SEDIMENTARY ROCKS and METAMORPHIC ROCKS

SEDIMENTARY ROCKS Compaction and cementation of layers of sediment leads to formation of sedimentary rocks. A characteristic feature of these rocks is their layered
arrangement, the layers being collectively called strata. .
Layers of different textural composition are alternated or inter-layered. The planes of separation between these layers are called bedding planes. All sedimentary rocks are non­crystalline and contain fossils. Sedimentary rocks fall into three main groups.
(i) Mechanically formed These are called clastic sedi­mentary rocks; the sediments are largely derived from pre­existing rocks that have been broken down and then transported by water (e.g. loess), wind (e.g. clay) or ice (e.g. moraines, gravel) to form rocks. They are further divided into three groups on the basis of the size of the constituent particles: (a) ludaceous or pebbly, e.g., conglomerate, (b) arenaceous or sandy, e.g., sandstone, (c). argillaceous or clayey, e.g., shale.
(ii) Organically formed These rocks are derived from remains of plants (e.g., peat, lignite, coal), or animals (e.g., chalk and coral).
(iii) Chemically formed Some examples are evaporitics, which are formed from sediments precipitated from a saturated solution and dried out by evaporation, such as rock salt, borax, gypsum, nitrates, potash and certain limestones. Shale, sandstone and limestone are the most abundant of sedimentary rocks.
Sedimentary rocks contain certain essential resources for man's industrial society: (i) they provide materials to build structures and highways; (ii) they supply compounds for induStrial processes that yield chemicals such as fertilisers and acids; (iii) they are the source of hydrocarbon com­pounds, e.g., coal, petroleum and natural gas that turn the industrial wheel and provide the fuel for heat and trans­portation.

METAMORPHIC ROCKS These rocks are formed when pre-existing sedimentary or igneous rock is altered as a result of changes in physical or chemical conditions. This process of metamorphism may be through intense pressure or stress caused by earth movements, increased tempera­ture caused by volcanic activity, or the action of gases and liquids of magmatic origin. There are three main types of metamorphism.
(i) Contact (thermal) metamorphism results from the intrusion of a mass of molten rock.
(ii) Dynamic (regional) metamorphism when rock 'layers undergo strong structural during mountain building.
(iii) Dislocation metamorphism occurs when pre­existing rocks undergo localised deformation along a fault plane or thrust plane. The mineral composition and the structure of the rock can be altered; for example, under stress some minerals like mica take on a parallel arrange­ment.

Each igneous and sedimentary rock has a metamorphic equivalent. Metamorphic rocks, generally speaking, are harder and more compact than their original types, except when derived from igneous rocks. Examples of metamor­phic rocks are marble (from limestone); slate (from shale); quartzite (from sand); graphite (from coal); gneiss (from granite).