Introduction of Rocks
“Rocks” are defined as the aggregates of one (or) more minerals. Eg: Limestone – aggregation of calcite mineral.
Rocks have no definite chemical, morphological composition or definite symmetrical form
Based on origin, rocks are classified as(Introduction of Rocks)
1. Igneous rocks
2. Sedimentory rocks
3. Metamorphic rocks.
I. Igneous rocks:(Introduction of 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(Introduction of 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:
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(Introduction of 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.
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