| Metamorphic, Igneous and Sedimentary rocks Metamorphic rocks are sedimentary or igneous rocks that have been altered by heat and/or pressure. As they are derived from previously existing igneous, sedimentary or even metamorphic rock, their appearance is variable. They are identified by the types of minerals they contain and their texture. Thermally metamorphosed rocks occur bordering igneous intrusions, which altered the surrounding rock originally because of their intense hear, resulting also in the formation of new minerals such as andalusite and garnet. Regionally metamorphosed rocks occur in the roots of mountain ranges, where intense pressures and high temperatures formed platy minerals (e.g. mica) and high-pressure minerals (e.g. staurolite). Igneous rocks are non-stratified intrusions or extrusions. they can be extremely coarse-grained (granite), fine (andesite) or glassy (obsidian). They are composed of minerals that have crystallised from molten rock. Sedimentary rocks are generally stratified, fine-grained or composed of fragments of older rocks from which these were derived, such as pebbles, sand, angular fragments of older rocks, broken shells, rounded mineral grains and alteration minerals such as clays. Limestones are easily identified because they effervesce in dilute hydrochloric acid. Many sedimentary rocks also contain fossils. |
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