WEEK 3 - Metamorphic Rocks Flashcards
Metamorphic Rocks
Formed by heat, pressure, and chemically active fluids transforming pre-existing rocks
Three Main Types of Metamorphism
Contact Metamorphism → Heat, uniform pressure
Regional Metamorphism → Heat, differential pressure
Metasomatism → Chemically active fluids
Contact Metamorphism
Rock is “baked” under low pressure near igneous intrusions
Creates a metamorphic halo (aureole) around the intrusion
Localized effect—doesn’t alter large regions
Regional Metamorphism
Happens on a large scale, affecting rocks deep underground
Mountain-building events cause intense heat and pressure
Rocks are compressed & buried, often forming foliation
Metamorphism & Recrystallization (The Two Pressures)
Uniform pressure → Randomly oriented mineral grains
Differential pressure → Minerals align, forming foliation
Rock composition remains mostly unchanged
Foliation: Preferred Grain Orientation
Contact metamorphism = Uniform pressure (random grain orientation)
Regional metamorphism = Differential pressure (minerals align)
Foliated rocks: Platy minerals (mica, pyroxene, amphibole) line up perpendicular to pressure
Foliation vs. Bedding
Foliation = Reoriented minerals due to metamorphism
Bedding = Original sedimentary layering
Don’t confuse the two! Bedding can be overprinted by foliation
Foliated Metamorphic Rocks
Formed by regional metamorphism under differential pressure
Contain platy or elongate minerals that align due to stress
Higher metamorphic grade → larger mineral grains, more pronounced foliation
Metamorphic Progression: Shale to Gneiss
Shale → Slate → Phyllite → Schist → Gneiss
Process Shale to Gneiss Explanation
Shale (sedimentary) undergoes heat & pressure
Slate forms (low-grade, fine-grained, slaty cleavage)
Phyllite (higher-grade, shiny due to mica growth)
Schist (medium-grade, visible minerals, strong foliation)
Gneiss (high-grade, banded appearance, most transformed)
Progressive Metamorphism: Increasing Heat & Pressure
🔥 More heat & pressure = bigger crystals & more foliation!
Shale (parent rock) → Metamorphic changes → Different textures & mineral sizes
Higher grade = More distinct foliation
Very Low Metamorphic Grade - Slate
Derived from shale, undergoes minimal metamorphism
Minerals start transforming into small, platy micas
Foliation Type: Slaty cleavage
Low to Medium Metamorphic Grade - Phyllite
More heat & pressure than slate
Silky sheen due to growing platy mica crystals
Foliation Type: Phyllitic foliation
Medium to High Metamorphic Grade - Schist
Larger, mirror-like mica crystals = sparkly texture ✨
Foliation Type: Schistose foliation
High to Very High Metamorphic Grade - Gneiss
Mica grains form elongated, tabular shapes
Minerals separate into bands (light & dark) → gneissic banding
Foliation Type: Gneissic foliation
Increasing Heat and Pressure: Key Concept
🔥 More heat & pressure → bigger minerals & stronger foliation!
What are Non-Foliated Metamorphic Rocks?
Do not show foliation
Often formed by contact metamorphism (heat + uniform pressure)
Mineral grains grow in size & interlock
Examples of Non-Foliated Metamorphic Rocks
Marble (from limestone, made of calcite)
Quartzite (from quartz sandstone, made of quartz)
Hornfels (from shale, forms under contact metamorphism)
Granoblastic Texture
Random grain orientation due to uniform pressure
Quartzite & marble show granoblastic texture
Fossils in original rock are obliterated during recrystallization
🔥 No platy minerals (like mica) = No foliation!
Granoblastic Rocks in Art & Architecture
Marble (a non-foliated metamorphic rock) is widely used in sculptures due to low hardness (calcite = 3 on Mohs scale)
Marble & other non-foliated rocks are used in architecture for entrances & floors
Color variations & fracture patterns reflect tectonic activity (folding & faulting)
What is Metasomatism?
A process where fluids (water or CO₂) react with a rock, changing its chemical composition
Can involve simple hydration (adding water) or chemical replacement of elements
Example:
- Olivine reacts with water → forms Serpentine
Metasomatized Metamorphic Rocks: Serpentine
Serpentine is a rock formed through metasomatism
Has a platy to fibrous crystal structure
Used in sculptures since ancient times
Soapstone Formation & Use
Serpentine reacts with dissolved quartz → forms Talc
Soapstone (mostly made of talc) is a metamorphic rock formed in this process
Widely used by Aboriginal artists of the Canadian Arctic for carving sculptures
🔥 Metasomatism alters rocks chemically, leading to new minerals like serpentine & talc!
What is Jade?
A Metasomatized Metamorphic Rock
Composition of Jade
Made of Jadeite (a pyroxene) or Nephrite (an amphibole)
Forms through metasomatism (chemical alteration by fluids)
Properties & Uses of Jade
Extremely tough, making it highly durable
Carved into intricate sculptures & jewelry due to its strength
🔥 Jade’s toughness makes it a prized material for art & ornamentation!
