(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: a hard, crystalline igneous rock, coarse-grained with large interlocking crystals of quartz, feldspar and mica; light in colour, resistant to weathering, non-porous and non-bedded (no layers or fossils).
• Formation: formed by the slow cooling and solidification of molten magma deep below the earth's surface (a plutonic/intrusive igneous rock). Slow cooling allows the large crystals to grow.

(ii) Limestone

• Characteristics: a sedimentary rock made largely of calcium carbonate; usually well-bedded (layered), often contains fossils, is porous and jointed, and reacts with (is dissolved by) dilute acid and rain water.
• Formation: formed by the accumulation and compaction of the shells and skeletons of marine organisms (or by chemical precipitation of calcium carbonate) on the sea floor, later cemented and hardened into rock.

(iii) Graphite

• Characteristics: a soft, dark-grey to black, greasy/smooth metamorphic rock (form of carbon); it is a good conductor of heat and electricity and marks paper (used in pencils).
• Formation: formed by the metamorphism (alteration by great heat and pressure) of carbon-rich rocks such as coal or organic sediments deep within the earth's crust.