L11: Metamorphic Rock Flashcards
What is metamorphism?
- Occur when a rock is subjected to new temperatures and pressures
- Rocks change mineralogically and texturally until its in equilibrium with the new set of temperatures and pressures
- Changes the composition of a rock
What are the 3 types of metamorphism?
- Contact
- Burial
- Regional
Contact metamorphism
- Transformed by heat
- Hot magma rises to the surface
Burial metamorphism
- Sediment is buried and accumulates under moderate temperatures and pressures
- Follows on from diagenesis
Regional metamorphism
- Buried deep into the crust, undergo elevated temperatures and pressures
- Commonly takes place during mountain building and it occurs on scales much larger than simple intrusions
- Think of subduction and mountain building!!
What do plate tectonics affect?
Greatly affects the metamorphic reactions because the process may move sediments and rocks from earth’s surface to interior, where the temperatures are higher
As you go ______, temperature goes ______.
- Down
- Up
What is a geothermal gradient?
- Increase in temperature with increasing depth
- Steeper where crustal thinning occurs
- Shallower where the continental lithosphere is old and thick
What do geologists use to study composition of rocks?
geothermometer/geobarometer
What is metasomatism?
Metamorphic process in which abundant fluids change the composition and texture of a rock
What are metamorphic textures?
- Foliated
- Granoblastic (non-foliated)
- Porphyroblastic
Foliated rocks
- Show alignment of minerals
- Have preferred orientation due to directed stresses
- Have clear texture
- Typically slate, phyllite, schist, gneiss
- Typical parent rocks = shale, sandstone
Granoblastic rocks (non-foliated)
- No clear texture
- Grow in equant shapes (such as cubes or spheres)
- Form from metamorphism in which deformation is absent (such as contact metamorphism)
- Hornfel, quartzite, marble
- Parents: shale, basalt, limestone or dolomite
Porphyroblastic rocks
- Large crystals set in a fine-grained matrix
- Surrounded by much finer grain matric
- Contact and regional metamorphism rocks
- Dissolved and reorganized
- Rock name: slate to gneiss
- Parent: Shale
What else increases as the intensity of metamorphism does?
So does crystal size and coarseness of foliatio
Examples of non-foliated rocks
- Marble
- Quartzite
- Hornfel
Define metamorphic facies
- The assemblage of minerals formed during metamorphism of a rock of a given composition subjected to a given temperature and pressure
- Partly depends on the composition of the parent rock
Zeolite minerals
- A class of silicates that contain interstitial water in cavities within the crystal
- AKA zeolite grade
- Basalt usually lowest grain minerals
Greenschists
A higher grade of metamorphosed rocks; abundant minerals include chlorite
Amphibolites
Contain amphibole and then the granulites (often contain pyroxene)
Blueschist
- High pressure, moderate temperature
- Typically blue in color
Ecologites
Produced in the highest temperature conditions
What are the 2 essential points that characterize the concept of metamorphic facies?
- Different kinds of metamorphic rocks form from parent rocks of different composition at the same grade of metamorphism
- Different kinds of metamorphic rocks form at different grades of metamorphism from parent rocks of the same composition
What are isograds?
Used to plot metamorphic grades over a regional metamorphic belt
