Metamorphism is the modification of pre-existing rocks in the solid state early out to transforms in temperature and pressure. Under increasing temperature and / or pressure existing minerals become unstable and break down to type new minerals. In the instance of regional metamorphism the rocks space subjected come tectonic forces which carry out the vital mechanisms because that metamorphism. Assets include schist and also slate. Contact metamorphism involves metamorphosis through heating by one intruding plutonic body. Hornfels is the result of this kind of metamorphism.

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Metamorphic rocks are classified according to the conditions under which castle recrystallised, recognized as their metamorphic facies. The crucial parameters in this category are index minerals i m sorry indicate specific pressure / temperature conditions. These minerals may only be present in small proportions, and in many instances are daunting to recognise in hand specimen. An different classification, beneficial in the ar or for hand specimens, is based upon the level of recrystallisation the the original minerals, and also so serial size and also the degree of foliation (see below) room important. As metamorphism wake up in areas undergoing deformation, look likewise for structures that show deformation, such as folding (often presented as crenulations or tiny crumpled folds), and little fractures or faults.

Metamorphic textures

The 2 distinctive metamorphic structure are:


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Foliation plane in gneiss

Foliation -This represents a distinct airplane of weakness in the rock. Foliation is led to by the re-alignment that minerals once they space subjected come high pressure and temperature. Individual minerals align themselves perpendicular to the stress field such the their long axes are in the direction of these planes (which might look like the cleavage airplane of minerals). Usually, a series of foliation planes deserve to be seen parallel to each various other in the rock. Well developed foliation is characteristic of many metamorphic rocks. Metamorphic rocks often break easily along foliation planes.

Granular - This defines a metamorphic rock consisting of interlocking equant crystals (granules), practically entirely of one mineral. A granular texture is arisen if a rock"s chemistry composition is close to that of a details mineral. This mineral will crystallise if the absent is subjected to high pressure and temperature. A granular structure is characteristics of part metamorphic rocks.

Note: together the grade of metamorphism boosts (more temperature and pressure), both crystal size and the coarseness the foliation increase. Therefore, gneiss represents much more intense metamorphism (or a higher grade) than does schist.

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Some fine-grained metamorphic rocks, e.g. Schist, have actually larger crystals present. This crystals are called porphyroblasts. Porphyroblasts represent minerals that crystallise at a much faster rate than the matrix minerals. Garnet is a typical porphyroblast mineral.


Grain size
FineMediumCoarse
Poorly foliatedHornfelsMarble, quartziteMarble, quartzite
Well foliatedSlateSchistGneiss
Well foliated and shearedMyloniteMylonite, schistAugen gneiss