Metamorhic Flashcards
What is the geological environment of contact metamorphism
- Adjacent to igneous intrusions/metamorphic/aureole/baked margin
- plutons, magma chambers, sills, dykes
- range high temperature to low temperature
What is the geological environment of regional metamorphism
- subduction zones/fold mountain root/deep burial orogenic
- associated with destructive plate margins/organic belt
- high temperature
Why is clay the most likely rock to give rise following metamorphism
- most chemically diverse rock
- composed of clay minerals
- clay is rich in Al, Si and O
- other rocks do not contain aluminium
- mica from clay minerals
Why can metamorphic processes not take place below 200C
not enough energy for recrystallisation
Why would the metamorphic aureole be less wide in wet limestone compared to dry limestone
-dry rock conducts heat/wet rock converts heat
-water allows faster transfer of heat by convection
-leaving for temp to allow met changes
What geological factors could influence the width of the metamorphic aureole around a pluton
- temperature of intrusion
- size of intrusion
- time since intrusion
- angle of contact with country rock
- nature of country rock (fractured/permeability/rock type)
- thermal conductivity if country rock
How might water and rock type affect the width of a metamorphic aureole
- narrower aureole (limestone) wider aureole (shale)
- limestone is permeable (joints) but shale is impermeable
- water allows transfer of heat by convection away from pluton (permeable limestone) but water unable to transfer heat by convection as far in impermeable shale (dryer)
- leaving insufficient/sufficient heat (or time) to allow metamorphic changes
Why might there be a zone of spotted rock near a granite pluton
- Part of the pluton closer to (but beneath) the surface
- irregular boundary of pluton
- reference to local topography
- thus lower grade metamorphic aureole exposed
- thermal conductivity if country rock
What are features of plutons that may influence the width of their metamorphic aureoles
- temperature of intrusion
- size of intrusion
- time since intrusion
- magma mixing/multiple injection
- depth of intrusion/angle of the contact with country rock
Where is the metamorphic rock hornfels most likely to form
Contact metamorphism
- high temperature/low pressure
What does porphyroblastic texture mean
- Large crystals e.g. garnet
- finer ground mass
- mica (foliations) wrapped around garnet
What evidence is there to suggest that the principle stress directions have changed during the metamorphism of a rock
- schistosity different orientation to inclusions in garnet
- indicates pressure max has changed over time
- s shaped nature of inclusions (suggesting continuing change)
- porphyroblasts may have rotated
How does evidence suggest that the granite is unlikely to be responsible for the metamorphism of floated sediments
- it came after - isograds no concentric around intrusion
- isograds cut by granite
- granite occurs in an area of mixed metamorphic grade
- contact metamorphism does not form a schists - regional
- no foliation with contact metamorphism
- scale of metamorphism - granite too small to metamorphose whole area
Explain schistose texture and it’s origin
- porphyroblastic/large and small crystals
- grain size
- foliation
- minerals recrystallised (under direct stress)
- long axes of crystals aligned at right angles to directional stress
- regional metamorphism/temperature and pressure
What are some tests that can be sued to determine where a rock is marble or metaquartzite
Test 1 - add drops of dilute hydrochloric acid to each specimen
Result - the marble effervesces and gives of carbon dioxide
Test 2 - try to scratch them with a steel pin
Result - the Marble scratches easily
