Metamorphic Rocks Flashcards
What is metamorphism?
The recrystallisation of pre-existing rock in a solid solution via pressure and/or heat
What is contact metamorphism?
Localised in a metamorphic aureole. The predominant force is heat (pressure present too). When magma intrudes country
Are we likely to see alignment/foliation with contact metamorphism?
No, as there is little pressure
What is regional metamorphism?
Larger regional area. The predominant force is pressure. Happens at convergent boundaries at subduction and collision zones
Are we likely to see alignment/foliation with regional metamorphic?
Yes, as there is significant pressure
What is dynamic metamorphism also known as?
Cataclasis
What is dynamic metamorphism?
Happens in fault zones, where rocks are moving past each other suddenly. Rocks are old, cold, brittle - causing rocks to be crushed, resulting in angular clasts. (cataclasis)
Then recrystallisation to hold clasts together
What is foliation?
Formed by the preferred alignment of flat, platy minerals (micas, clays). These must be present along with pressure. Usually formed during regional metamorphism
What is the sequence of regional metamorphic rocks starting with shale?
Shale - Parent rock
Slate, phyllite, schist, gneiss
What would happen if gneiss undergoes more pressure?
It would melt
What is shale made up of?
clays and quartz
What is slate?
Foliated and has slaty cleavage. Made of micas. Low grade metamorphism
What is phyllite?
Wavy foliation, made of micas
What is schist?
Foliated and schistosity. Made of micas. May have garnet porphyroblasts
If garnet is found, what rock type is it?
Always metamorphic
What is gneiss?
Dark and light bands. Foliation and gneissose banding. Made of micas
What is the daughter rock of shale in contact metamorphism?
Spotted rock. But at an intrusion, hornfels (closest) then andalusite then spotted rock
When undergoing contact metamorphism, sandstone becomes…?
metaquartzite
When undergoing contact metamorphism, limestone becomes…?
Marble
What is marble?
White, granoblastic with medium grain size, but the size increases with metamorphic grade. Calcite crystals
What is metaquartzite?
White or grey. Granoblastic with medium grain size, but increases with metamorphic grade. Quartz crystals
What is spotted rock?
Darker spots with poorly formed minerals. Poorly developed with slaty cleavage and fine grained. Clay and micas
When does diagenesis occur? (According to the metamorphic grade graph)
<200°C
<10 km depth pressures
When does contact metamorphism happen? (According to the metamorphic grade graph)
> 200°C
Shallow depth and pressure. Heat is the main source
When do igneous rocks form? According to the metamorphic grade graph)
When pressure and temps are both high.
>600°C. They can form at a range of depth depending on pressure and heat. It can happen at lower temps if pressure is significant
What did George Barrow discover?
Index minerals in metamorphic rocks
What index minerals did George Barrow discover in metamorphic rocks?
Chlorine (low)
Biotite
Garnet (mid)
Kyanite
Silimanite (high)
What is granoblastic?
Metamorphic texture. Interlocking equidimensional crystals
What is schistosity?
Metamorphic texture.
Medium-coarse rocks. Preferred alignment of flat/tabular minerals. Alignment perpendicular to stress. No traces of original bed remain
What is gneissose banding?
Light and dark layers. Mm or cm thick.
Light = granoblastic
Dark = schistosity
What is the difference between porphyritic and porphyroblastic?
Porphyritic = igneous texture. Two stage cooling.
Porphyroblastic = metamorphic. Recrystallisation
What is a common metamorphic porphyroblast?
Garnet in mica schist
