Metamorphism Flashcards
Define metamorphism.
Rocks changing in response to changes in temperature and pressure
What factors does the product rock of metamorphism depend on?
Chemical composition of original rock, temperature, pressure, and stress
What are the two ways a rock can be heated as part of metamorphosis?
Burial and nearby igneous intrusion
What is the temperature range of heat metamophism?
Starts around 200ºC and ends around 650-800ºC
Why does metamorphism stop around 650-800ºC?
Because that’s the temperature where crustal rocks start to melt
What types of rocks does metamorphism usually affect?
Sedimentary
What happens to carbonates when they are heated?
Carbon dioxide is expelled.
What happens to clays when heated as part of a metamorphic process?
Water is expelled to produce mica
What generally happens to the grainsize of a mineral under metamorphosis?
Becomes coarser.
With increasing pressure, _________ minerals are formed.
Denser
What is the distinction between high and low grade metamorphism?
Low = 200-500ºC and mostly low pressures
High = >500ºC with high pressures
What temperatures does low grade metamorphism usually occur at?
200ºC to 500ºC
Above what temperature is metamorphism usually high grade?
500ºC
Why does high temperatures usually mean high pressures too, in terms of metamorphism?
High temperature metamorphism is usually the result of burial, which usually has higher pressures.
What is differential stress?
When pressure on a rock varies in different directions.
What does differential stress cause?
Foliation
What is foliation?
A planar fabric where grain aligns at a normal to the direction of highest pressure
What is foliation on a large scale?
Cleavage
What is foliation called in lower grade metamorphism?
Slaty cleavage
What is foliation called in higher grade metamorphism?
Schistosity
What are the 3 types of low grade metamorphism?
Cataclastic metamorphism
Burial metamorphism
Contact metamorphism
Describe cataclastic metamorphism. Is it low or high grade?
Mechanical deformation of a rock with only minor recrystallisation.
- low grade
Describe burial metamorphism. Is it low or high grade?
Caused by an increase in temperature with depth, but only a small increase in pressure, only minor recrystallisation.
- Can be both
Describe contact metamorphism.
Occurs when hot magma intrudes into cool rocks, mechanical deformation is minor or absent
What is a metamorphic aureole? What size is it normally?
The heated region around an igneous intrusion
Usually <100m
What are the Siberian Traps?
A region of Siberia dominated by coals, hydrocarbon course rocks, evaporites and remnants of large scale volcanic activity
What did contact metamorphism produce post-eruption in the Siberian Traps? What was concerning about this?
Organohalogens
Which were linked to Stratosphere ozone destruction
What is regional metamorphism? Is it high grade or low grade?
A type of metamorphism caused by mountain building and continental changes which causes very pronounced foliations and new minerals.
High grade
What are metamorphic rocks?
A way of grouping metamorphic rocks by what temperature and pressure their minerals were formed at
What are the 3 metamorphic facie groups?
1) High T, low P usually in the vicinity of igneous intrusion in the shallow crust
2) High T, high P most common
3) Low T, high P very rare
What do shale/mudstone metamorphose to? (in order)
Shale -> Slate -> Phyllite -> Schist -> Gneiss
What does sandstone/sandy sediments metamorphose to?
Psammite
What does quartz sandstone metamorphose to?
Quartzite
What does limestone metamorphose to/
Marble
Which type of igneous rocks show a change after metamorphism? Examples?
Less silica rich igneous rocks
e.g. Basalt or Gabbro
What does basalt/gabbro metamorphose to? What is this composed of?
A metabasite
made of amphiboles
What is contact metamorphic rock known as?
Hornfels
