Lecture 7 Flashcards
What is metamorphism?
Changes when a rock is subjected to temperatures or pressures very different from where they formed.
What types of rocks can serve as parent rocks in metamorphism?
Metamorphic, igneous, or sedimentary rocks.
Does the rock remain solid during metamorphism?
Yes, the rock remains solid.
What determines the degree of change during metamorphism?
The degree of the processes involved.
What are the controlling factors in metamorphism? (IMPORTANT)
Composition of the parent rock, heat, pressure, and chemically active fluids.
How can clay minerals and calcite change during metamorphism? (IMPORTANT)
Clay minerals can re-crystallize to form micas, and calcite in marble is derived from a limestone parent.
Do metamorphic rocks usually change composition from their parent rocks? (IMPORTANT)
No, except in extreme cases of fluid migration.
Why is heat considered the most important agent of metamorphism? (IMPORTANT)
It influences the mobility and reactivity of chemically active fluids.
What are the two sources of heat in metamorphism? (IMPORTANT)
Intrusive igneous bodies and Earth’s internal heat.
At what temperature do clay minerals become unstable and start to change to micas? (IMPORTANT)
Beyond 200°C.
What are the two types of pressure in metamorphism? (IMPORTANT)
Confining pressure (uniform stress) and directed pressure (differential stress).
How does confining pressure affect rocks? (IMPORTANT)
It applies forces equally in all directions, making rocks harder and denser.
What does directed pressure result in? (IMPORTANT)
Distortion of a body and the formation of foliation.
What are the three basic types of directed pressure?
Compressional, tensional, and shear.
How do rocks behave under pressure at the surface versus at depth?
Rocks are brittle at the surface and fracture, while they are ductile at depth and can flatten and elongate.
What is foliation? (IMPORTANT)
A layered or banded texture resulting from the preferred orientation of platy and elongate minerals in a metamorphic rock.
What three factors influence the development of foliation? (IMPORTANT)
- Rotation of platy and elongate mineral grains into a new orientation.
- Changing the shape of equi-dimensional grains into elongate shapes.
- Re-crystallization of minerals to grow new grains in the direction of the preferred orientation.
What causes minerals to dehydrate at depth?
Higher temperatures and pressures.
What is metasomatism?
The process where fluids with a different chemical makeup change the composition of surrounding host rock.
What is metamorphic grade?
The intensity or degree of metamorphism that rocks have experienced.
How is metamorphic grade indicated?
By the sequential appearance of index minerals and by textures.
What are index minerals?
Minerals stable under specific ranges of pressure-temperature (P-T).
What are examples of low- and high-grade index minerals?
Low grade: Chlorite (~200ºC).
High grade: Sillimanite (~600ºC).
What are the controlling factors that determine the type of metamorphism and the texture of the rocks that are formed?
The type of metamorphic rock that is formed depends on the pressure, temperature, fluids, and original parent rock composition.
