Metamorphism

When does metamorphism occur?

• above diagenesis

• at 200 - 800 degrees Celsius

• At depths of <2km - 45km

What are the effects of metamorphism?

• fossils and sedimentary structures are destroyed or distorted.

• Rock hardens - more resistant because it is more crystalline.

• Growth of new metamorphic minerals

• Alignment of minerals in th erock

What does the term metamorphic grade refer to?

The extent to which the pre-existing rocks have been changed/altered in form.

What are the different types of metamorphic grade?

• Low grade - slight altercation

• Medium grade - significant altercation

• High grade - extensive/total altercation

What are the chemical changes that happen during metamorphism?

• rock is assumed to be isochemical, meaning that the chemical composition of the parent rock and meta. rock are the same.

• Only loss from the system is water as hydrous clay minerals are dehydrated by a rise in temp.

What are the factors which would increase the susceptibility to metamorphism?

• Argillaceous rocks undergo the greatest mineralogical change

• Rocks that are rich in clay minerals which are only stable at low temp. + pressure

• Clay minerals - they are chemically complex and a range of mineral types can form by recrystallisation.

What are the types of metamorphism?

• Contact - heat only

• Regional - changes due to heat and pressure

What are the differences between the two types of metamorphism?

contact - smaller crystal sizes, minerals are non-foliated (random orientation)

Regional - rocks show foliation as they crystallised under directed tectonic stress.