(a) Enumerate four economic importance of rocks: (i) to man; (ii) in agriculture. (b) State two characteristics of each of the following (1) igneous rock (i...

Answer Details

(a) Economic importance of rocks:

(i) To man:

• They are quarried for building and construction materials (stones, gravel, sand, cement).
• They are sources of valuable minerals and metals (gold, iron, tin, coal).
• They are used to make roads, dams and other engineering works.
• Some rocks yield mineral fuels and salts used in industry.

(ii) In agriculture:

• They weather to form the soil in which crops grow.
• They release and supply plant nutrients (such as phosphorus, potassium and calcium) to the soil.
• Rock materials such as limestone provide liming materials to reduce soil acidity.
• Rock phosphate and other rocks serve as raw materials for making fertilizers.

(b) Two characteristics of each rock type:

• (i) Igneous rock: It is formed from cooled and solidified molten magma; it is usually hard, crystalline and contains no fossils. Examples are granite and basalt.
• (ii) Metamorphic rock: It is formed when existing rocks are changed by great heat and pressure; it is hard, often has a banded or layered (foliated) appearance and is more compact than the parent rock. Examples are marble and slate.

(c) Mode of formation of sedimentary rocks: Sedimentary rocks are formed from the products of weathering and erosion of existing rocks. Rock fragments, sand, mud and the remains of plants and animals are carried by wind, water and ice and deposited in layers in low areas such as river beds, lakes and the sea. Over a long period these deposits (sediments) accumulate, the upper layers press down on the lower ones (compaction), and dissolved minerals cement the particles together (cementation). The compacted and cemented layers gradually harden into sedimentary rock. They are typically layered (stratified) and often contain fossils. Examples are sandstone, limestone and shale.