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October 12, 2017 at 5:32 pm #9275AnonymousGuest
The Geology Group met at 10.30 am on Wednesday 11 October 2017 at merlin's Bridge Community Hall. The topic was METAMORPHIC ROCKS.
Metamorphism is the process by which existing rocks are changed in form due to the influence of heat and/or pressure. There are essentially two main types of metamorphism as follows:
1. Contact or Thermal Metamorphism
This takes place around an igneous intrusion where the heat from the molten magma is dissipated into the surrounding rocks. The zone of baking surrounding the intrusion is known as the metamorphic aureole. The intensity of metamorphism declines away from the intrusion as the temperature drops. New minerals are formed by chemical changes within the aureole and these can be used to identify concentric metamorphic zones. For example, a characteristic index mineral is garnet which may be formed when volcanic rocks are thermally metamorphosed.
Mudstones are often changed to a tough flinty rock called hornfels (‘hornstone’). The Borrowdale volcanics alongside the Shap granite have been baked to produce hornfels. Limestone is recrystallised to produce marble which may be white like Carrara marble or green as in Connemara marble; impurities such as serpentine or iron minerals produce the colouring.
2. Regional Metamorphism
This is a large scale process involving continental collision (subduction) resulting in partial melting and the deformation of pre existing rocks. The following common metamorphic rocks are formed as the grade of metamorphism increases ie/ the temperature and pressure increases.
i). Slate is produced when fine grained mudstone is subjected to intense pressure and the platy minerals such as mica are orientated at right angles to the direction of pressure. This creates cleavage planes along which the slate splits. These cleavage planes are often at an angle to the original bedding planes of the mudstone.
ii).Quartzite results from the recrystallisation of sandstone when it is subjected to heat and pressure.
iii) Schist is formed when coarse grained sediments are subjected to higher temperatures and pressures. Mica is aligned along undulating foliation planes. Garnets often grow along foliation planes in schists during metamorphism.
iv) Gneiss is a high grade metamorphic rock that is often derived from the partial melting of igneous rocks such as granite. This coarse grained rock is characterised by the segregation of its felsic and mafic minerals into bands.
v) Migmatite. Under extreme conditions of regional metamorphism, deep within an orogenic zone, a gneiss will partially melt and recrystallize to form a coarse grained igneous rock called migmatite. It usually consists of alternate light bands of quartz and orthoclase feldspar. and darker bands rich in amphibole and biotite. These bands often appear in tight ptygmatic folds.
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