Notes and diagrams explaining the processes of metamorphism - both regional and contact - as well as the rocks produced by each. Includes how to describe the metamorphic rocks and create thin section drawings.
• Original composition of rock
• Amount of change in pressure and temperature
• Length of time
Metamorphism: The Process
Must know that:
• Increases in pressure and temperature are factors responsible for changes
• Metamorphic reactions taking place in the solid state (the rock never melts)
• Metamorphism is an isochemical process – the chemistry of the rock doesn’t change, and no new
elements are added
With increasing pressure and/or temperature a rock is no longer stable. It changes through metamorphic
reactions to restore equilibrium
Contact Metamorphism
Contact Metamorphism – occurs when the country rock surrounding an intrusion is heated causing the rock to
recrystallise
There is NO change in pressure so the crystals in the metamorphic rock remain randomly orientated
• Just increase in temperature – the higher the temperature the rock gets, the greater the
metamorphism
• Adding heat to the system causes the country rock to bake or recrystallise
• Affects rock lying next to an intrusion in the country rock
• Heat is conducted into the country rock
• Once the intrusion has cooled below a certain temperature metamorphism will stop
, Contact Metamorphism Rocks
Original Rock Metamorphic Rock
Sandstone: Metaquartzite:
• Clastic texture • Crystalline
• Sand (medium) size grains (approx. 1mm) • Non-foliated
• Well sorted • Granoblastic texture
• Sub-rounded to sub-angular grains Becomes • Crystals are subhedral
• Compositionally orthoquartzite as more • Size depends on amount of
than 90% quartz recrystallization
• There is a very fine cement holding the • No reaction with acid.
grains together
Oolitic Limestone: Marble:
• Clastic texture • Crystalline
• Sand (medium) size grains (approx. 1mm) • Non-foliated
• Well sorted • Granoblastic texture
• Very well-rounded grains Becomes • Crystals are subhedral
• Compositionally oolites are calcite • Size depends on amount of
• There is a very fine cement holding the recrystallisation
grains together with occasional shell • Reacts with acid
fragments
Shale: Spotted rock:
• Clastic texture • Crystalline
• Clay (fine) size grains (approx. <1/256mm) • non-foliated
• Well sorted Becomes • Porphyroblastic texture
• Too fine to see the shape • Porphyroblasts of andalusite are
• There are irregular laminations with the euhedral and coarse grained
rock picked out by larger quartz grains • Groundmass is fine grained
Shale: Hornfels:
• Clastic texture • Crystalline
• Clay (fine) size grains (approx. <1/256mm) • Non-foliated
• Well sorted Becomes • Granoblastic texture
• Too fine to see the shape • Subhedral
• There are irregular laminations with the • Coarse grained
rock picked out by larger quartz grains
Metamorphic Aureole:
The Size of the Metamorphic Aureole Depends on:
• Initial temperature of the igneous body (linked to its composition)
• The size (volume) of the igneous body
• Composition of the country rock
• The amount of water in the country rock
• The dip of the contact affecting outcrop width on a map
Within a Metamorphic Aureole:
• A trend can develop – the higher the temperature reached by the country rock, the greater the
amount of recrystallisation can take place and the bigger the crystals will become
For contact metamorphism to take place, the country rock needs to reach a certain temperature for
recrystallisation to start. Below this temperature there isn’t enough energy for the chemical reactions
Indications that Metamorphism has Started – development of porphyroblasts of minerals e.g. andalusite
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