6: Metamorphic Rocks & Processes Flashcards
What is metamorphism?
Definition: Mineralogical and structural change of rock due to physical (temperature and pressure) and chemical conditions
What are the agents of metamorphism?
Heat:
- Most important agent
- Drives recrystallization, creating new/more stable minerals
Pressure:
- increases with depth and with tectonic activity
Chemically active fluids:
- They transport heat and ions in solution and promote recrystallziation
What is the average geothermal gradiant?
25C/km, meaning it increases by 25C every km you go down!
What happens to a rock when it gets metamorphosed?
Minerals change:
- From less stable to more stable
Crystals grow larger
Crystals realign, creating foliated and / or banding
rock density increases
Note: The starting rock (parent rock, aka protolith) is IMPORTANT
What does metamorphic rocks start with and end as?
Metamorphic rocks change grades from low to high and ROCKS REMAIN SOLID DURING METAMORPHISM!
- As T and P increase, rocks increase grade:
Sedimentary Rock (shale)
START
Slate
Phyllite
Schist
Gneiss
Migmatite
END
Igneous Rocks (granite)
In Metamorphic rock, what difference does the “parent rock” (protolith) make
Metamorphic rocks have the same chemical composition as the rocks they were formed from
Different minerals, but made of same atoms
sandstone-> quartzite
Granite -> Gneiss
Limestone -> marble
shale -> slate -> schist
What are metamorphic textures / foliation?
Textures refer to the size, shape, and arrangement of mineral grains/crystals within a rock
Foliation forms when pressure squeezes the flat or elongate minerals within a rock so they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.
How can foliation form?
They can be formed through:
- Rotation of platy of elongated minerals
- Recrystallization of minerals in a preferred orientation
- Changing the shape of 3D-grains into elongated and aligned shapes
describe Slate:
Slate (1)
- very fine grain
- excellent rock cleavage
- From metamorphism of low-grade shale
describe phyllite:
Phyllite (2)
- Made of small platy minerals
- glossy sheen
describe schist
Schist (3)
- Medium to coarse grain (visible minerals)
- Comprised of platy minerals (mica) and other silicates
- among most abundant types of metamorphic rocks
describe gneiss:
Gneiss
- medium to coarse grain
- High grade metamorphism
- Banded appearance
- Poor Foliation (does not split easily
What happens if only heat is involved?
NO FOLIATION
- need pressure for that
You get Non-foliated metamorphic rocks
- Quartzite: metamorphosed primarily quartz sandstone (quartz)
- Marble:
metamorphosed primarily calcite (limestone)
Describe the non-foliated metamorphic rock Quartzite:
- Formed from sandstone as a parent rock
- quartz grains are fused together
- some of the strongest rocks encountered in engineering
Describe the non-foliated metamorphic rock Marble:
- Formed from limestone as a parent rock
- Coarse, crystalline
- calcite grains are fused together
