(a) Give an account of the economic importance or rocks (b) Describe the characteristics and mode of formation of igneous rocks

Answer Details

(a) Economic importance of rocks

• Source of building materials: Granite, limestone, sandstone and gravel are quarried for construction of houses, roads and bridges; limestone is the raw material for cement.
• Source of minerals and ores: Rocks contain valuable metallic and non-metallic minerals (iron ore, tin, gold, copper, coal, petroleum) that support mining industries and exports.
• Formation of soils for agriculture: The weathering of rocks produces the soils on which crops are grown, and some rocks (e.g. volcanic) give very fertile soils.
• Source of energy and industrial raw materials: Coal, petroleum and natural gas found in sedimentary rocks provide fuel and power; other rocks supply raw materials for glass, ceramics and chemical industries.
• Underground water and tourism/employment: Porous rocks store underground water (aquifers) tapped by wells; striking rock scenery attracts tourists, and quarrying and mining create employment and revenue.

(b) Characteristics and mode of formation of igneous rocks

Mode of formation: Igneous rocks are formed from the cooling and solidification of molten material (magma below the surface or lava on the surface).

• Intrusive (plutonic) rocks form when magma cools slowly deep underground, allowing large crystals to grow (e.g. granite).
• Extrusive (volcanic) rocks form when lava cools quickly at the surface, giving small or fine crystals (e.g. basalt).

Characteristics:

• They are crystalline, made of interlocking crystals; slow cooling gives large crystals, fast cooling gives fine ones.
• They are usually hard and non-porous, and generally lack stratification (no layers).
• They contain no fossils, because they solidified from molten material.
• They occur in masses such as batholiths, dykes and sills, and many are resistant to weathering.