Rocks • distinguish between igneous, sedimentary and metamorphic rock groups in terms of their origins and common mineral composition
Types of rocks There are 3 major types of rocks: igneous, sedimentary and metamorphic. Each rock type is formed in different ways. Rocks may change over time, through the rock cycle, into different types of rocks. Rocks are described in terms of their mineral composition and texture.
Igneous rocks Igneous rocks form from the crystallisation of magma (molten rock) formed in the Earth’s mantle or lower crust. Intrusive igneous rocks form inside the earth’s crust. Extrusive igneous rocks are formed through volcanic action. The slower the rate of cooling, the larger the mineral crystals – extremely fast cooling igneous rocks may have a glassy texture (e.g. perlite). Light coloured igneous rocks will be high in orthoclase feldspar and quartz, dark coloured igneous rocks will be high in olivine and pyroxene.
Sedimentary rocks The breaking down of larger rocks through mechanical (e.g. wind, water) and/or chemical means (e.g. salt water) produces sediments. Over time these sediments can become compacted and cemented together to form sedimentary rocks (e.g. sandstone). Clastic sedimentary rocks form from the weathered and eroded sediments of other rocks (Table 3.2.4, p. 137). Chemical sedimentary rocks are formed by chemical precipitation or biological activity (Table 3.2.5 p. 137).
Metamorphic rocks Metamorphic rocks are ones in which the minerals have changed, usually due to intense heat and pressure. Foliated metamorphic rocks contain mica minerals forming distinct layers. Non-foliated metamorphic rocks lack this layering effect. Metamorphic rocks often have larger crystals than the original rock they were formed from. See table 3.2.6, p. 136
Homework Submit by Friday 14/2/11 Read ‘Describing Rocks’, pp. 133-139. Answer questions 1, 3, 4, 5 and 8. Copy any 1 rock each from Table 3.2.4 and Table 3.2.5 and any two rocks from Table 3.2.6 Research any two igneous rocks. What are their properties and how are they formed?