Metaphoric Rocks Flashcards
Protolith
Parent rock - the type of rock that was metamorphosed into a metamorphic rock
What causes metamorphism?
Heating, squeezing under pressure, and hot fluids
Over which temperatures and pressures does metamorphism occur?
Occurs over a set range of P&T conditions (pressure and temperature), the higher the P&T, the higher the degree of metamorphic grade
When does the parent rock start metamorphing?
Around 200*C
At what temperature does metamorphism (all grades) occur, when the rock is under low pressure in the shallow crust?
200C to 850C
What happens to the required temperature for metamorphism as the pressure increases?
The temperature required for metamorphism decreases as pressure increases
At what temperature do rocks start melting?
850*C
Three ways that metamorphism can occur
Heating, pressure, and hot fluids
The average (normal) geothermal gradient in the crust
Around 25*C/km
Where do you find very high temperatures near the surface?
At mid-ocean ridges, because that’s where you would find magma near the surface, and magma is melted rock which is above 850*C
Contact Metamorphism
When hot magma body “bakes” the surrounding country rock
Contact Aureole
The zone of contact of metamorphism is narrow (mm to 10mm) and completely surrounds the intrusion
Metamorphism by hot fluids
- water is everywhere, so it is involved in nearly all metamorphic reactions
- water speeds up metamorphic reactions
- hydrothermal fluids often contain dissolved elements rejected from normal silicate minerals that form as magma solidifies
- fluids migrate into cracks in the surrounding rocks, changes in p & t cause minerals to precipitate out of solution
Hydrothermal metamorphism
When water is the main agent of metamorphism
Lithostatic stress
When pressure on a rock increases evenly in all directions
Directed stress
Stress that is higher in one direction
Regional metamorphism
Directed stress in large areas (the crust adjacent to convergent plates always experiences directed stress, this compression causes crust to thicken, producing mountains)
How do folded rocks form?
Directed stress
What kinds of metamorphism occur at subduction zones?
Contact, hydrothermal, and regional metamorphism
How does metamorphism affect composition?
Elements get rearranged (not added or removed), this happens because minerals that are stable at the surface of the Earth become unstable as P/T rise
How does temperature affect the composition of a mineral?
Some minerals are stable at low T, some at high T; minerals are always more stable at higher temperatures when rocks are under pressure
How does pressure affect the composition of a mineral?
Pressure can cause atoms in minerals to become more closely packed, which is why metamorphic rocks tend to be more dense
At what state (solid, liquid, gas) do metamorphic reactions happen?
Solid state only (no melting!!!)
Recrystallization
Minerals change size and shape without changing identity (ex - from quartz arenite to quartzite)
Neocrystallization
New minerals grow that were not present in the protolith (ex - from shale to phyllite)
Minerals that indicate metamorphic grade (index minerals)
Chlorite, muscovite, biotite, garnet, kyanite, sillimanite
Remnant clastic texture
Rock that appears granular or sugary, but the grains won’t rub off
Non-foliated vs foliated texture
Non-foliated rocks do not have layers, foliated rocks do
Where does slate form?
Shallow depths
Where does gneiss form?
Deep in the cores of mountains
