2 Metamorphic Rocks Flashcards
Metamorphisis
Change of State
Unseen Metamorphism
Occurs deep in the crust
How can we see it?
Erosion and uplift
Metamorphic rock formation
Formed from pre-existing rocks
Protolith
SOLID state recrystallization
From changes in the temperature, pressure, or chemically active fluids
New minerals from old
+ TIME
Formed from pre-existing rocks
Protolith
SOLID state recrystallization
Temperature
Pressure
Directional or uniform
Chemically active fluids
+ TIME
This is a S-L-O-W process (usually)
Millions of years
Solid State Recrystallization: Clay into Ceramic
Atoms of a mineral are reorganized and rearranged
Particles of clay are fused and quartz recrystallizes
Water, carbon, and sulfur will be driven off
New minerals are formed
Minerology and texture
Protolith to metamorphic rock:
Protolith: Red Shale
quartz, clay, iron oxide
Metamorphic rock: Gneiss
quartz, felspar, biotite, garnet
Protolith: Fossiliferous limestone
fossil fragments, shells, etc
Metamorphic rock: Marble
interlocking grains of intergrown calcite
metamorphic processes
Recrystallization
Size and shape change, composition does not
Neocrystallization
New minerals grow through chemical reactions
Plastic deformation
Mineral grains soften and deform, no breakage: affects entire grain whole thing deformed
Pressure solution
Grains partially dissolve where surfaces touch: middle of the grain remains untouched like a jawbreaker
Contact Metamorphism
Around hot igneous intrusions
Narrow bands (thin halo around igneous rocks) of altered/baked not melted rocks
Between 1 m – 100s m thick
Notch Peak, Utah
Dynamic Metamorphism
Fault zones
Shallow
Brittle, no metamorphism
Deep (+15km)
Ductile, smear like taffy
Forms a metamorphic rock called mylonite
rock becomes long and skinny streaks because it is being squashed and smeared
Regional Metamorphism
The biggest deal to us
Deep in the crust
Usually convergent plate boundaries
Mountain building events
Appalachians, Himalaya
Wide, deep, extensive
Directional stress, two plates coming together to form mountains
Exposed Continental shields
Roots of folded mountain belts
orogeny
mountain building events that gives us clues about what has happened in the past
Metamorphic classification
Once again: Texture and Composition
Texture
Non-foliated Rocks (be able to turn in circles and not be able to see the switch in directionality in it)
Foliated Rocks (directional, lineation on rock)
Foliation
Visible direction of mineral growth
- Alignment of platy minerals or alternating light/dark bands
“Layers”
- But not sedimentary strata!
Subjected to differential stress
foliated Metamorphic Rocks: Slate
from shale
Slate
Fine grained rock
Micas grow in response to stress (they are being squashed in a certain direction of growth, breaks like the pages of a book)
Low grade metamorphism
foliated Metamorphic Rocks: schist
from slate
Schist
Strongly foliated
“Schistosity”=Shimmer of platy minerals (micas)
Mid-grade metamorphism
Platy minerals (bioite and muscovite micas) aligned with each other=shiny under a microscope and without sometimes
foliated Metamorphic Rocks: gneiss
from schist
Gneiss
Extremely foliated (gneissic banding)
Light and dark minerals group together (mafic and felsic)(through stress and chemical/physical weathering)
Quartz & Feldspar together
Amphibole and biotite together
High metamorphic grade
foliated Metamorphic Rocks: Migmatite
from gneiss
Migmatite
Extremely foliated
Actual melting of certain minerals: Quartz & Feldspar
Not melted: mafic
Igneous and metamorphic mixed
creates ribbons of melted rock in the metamorphic rock
foliated Metamorphic Rocks: stretched pebble conglomerates
Metamorphism of a conglomerate
“Stretched pebble conglomerates”
Raft River Mountains
round river rocks Flattened by pressure solution and plastic deformation
Non-foliated Metamorphic Rocks: marble
Marble
Interlocking large grains of calcite
Recrystallization of limestone
Organic carbon either cooked out, or concentrated into dark veins
looks kind of foliated, but it’s not
Non-foliated Metamorphic Rocks: Quartzite
Quartzite
Metamorphism of quartz sandstone
Entire rock recrystallized
Sometimes, sedimentary features can still be seen
Used in buildings, flagstones, patios, fireplaces…very durable and sturdy
Metamorphic Fact #1: You can’t make something from nothing:
You can’t make something from nothing: (sand grains=quartz)
Composition of the protolith dictates the type of metamorphic rock
If the original rock is sandstone, then what is it made of?
What do we have to make new minerals?
Metamorphic Fact #2: Heat Matters
Temperature change causes metamorphism
-High temperatures causes different minerals to be stable
-May also allow minerals to grow bigger
Causes of temperature change
-Geothermal gradient: shove down into the earth
-Upwelling hot magma: hot stuff around it
Metamorphic Fact #3: pressure causes change
Higher pressure causes
-metamorphism
High pressures causes different minerals to be stable
-New minerals will be more dense
Causes of pressure
-The deeper, the more pressure
-Directional stress
Newly formed minerals will be elongated
Metamorphic Fact #4: hot fluids are super reactive
Chemically active fluids cause metamorphism
Mostly water and carbon dioxide
Carries ions through the rock to create new minerals
Sources of water
Trapped in sedimentary protolith
Given off by cooling intrusive rocks
rain
Can create ore deposits
Kennecott Copper
Metamorphic Fact #5: geologic time is needed
None of these conditional changes happen fast
Long periods of time are needed
Environments of Metamorphism: contact metamorphism
Contact Metamorphism:
High temperature
Low pressure
Fluids
Environments of Metamorphism: Seafloor metamorphism
Seafloor Metamorphism
Moderately high temperature
Low pressure
Fluids playing the biggest role in temp change and formation
Environements of metamorphism: regional metamorphism
Regional Metamorphism
Need convergent (continental) plate boundaries High temperature
High pressure
Fluids
“Roots” of mountains
Location of most intense metamorphism
Intensity of metamorphism decreases away from the mountains
