Metamorphic Rocks

Question 1

metamorphism

Answer 1

processes of mineralogical and textural change that occur in a rock when it is subjected to pressure, temperature, and fluid conditions different than those when the parent rock formed

Question 2

parent rock

Answer 2

original rock before metamorphism

Question 3

rock cycle and metamorphic rocks

Answer 3

• P and T conditions are above those of sedimentary lithification and below those of melting
• Typically between 200-850 degrees celcius and >300Mpa
• Magmas are the only thing that can’t be directly turned into a metamorphic rock -> needs to cool and become a rock first

Question 4

what forces drive metamorphism?

Answer 4

1. changes in temperature

2. changes in pressure

Question 5

changes in temperature

Answer 5

• Average geothermal gradient 25degC/km depth
• Geothermal gradient = amount of increase in temperature with depth in the earth
• Varies depending on geologic setting (ie. Higher gradients close to mid-ocean ranges or close to igneous extrusions)

Question 6

changes in pressure

Answer 6

• Lithostatic (equal in all directions) increase in pressure due to depth
• Differential (directed stress, which may be compressive or shear -> some areas receive more stress than others)
• Compressive differential stress produces a foliation -> needs platy minerals to form
• Minerals grow at 90 degrees to the pressure source -> ie. If the pressure is coming from top and bottom, layers will be horizontal

Question 7

2 classes of metamorphic rocks

Answer 7

• foliated

- non-foliated

Question 8

foliated rocks

Answer 8

Ex. Schist (metamorphosed mudstone) with strong foliation = differential pressure and platy minerals

Question 9

non-foliated rocks

Answer 9

• Ex. Marble (metamorphosed limestone) with no foliation = lithostatic pressure (equal pressure in all directions) only OR no platy minerals
• Pure limestones only consist of carbonates, impure limestones have some clay or mud in them

Question 10

changes during progressive metamorphism

Answer 10

• Recrystallization (coarsening)
• Formation of new minerals
• Foliage (cleavage)
• Metamorphic rock is typically denser

Question 11

metamorphic process of shale

Answer 11

• Shale -> slate -> phyllite -> schist -> gneiss -> migmatite (right before it melts)
• As we go along, pressure and temperature increase

Question 12

slate

Answer 12

rock takes on a platy breaking habit, >1mm planes

Question 13

phyllite

Answer 13

fine-grained micas forming; gives a “sheen” on foliation planes

Question 14

schist

Answer 14

individual minerals (micas, garnet) are recognizable

Question 15

gneiss

Answer 15

medium to coarse-grained with alternating layers of mafic (dark) and felsic (light) material

Question 16

migmatite

Answer 16

• rock begins to melt in situ = “mixed rock” with veins and patches of granitic melt preserved
• Form where high-grade metamorphic rocks exceed the melting temperature of the felsic layers but not of the mafic layers

Question 17

metamorphic facies

Answer 17

• Particular metamorphic rocks form in particular P-T regimes/spaces
• Some facies are restricted in tectonic setting -> different metamorphic rocks form at different tectonic settings

Question 18

exposure of metamorphic rocks

Answer 18

• Crust is like a memory-foam mattress -> thick crust with heavy mountains will sag a bit, then as mountains erode and crust gets lighter, is rises up to compensate -> rocks that were once deep in the earth are now close to the surface
• This process = Isostasy – an isostatic rift
• another analogy: Like an iceberg

Question 19

where does metamorphism occur?

Answer 19

• shock metamorphism
• contact metamorphism
• regional metamorphism

Question 20

shock metamorphism

Answer 20

• Impact craters
• Brief but extremely high pressure and temperature
• Effects: recrystallization of quartz to high P polymorphs, ejected blobs of molten rock called tektites

Question 21

contact metamorphism

Answer 21

• Adjacent to intrusions in shallow crust (<5km)
• Country rock heated by conduction and hydrothermal convection
• Generally low P so non-foliated
• Often associated with hydrothermal alteration
• Results: recrystallization, new minerals, veins form

Question 22

regional metamorphism

Answer 22

• Occurs over large areas of the crust due to increased P-T at depths >5km
• Effects: recrystallization, new minerals form, metamorphic foliation develops

Question 23

index materials in regional metamorphism

Answer 23

• Polymorphs: forms of minerals with same composition but different arrangement of atoms
• Stable at different P and T conditions

Question 24

temperature vs. depth vs. tectonic settings

Answer 24

• For the same depth (P), the temperature – depending on tectonic setting
• This for a given depth, different metamorphic rocks form in different settings

Question 25

4 broad metamorphic settings in a convergent margin

Answer 25

A) Subduction zone (low T high P)
B) plate interior (normal geothermal gradient)
C) volcano-plutonic complex (near arcs – high P and T)
D) Shallow depths (contact metamorphism – low P very high T)

Question 26

fluids and metamorphism

Answer 26

• H20 in pore spaces carries dissolved ions in solution
• Fluids increase rates at which minerals form
• Rock veins: water can get into veins and travel down near magma -> evaporates and travels up -> cools down as it travels up and precipitates gold and quartz

Question 27

hydrothermal metamorphism

Answer 27

• Important at mid-ocean ridges
• Ocean water seeps into ocean crust – heats up and rises
• Returned to ocean via vents (eg. Black smokers) - rich in mineral sulfides
• As water moves through crust, it chemically hydrates the rocks (basalts and gabbros)
• Olivine and pyroxene get converted to hydrous minerals like amphiboles
• Rocks turn green -> “Greenstones”
• Water released from these at subduction zones promotes melting and magma generation