Lecture 12 Flashcards
What are the main agents of metamorphism that transform a protolith into a metamorphic rock?
1) Temperature: Elevated heat drives recrystallization. 2) Pressure: High pressure favors denser mineral phases. 3) Differential stress: Leads to aligned minerals/foliation. 4) Hydrothermal fluids: Hot, reactive solutions can cause metasomatism.
How do we distinguish foliated from nonfoliated metamorphic rocks?
Foliated rocks show a layered or planar fabric from aligned platy minerals (e.g., slate, schist, gneiss). Nonfoliated rocks lack such alignment, often because minerals are equant or there’s little differential stress (e.g., marble, quartzite).
Why does foliation form in some metamorphic rocks?
Foliation develops under differential stress, causing minerals to align or segregate into bands. This produces a layered, banded, or streaked appearance distinct from the random orientation in nonfoliated rocks.
What are common foliated metamorphic rocks, and how do they differ?
1) Slate: Fine-grained, low-grade with slaty cleavage. 2) Phyllite: Fine-grained with a shiny sheen. 3) Schist: Medium/coarse with visible mica flakes. 4) Gneiss: Light/dark bands (gneissic banding). 5) Migmatite: Partially melted gneiss, mixing igneous and metamorphic traits.
Can you give examples of nonfoliated metamorphic rocks and their protoliths?
Quartzite (from quartz sandstone), Marble (from limestone), Hornfels (from various protoliths, often ‘baked’ by contact metamorphism). These rocks lack a preferred orientation of minerals.
Why are certain minerals (like clay) unstable under metamorphic conditions, while others (like quartz) persist?
Minerals have specific temperature-pressure stability ranges. Clay is stable at low T–P near the surface but breaks down under metamorphic conditions. Quartz is stable over a wide range and typically survives or simply recrystallizes.
What is metamorphic grade, and what does it tell us?
Metamorphic grade reflects the intensity of metamorphism, mainly related to temperature (and pressure). Low-grade rocks (e.g., slate) undergo subtle changes; high-grade rocks (e.g., gneiss) show extensive recrystallization and coarser textures.
Describe the main types/settings of metamorphism.
1) Burial: Moderate T–P at depth. 2) Regional: Large-scale T–P during mountain building. 3) Contact: Local heating near an intrusion. 4) Hydrothermal: Hot fluids alter rock. 5) Subduction-zone: High P, low T (blueschist). 6) Dynamic: Shearing in fault zones. 7) Shock: Meteorite impacts.
How does regional metamorphism relate to mountain building?
Mountain building (orogeny) thickens the crust, generating high pressures, and tectonic collisions produce high temperatures. This widespread metamorphism creates large volumes of foliated rocks (e.g., schist, gneiss) at depth.
Why might a single protolith (like mudstone) form different metamorphic rocks (slate, schist, gneiss)?
As T–P conditions increase, the same protolith undergoes progressive metamorphism: mudstone → slate (low grade) → phyllite → schist (medium grade) → gneiss (high grade). Each stage reflects different mineral and textural changes.
