## THE MAJOR ROCKS OF THE EARTH

The earth crust consists of rocks.A rock is any mineral material of the earth. it may be a combination of different minerals elements such as silica which contains silicon and oxygen.
All the rocks differ from one another in texture, structure, colour,permeability,mode of occurence and degree of resistance to denudation.

TYPES OF ROCKS
All rocks are classified into three major types-igneous rocks,sedimentary rocks and metamorphic rock based on their origin (mode of formation) and appearance.

IGNEOUS ROCKS:
CHARACTERISTICS
(I)-Igneous rocks are crystalline in structure i.e they contain crystals.
(ii)-they do not occur in layers i.e they are non-stratified rocks.
(iii)-they do not contain fossils.
(iv)-they are usually hard and imprevious.
(v)-they are resistant to erosion.

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MODE OF FORMATION
Igneous rocks are formed by the cooling and solidification of molten rocks called MAGMA,ejected from beneath the earth crust.The magma which results from high temperature and pressure beneath the earth, forces itself towards the earth surface through cracks.As the magma moves towards the surface, it comes in contact with lower temperature, hence it cools and solidifies to form igneous rocks.

THERE ARE TYPES OF IGNEOUS ROCKS
(A) PLUTONIC (INTRUSIVE)IGNEOUS ROCKS:
These are rocks formed when the molten magma cools and solidifies slowly before it gets to the surface of the earth to form large crystals. As a results of prolonged erosion,the PLUTONIC igneous rocks will later be exposed to the surface,examples of PLUTONIC igneous rocks are granite,grabbro,diorite.

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(B)VOLCANIC (EXTRUSIVE)IGNEOUS ROCKS:
these are rocks formed when the molten magma cools and solidifies rapidly on getting to the surface of the earth to form small crystals.examples of volcanic rocks are basalt,obisidian,rhyolite.

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SEDIMENTARY ROCKS
CHARACTERISTICS:
(I) - They occur in layers
(ii)-the rocks may be coarse,fine, soft or hard
(iii)-they are not crystalline in nature
(iv)-they are not resistant to erosion

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MODE OF FORMATION.
sedimentary rocks are rocks formed from sediments deposits either by water, wind or ice.these sediments are deposited in layers one on top of the other and after a long period of tine,they will become hardened by compression to form sedimentary rocks.sedimentsry rocks are stratified rocks and the plane between two strata is called BEDDING PLANE.

There are three types of sedimentary rocks based on their process of formation.these are
(a) MECHANICALLY GORGED SEDIMENTARY ROCKS :These are formed from the sediments of other rocks that have accumulated and cemented together over a long period of time.examples includes sandstone,breccia,shale,clay,and conglomerate.

(b) ORIGINALLY FORMED SEDIMENTARY ROCKS:
These are rocks formed from the remains if living organisms. when sedimentary rocks are formed from the remains of animals like corals or shellfish,such rocks are called CALCAREOUS ROCKS,eg limestone and chalk. but when sedimentary rocks are formed from the remains of vegetables matter such as swamps and forests they are called CARBONACEOUS ROCKS,eg coal,peat,lignite,petroleum

(c) CHEMICALLY FORMED SEDIMENTARY ROCKS:
these rocks are precipitated chemically from rocks solution. example includes potash,soduim chloride(common salt), nitrates, gypsum,and dolomite.

IMPORTANCE OF SEDIMENTARY ROCKS TO MAN
(i) sedimentary rocks are source of materials for construction.
(ii) they are source of minerals eg petroleum, coal,gold.
(iii) they are used for medicine.
(iv) they are source of food.
(v) they are used for scientific purpose.

METAMORPHIC ROCKS
CHARACTERISTICS :
(i) some may occur in layers
(ii) the rocks my be hard or soft.
(iii) they are not crystalline in structure.
(iv) they contain fossils.

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MODE OF FORMATION
Metamorphic rocks are changed rocks, the rocks are formed from pre-existing igneous rocks or sedimentary rocks which have been changed as a results of great heat and pressure. the original character and appearance may be greatly altered by such forces, examples of metamorphic rocks include slate,mable,quartize,gneiss, graphite.
slate is formed from clay, marble is formed from limestone,quartzite is formed from sandstone and graphite is formed from coal.

ECONOMIC IMPORTANCE OF ROCKS TO MAN
(1) SOURCE OF MINERAL:
some rocks are sources of minerals such as gold,diamond,limestone,petroluem which can be exported to provide foreign exchange to a country.

(2) SOURCE OF FUEL:
sedimentary rocks like petroluem and coal are sources of fuel for domestic and industrial use.

(3) TOURIST CENTRES:
huge rocks masses on mountains serve as tourist centres eg Olmos rock in absolute and Zuma rocks in suleja.

(4) RAW MATERIALS FOR INDUSTRIES:
some sedimentary rocks are used as raw material in industries eg limestone. is used for making cement,dolomite and marble are used for manufacturing of glasses and house paints.

(5). SOURCES OF FOOD NUTRIENT:
Rocks salts such as soduim chloride (table salt) from sedimentary rocks provide minerals used in cooking.

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