Theory question 203
Rocks(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
Show the model answer
(a)(i) Granite: igneous, coarse-grained with large crystals (quartz, feldspar, mica); hard and resistant; light coloured; no layers or fossils; jointed
(ii) Limestone: sedimentary; mainly calcium carbonate; in layers (bedded); may contain fossils; jointed and permeable; dissolves in rainwater (carbonation)
(iii) Graphite: metamorphic; soft, greasy, black/grey; good conductor; made of carbon; often foliated
(b) Granite: magma cools slowly deep underground (intrusive) giving large crystals
Limestone: shells and skeletons of sea creatures (corals) accumulate on the sea floor, or calcium carbonate precipitates, and is compacted and cemented
Graphite: carbon-rich rocks (coal/carbonaceous sediments) changed by great heat and pressure (metamorphism)
Say which rock group (igneous, sedimentary, metamorphic) each belongs to.