(i) A rock can be defined as an aggregate of minerals that forms a more or less definite unit of the lithosphere. It is a consolidated and compact mass of more than two minerals.
On the basis of their mode of formation, rocks can be classified into the following categories:-
• Igneous Rocks (Primary Rocks)
• Sedimentary Rocks (Secondary rocks)
• Metamorphic Rocks.
(ii) The word ‘igneous’ means ‘ire’ derived from the Latin word ‘ignis’ or Sanskrit word ‘agni’.
Igneous rocks are of thermal origin and are formed by the process of solidification of molten rock material called magma. Such molten magma originates from the interior of the Earth. Such magma erupts during volcanic eruptions and on reaching the Earth’s surface, gets solidified by cooling. So they are referred to as primary rocks.
Characteristics of Igneous rocks:-
• Formed by cooling and solidification of Lava
• Hard in nature
• Crystalline and have texture
• Non-porous rock.
(iii) Sedimentary is derived from a Latin word which means ‘settling down’- ‘sedimentum’. Sedimentary rocks are called secondary rocks or derived rocks because they are derived by the denudation of other pre-existing primary or parent rocks.
Sedimentary rocks are formed by denudation, weathering, erosion, transportation and deposition. These consolidated layered strata are known as stratified rocks. Lithification is the cementation, compaction, and hardening of sediments into sedimentary rocks. As the sediments are deposited in water bodies, they get sorted out according to their size. These sediments accumulate in different layers or strata arranged one above the other and thus form sedimentary rocks.
Characteristics of Sedimentary rocks:
• They accumulate in different strata one above the other and thus called stratified rocks.
• Presence of plants and animals between layers of sediments i.e. fossils.
• Formed by agents of denudation.
• Derived from pre-existing primary or parenting rock.
(iv) The changing of the forms of rocks from one to another is known as rock cycle. The changes of these rocks are interrelated. Igneous rocks are primary rocks and it can be changed to form sedimentary or metamorphic rocks. Normally sedimentary rocks form from igneous rocks. These sedimentary rocks themselves can turn into fragments and form another type of sedimentary rocks. The crustal rocks can once again carry down to mantle of the earth by subjugation process and again melt in lava. This lava while erupting will again form igneous rocks and so on.
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