Earth’s internal composition and Landforms — Social Science STD 7 — Question
Gujarat BoardEnglish MediumSTD 7Social ScienceEarth’s internal composition and Landforms4 Marks
Question
Explain the types of rocks giving examples.
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Answer
On the basis of formation process, the main types of rocks are as follows :$(i)$ Igneous rocks, $(ii)$ Sedimentary or Stratified rocks and $(iii)$ Metamorphic rocks.$(i)$ Igneous rocks : During a volcanic eruption the hot magma (lava) in the interior of the earth spreads on the crust or on the surface of the earth. The rocks which are formed when this lava is cooled down are called 'Igneous rocks'. Igneous rocks are of two types :$(1)$ Extrusive igneous rocks and $(2)$ Intrusive igneous rocks.At the time of volcanic eruption, the rocks which are formed due to rapidly cooling down of the lava thrown out towards the surface of the earth are called 'Extrusive igneous rocks'.
These rocks have finer granular structure.
Basalt is the best example of extrusive igneous rock. If the magma or lava in the interior of the earth cools down in the earth's crust $(SiAl)$ or deep inside, then the solid rocks formed at that place are called 'intrusive igneous rocks'. Due to slow cooling of the lava, it forms larger granules.
Granite is the best example of intrusive igneous rocks. It is used mainly to grind cereals, grains or condiments and spices.
$(ii)$ Sedimentary rocks : The process of erosion the surface of the earth is continuously carried out by factors like rain, river, glacier and wind. The rock material, clay, vegetation, residue of animals, etc.
separated due to erosional process are carricd away by running water and are collected at another place.
Thus, one layer is formed over another layer by the material deposited in the water.
These layers are suppresed bccause of the pressure of overlying rock layers. With the passing of time, stratified solid rocks are formed out of this. That is why these rocks are called 'Sedimentary' or 'Stratificd rocks'.
E.g., sandstone is formed from the sand granules.
Fossils are formed out of vegetation, animals and minute bacteria.
$(iii)$ Metamorphic rocks : When igneous rocks and sedimentary (stratified) rocks come under extremely high heat and intense pressure, their basic composition, structure and shape change totally, and the rocks thus formed are called 'Metamorphic rocks'. E.g., sticky clay turns into slate and limestone into marble.
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