Overview
This lecture covers the processes, environments, and distinguishing features of metamorphic rocks, emphasizing metamorphic grades, textures, and the geological stories they tell.
Contact Metamorphism
- Occurs near igneous intrusions where hot magma heats surrounding rock.
- The closer the rock is to the intrusion, the higher its metamorphic grade.
- Fluids from the intrusion or groundwater can cause mineral recrystallization.
- Results in unique minerals ("star minerals") and economically significant deposits, e.g., copper and iron mines.
Regional Metamorphism
- Driven by plate tectonics: subduction zones, mountain building (orogeny), and large-scale crustal movement.
- Involves high pressure and temperature over large areas, leading to extensive recrystallization.
- Highest grade rocks are found at the core of mountain chains.
- Metamorphic grade decreases outward from the core.
Metamorphic Grades and Index Minerals
- Low grade: slate (from shale), with fine grains and laminated foliation.
- Medium grade: phyllite (wavy foliation, more mica), schist (shiny, flaky, larger mica and garnets).
- High grade: gneiss (coarse, banded foliation), migmatite (partially melted, most deformed).
- Metamorphic grade increases with temperature and pressure; grain size and mineral changes reflect this.
Metamorphic Textures
- Foliated: Layered appearance due to differential (non-uniform) stress, e.g., slate, phyllite, schist, gneiss.
- Non-foliated: Lacks visible layering, formed under uniform stress, e.g., marble (from limestone), quartzite (from sandstone).
- Foliation results from minerals aligning perpendicular to stress.
Sequence of Metamorphic Changes
- Progression: shale → slate → phyllite → schist → gneiss → migmatite.
- Each step involves increased metamorphic grade, larger grains, and new minerals.
Practical Identification
- Foliated rocks: Look for visible layers, banding, or a scaly, shiny appearance.
- Non-foliated rocks: Typically massive, lack patterns; hardness or acid reaction can distinguish marble from quartzite.
Field Observations & Interpretation
- Geologists observe outcrops, take notes, and sample rocks to reconstruct geological history.
- Rock properties and structures reveal information about past tectonic and metamorphic events.
Key Terms & Definitions
- Metamorphism — transformation of rocks through heat, pressure, and fluids.
- Contact Metamorphism — local heating and alteration of rocks by magma.
- Regional Metamorphism — large-scale alteration due to tectonic forces.
- Foliation — planar arrangement of mineral grains or structural features in rock.
- Slate — low-grade, fine-grained foliated metamorphic rock from shale.
- Phyllite — fine-grained, wavy foliated metamorphic rock with mica.
- Schist — medium-grade, shiny, mica-rich foliated metamorphic rock.
- Gneiss — high-grade, coarse-grained, banded metamorphic rock.
- Migmatite — partially melted, highest-grade metamorphic rock.
- Marble — non-foliated rock from limestone, mainly calcite.
- Quartzite — non-foliated rock from sandstone, mainly quartz.
Action Items / Next Steps
- Review posted lecture slides on Canvas for images and diagrams.
- Practice identifying metamorphic rocks and textures using samples if available.
- Prepare for exam questions on metamorphic grade progression and rock identification.