metamorphic rocks Flashcards
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What is metamorphism?
The process that alters rocks through heat, pressure, and chemical fluids
What are the two main types of metamorphism?
1) Regional Metamorphism: Affects large areas due to tectonic forces
2) Contact Metamorphism: Occurs near igneous intrusions
Describe slate and its characteristics
- Fine-grained, formed from shale
- Exhibits slaty cleavage
- Composed of clay minerals and mica
- Result of low-grade metamorphism
What are the characteristics of phyllite?
- Fine-grained with silky sheen
- Wavy or crinkled foliation
- Contains muscovite, chlorite, and quartz
- Low-grade metamorphism
- Grey or pale green in color
Describe the key features of schist
Medium to coarse-grained (1-5 mm)
* Contains porphyroblasts (large crystals like garnet)
* Shows schistosity (aligned mica crystals)
* Contains muscovite, biotite, and quartz
* Medium-grade metamorphism
What are the characteristics of gneiss?
- Coarse-grained (>5 mm)
- Light and dark bands (gneissose banding)
- Contains feldspar, quartz, biotite, and kyanite
- Result of high-grade metamorphism
Describe marble and its properties
- Composed of calcite or dolomite
- Reacts with acid
- White or light-colored
- Formed from limestone
What are the key characteristics of metaquartzite
Composed of quartz
* White or grey in color
* Formed from sandstone
Define porphyroblasts
Large crystals formed during metamorphism
What is schistosity
Parallel alignment of platy minerals in medium to coarse-grained metamorphic rocks
Define gneissose banding
Alternating layers of light and dark minerals
What is metamorphic grade
A measure of the intensity of metamorphism, determined by temperature and pressure conditions.
What is country rock
Pre-existing rock that has been affected by metamorphism.
Define metamorphic aureole
The zone around an igneous intrusion where rocks have been altered by heat. Can extend up to 10 km from the intrusion.
What are index minerals
Minerals that form under specific temperature and pressure conditions, indicating metamorphic grade
What characterizes contact metamorphism?
- High-temperature, low-pressure conditions
- Creates metamorphic aureole
- Forms spotted rocks with dark mineral clusters
What are the key features of regional metamorphism?
Associated with high pressures and temperatures
- Typically occurs in mountain-building regions
- Metamorphic grades increase with depth
How does thermal gradient work in metamorphic aureoles?
Temperature decreases with distance from the intrusion, producing a sequence of index minerals that indicate different metamorphic grades.
What are the three grades of regional metamorphic rocks and their characteristic minerals?
- Low-grade: Contains minerals like chlorite
- Medium-grade: Contains minerals like garnet
- High-grade: Contains minerals like sillimanite
How do index minerals help geologists?
They help determine the metamorphic history of a rock because each mineral forms at a specific range of temperature and pressure.
What is slaty cleavage?
Low-grade metamorphic fabric where rocks split into thin sheets due to aligned platy minerals under compression
What characterizes schistosity?
Visible alignment of mica crystals creating a shiny appearance in medium-grade metamorphic rocks like phyllites and schists.
Describe gneissose banding
Alternating light (quartz, feldspar) and dark (biotite, mica) bands in high-grade metamorphic rocks.
What is porphyroblastic fabric?
Large crystals (porphyroblasts) surrounded by finer-grained matrix, showing evidence of metamorphic growth.
What is granoblastic fabric?
Random orientation of equal-sized grains with sugary texture, typical in non-foliated rocks like marble and quartzite.
What is crenulation cleavage?
Wavy or crinkled textures formed when older foliations are folded during multiple deformation events.
what dos fabrics mean interms of classifying metamorphic rock?
the texture of the rock
What are the main categories of fabric identification in metamorphic rocks?
- Fine-grained (e.g., slate, phyllite)
- Medium-coarse grained (e.g., schist, gneiss, quartzite, marble)
- Rocks without preferred alignment (e.g., spotted rock)
How do fabrics help geologists understand metamorphic rocks?
Fabrics reveal:
* Conditions of temperature and pressure during metamorphism
* History of deformation events
* Help reconstruct metamorphic histories, especially during mountain-building events
What are the key factors that influence metamorphic fabrics?
Reaction Rates:
* Temperature levels
* Presence of water
* Speed of ion movement
2. Time and Deformation:
* Stress levels
* Rate of deformation
* Duration of metamorphic processes
How do stress-related changes affect metamorphic fabrics?
Stress affects fabrics in two main ways:
* Rapid stress (like faulting) creates cataclasites
* Slower stress forms foliated textures
These changes help reveal the history of metamorphic pressure and temperature
How can mineral timing be used to understand metamorphic fabrics?
- Pre-kinematic: Formed before deformation
- Syn-kinematic: Formed during deformation
- Post-kinematic: Formed after deformation
This timing can be determined through microscopic analysis of features like folded micas, inclusions, and mineral alignment
What are index minerals and what do they indicate?
Index minerals are specific minerals that indicate metamorphic grade:
* Low-grade: Chlorite
* Medium-grade: Biotite, garnet
* High-grade: Kyanite, sillimanite
They form at specific temperature and pressure conditions, helping geologists determine metamorphic history.
What is retrograde metamorphism?
Retrograde metamorphism is:
* The recrystallization of rocks during cooling
* Occurs when fluids like H₂O and CO₂ are reintroduced
* Results in hydration and oxidation reactions
* Has slower chemical reactions due to decreasing temperatures
What are the different geothermal gradients in metamorphism?
Contact metamorphism
* Regional metamorphism
* Subduction zones
Each represents different temperature-pressure pathways in metamorphic environments.
