(a) Describe the characteristics of each of the following rock types (i) Granite (ii) Limestone (iii) Graphite (b) Explain how each rock type is formed

Answer Details

(a) Characteristics and (b) formation of each rock type

(i) Granite

Characteristics:

• It has large, visible interlocking crystals.
• It is hard, coarse-grained and usually light-coloured.
• It does not occur in layers or strata and does not contain fossils.

Formation: Granite is formed by the slow cooling and solidification of molten magma deep below the earth’s surface. It is therefore a plutonic or intrusive igneous rock.

(ii) Limestone

Characteristics:

• It occurs in layers or strata.
• It often contains fossils of plants and animals, such as shells and corals.
• It is mainly composed of calcium carbonate and reacts with dilute acid.

Formation: Limestone is formed from the remains of marine organisms such as corals, shellfish and other animals. Their shells and skeletons accumulate on the sea bed, then become compacted and cemented together to form a sedimentary rock.

(iii) Graphite

Characteristics:

• It is dark grey to black in colour and is soft and slippery to the touch.
• It is a crystalline form of carbon and can leave marks on paper.
• It conducts heat and electricity; it does not contain fossils.

Formation: Graphite is formed when carbon-rich materials, such as coal or organic sediments, are changed by intense heat and pressure deep within the earth’s crust. It is a metamorphic rock.