slate, Limestone – Marble, Granite – Gnesiss, Basalt – Schist, Coal – Graphite.

Minerals:

A mineral is a naturally occuring, homogenous element or inorganic compound that has a definite chemical composition and a characteristic geometric form. The minerals can be identified by many of their physical properties like colour, lustre, streak, hardness etc.

  • · Lustre – general appearance of mineral in reflected light.
  • · Fracture – property of the mineral to break along an irregular surface i.e., surface produced when the mineral breaks in direction other than cleavage plane.
  • · Streak – The colour of the powder of the mineral that is obtained by rubbing the mineral against the unglazed porcelain plate.

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