Explanation

(a) Economic importance of rocks:

(i) The appearance and characteristic features of the land are influenced by the type of underlying rock

(ii) Rocks give rise to either highlands or lowlands e.g basement complex: rock give rise to highlands, while sedimentary rocks gi ie rise to lowlands

(iii) Most of the rock types are rich sources of minerals e.g petroleum is formed from sedimentary rock while gold is from igneous rock

(iv) Rocks Gan be quarried to form gravel used in house and road construction

(b) Characteristics of Igneous rock are:

(i) They exist in crystals i.e they are crystaline in structure

(ii) They do not occur in layer or strata

(iii) They do not contain fossils Mode of formation of Igneous rock: Igneous rock is formed from the cooling and solidification of molten magma ejected from below the earth crust. If the molten magma cools before it reaches the surface of the soil, it is called plutonic or intrusive igneous rock, e.g granite. But if it cools on the surface of the earth, it is called extrusive or volcanic igneous rock, e.g Basalt rock. 
 

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