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
