Unit 2 Rocks: Note 8 Metamorphic Rock Formation

Metamorphic Rocks: pre-existing rock changes into new rock because of increased pressure, temperature, and/or chemically reactive fluids

• cannot occur if rock is magmatizied (too much temperature increase or completely fractured (too much erosion)

Metamorphism: the process by which rock is transformed into a new and different type of rock

Metamorphic Terminology:

1. Grade: the degree to which a rock is metamorphosed

2. progress from low to high grade, high grade means high temperature and pressure

3. the more a grade increase, the more new mineral growth occurs and the growth of grain size

Metamorphism & Plate Tectonics

1. Temperature changes as plates shift, the rock within the plate changes

2. Subduction will cause plate to undergo higher temperature and pressure change

Controls of Metamorphism

1. Parent Rock: called the protolith, this is the rock type prior to the metamorphism

   • most rock keeps the same overall chemical composition as their parent rock

   • mineral composition influences the degree to which each metamorphic agent causes change, this is how the same elements can produce different minerals

2. Temperature: influences the mineral stability and provides energy for crystallization

   • some minerals occur in higher temperatures, other cannot form when in high temperatures

3. Pressure: influences mineral stability and the resulting texture of a rock, the greater the depth the greater the pressure

   1. Confining pressure- equal pressure from all directions (this happens when rock is buried and pushed at depth)

   2. Directed stress- when much high pressure is exerted onto one side compared to the others (this happens at convergent plates)

4. Polymorphs: rocks that have the same chemical formula, but different structures,

   1. This is dependent on temperature and pressure

   2. Example: kyanite, andalusite, and sillimanite all have the same chemical makeup

      • Kyanite exists at lower temperatures with a medium depth, Andalusite occurs at a moderate temperature with shallow depth, Sillimanite occurs at high temperatures with a medium to low depth

      • There is a point where all three minerals are stable, but this specific point is small so the 3 are likely to form independently

• We can use minerals to determine how much pressure and temperature the rock experienced

• We can use temperature and pressure to predict which polymorph will be present as long as all the elements are occurring

Metamorphic Textures: Determined by Pressure

1. Texture: size, shape, and arrangement of grains

2. Foliation: any planar arrangement of mineral grains or structural features within a rock

   • can be scales, color bands, squished pebbles

   • must be parallel to be planar

3. Low Grade metamorphism: small temperature and pressure increase

4. High Grade metamorphism: exposed to high levels of pressure and temperature increase

Foliated Rocks (Layering):

1. Slate: parent rock is shale/mudstone

   1. low grade metamorphism

   2. fine grained

   3. slated cleavage: breaks along flat plants

   4. Ping test: when hit with a metal slate pings, mud thuds

2. Phylite: parent rock is slate

   1. Mienral grains are a bit bigger

   2. New crystals appear because of higher temperature and pressure, has a glossier sheen and waves

   3. mainly muscovite and chlorite

3. Schist: parent rock is phylite

   1. Medium coarse/grain

   2. plate minerals are predominant

   3. many new minerals occur in higher temps and pressures

   4. Garnets, micas, kyanite

   5. Schistosity: describes the texture

4. Gneiss: parent rock is schist, granite, or volcanic rocks

   1. medium/coarse grain

   2. bands of colors, alternating between light felsic layers and dark mafic layers

   3. high grade metamorphism

Causes of Foliation

1. Directed Stress: stress in the directioon of the collision, which impacts the minerals and texture of rock

2. Layering: ways minerals respond to stress

   1. Rotation- of plated or elongated minerals

   2. Change- of haphazard grains into elongated aligned grains

   3. Recrystallization- of minerals due to direct stress

Non Foliated Rock Textures: Develops in environments where deformation is minimal

• commonly in minerals with equidimentional crystals

Marble: limestone which is from a marine environment, undergoes metamorphism

1. Grains in calcite get bigger, interlocking

2. Composition remains the same, still fizzes with acid

Quartzite: sandstone which is sedimentary undergoes metamorphism

1. larger grains, fused by hard quartz

2. remains a quartz-sandstone

Equidimentional: equal pressure from multiple directions

1. Anthracite- coal from organic material undergoes metamorphism

   • compacts, becomes more concentrated in carbon and is very fine

Impact of Fluid & Time

1. Water: facilitates transport of ions and can speed up reaction reacts

2. Time: metamorphic reactions are relatively slow on the order of millions of years

   • if changes occur before the appropriate time has passed, the reaction will not reach equillibrium