Theory question 203
Rocks(a) Give an account of the economic importance of rocks
(b) Describe the characteristics and mode of formation of igneous rocks
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(a) Source of minerals (gold, tin in igneous rocks; coal, petroleum in sedimentary rocks); building and road materials (granite, gravel, sandstone, marble); limestone for cement; clay for bricks and pottery; weather into soils for farming; store underground water; landscapes for tourism
(b) Characteristics: crystalline; hard and resistant; no layers (strata); no fossils; mostly impermeable; acid (granite) or basic (basalt)
Formation: magma from inside the earth rises and cools; intrusive/plutonic rocks cool slowly underground with large crystals (granite); extrusive/volcanic rocks cool quickly at the surface with small crystals (basalt)
Link crystal size to the speed of cooling.