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Basic Geology. Rock groups. 1. Igneous rocks: Are formed directly from magma (formed in the crust and upper mantle) and when exuded at the surface is called lava.
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Rock groups 1.Igneous rocks: Are formed directly from magma (formed in the crust and upper mantle) and when exuded at the surface is called lava. 2. Sedimentary rocks: formed from the deposits of other rocks, biological action e.g. limestone, coal etc, evaporation of dissolved mineral solutions e.g. rock salt.
Rocks cont.d 3. Metamorphic rock: Formed when heat or pressure changes the original form whatever i.e. igneous, sedimentary or even previously metamorphic. (meta = change, morph = form. Minerals: A naturally found chemical compound that rocks are composed of. E.g. quartz (sand) SiO or silicon oxide.
Igneous rock types Depends on the rate of cooling and the origin of the lava or magma. Slow cooling e.g. deep below ground results in the growing of large mineral crystals and therefore a coarse grained rock such as the granites results. Rapid cooling e.g. in air or sea results in fine grains, the finest being the glassy rocks that are ejected from volcanoes.
The composition of igneous rocks The rocks that are exuded from constructive plate margins are probably the nearest to the original mantle composition and are known as Mafic being rich in iron and magnesium and dark in colour. Called Basic in the past. The other type, Felsic after feldspars and silica are usually lighter both in colour and weight and used to be called Acidic rocks but this was a misnomer.
Types of Mafic rocks As mentioned the rate of cooling affects the texture of the subsequent rock e.g. Slow cooled: The gabbro from Skye,coarse grained. An intrusive rock. Faster rate: The dolerites, medium grained. Also intrusive. Rapid cooling: The basalts, fine grained to glass-like. e.g. Olivines, found on Iceland, itself a product of a constructive plate margin.
Types of Felsic rocks Slow cooled: The granites, coarse grained sometimes very large crystals (phenocrysts) Medium rate: Microgranite a medium grained rock. Rapid cooling: The rhyolites e.g. the Lake District rocks formed from volcanic outporings.
Sedimentary rocks (the main types) • Clastic (greek. broken) sedimentary rocks: Formed from eroded other rocks and transported, then undergoing lithifaction or making into rock usually by some cementing agent and pressure. Usually in estuaries and under the sea. 2. Organic sedi. rocks: e.g. limestone, formed from sea creatures that extract Ca from the sea water to make their skeletons or hard parts. e.g. corals or creatures with shells.
Sedimentary cont.d 3. Chemical sedi. rocks: Rocks that are evaporated out of solution such as salt beds found in Cheshire. Formed by land locked lakes or lagoons drying out leaving layers of salt. Bedding: Nearly all forms of sedimentary rocks have some form of bedding or layering that indicate seasonal deposition or slight changes in the environmental regime. (This may not always be visible to the naked eye.)
Types of clastic sedimentary rocks. 1. Coarse grained: Contain fragments over 2mm in dia. e.g. conglomerates, breccias and some gritstones. 2. Medium grained: Contains fragments between 2mm and 0.05mm e.g. sansstones. 3. Fine grained: Particles less than 0.05mm e.g. siltstones, mudstones, shales and clays. Examples of cementing agents are CaCO3, Fe(OH)3 and dissolved SiO2.
Metamorphic rocks Rocks that have been changed from their original form by heat or pressure or both. These can have been sedimentary of any type e.g. clastic, organic or igneous rocks. Even some metamorphic rocks that come in for a second dose of treatment. Two main types: Contact metamorphism and regional metamorphism.
Cont.d Contact or thermal metamorphism is associated with igneous activity e.g. the intrusion of dykes, sills and batholiths into the country rock. The surrounding rock that is affected is called the metamorphic aureole. Within this area various grades of metamorphism are found. Examples include limestones changed to marble or, at the lowest grade, spotted rocks are found where the minerals with the lowest melting points are allowed to concentrate.
Regional metamorphism Low grade regional metamorphism is characterised by high pressure but relatively low temperatures. E.g. mudstone converted to slates, minerals develop a preferred orientation at right angles to the direction of pressure that can form a cleavage plane making them easy to split. Medium grade metamorphism occurs during mountain building and results in the formation of schists from the above deposits.
Cont.d High grade metamorphism results again from mountain building but at greater depths producing gneiss. Here total re-crystalisation takes place at close to the rocks melting point and can result in, depending on the cooling rate, the recreation of granites.
