Why would i expect a metamorphic rock to form at a temperature lower than the melting point of rock?


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Ebony Nash answered
'Metamorphism' is a Greek term which basically means 'change form', and so is a process involving crystalline structures during which change form in some way. In Geology this refers more specifically to changes in mineral assemblage and texture which occur as a result to exposure to varied pressures or temperature intensities, different to those that the rock itself was exposed to when it formed.

• The melting point of rock

It is hard to classify, given that there are vast amounts of rock on Earth that we know of and each have their own melting points. However, all rocks become molten at 1200 Celsius. Just as there are different rocks there are also different types of magma which typically become a solid rock structure when cooled to temperatures of 600 Celsius and lower.

• Low-grade and high-grade metamorphism

Low-grade metamorphism takes place at temperatures between 200 and 320 Celsius with usually low pressures and high-grade takes place at anything higher than 320 Celsius with higher pressures. Low-grade candidates are categorized by a great number of hydrous minerals being present in the rock and high-grade candidates are pretty much opposite given that hydrous minerals become less hydrous at higher temperatures and pressures.

Metamorphism is controlled by a number of factors. The process only occurs because some minerals only stay stable at certain pressures and temperatures. When such factors change, a series of chemical reactions begin and so a different assemblage of minerals form to help protect the rock in a way that it can remain stable at such changed temperatures and pressures. A 'fluid phase' will often change the rate of metamorphism greatly in a rock. The phase refers to their being waters and other fluids present within the rock at a time when metamorphisms occur, and these fluids provide dissolved ions which can increase the rate at which chemical reactions happen.

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